INTERNATIONAL HYDROGRAPHIC ORGANIZATION

HYDROGRAPHIC DICTIONARY

Part I

Volume I, English

Special Publication No. 32

FIFTH EDITION

MONACO1994

600-XII-1994 S-32

INTERNATIONAL HYDROGRAPHIC ORGANIZATION

HYDROGRAPHIC DICTIONARY

Part I

Volume I, English

Special Publication No. 32

FIFTH EDITION

MONACO1994

P R E F A C E

The preparation of the Fifth Edition of the Hydrographic Dictionary was undertaken by a WorkingGroup established in accordance with the Bureau's Circular Letter CL 45/1984 of 13 December 1984.

The Working Group was comprised of staff members of the Hydrographic Offices of Argentina,Croatia (associate member), France, and the United States of America (NOS). It carried out its work underthe chairmanship of Captain H.-P. Rohde, IHB.

The Hydrographic Dictionary is published in two parts. Part I contains terms and definitions in thetwo official IHO languages and is published in separate volumes for each language. The definitions areintended to give concise explanations of terms, without necessarily considering specific applications orinterpretations as, for example, for legal matters. Definitions of terms in Part I which were taken frompublications of other authorities, are given without quoting these authorities. Other IHO publicationscontaining glossaries were consulted to ensure consistency.

Part II will consist of separate supplements that provide a translation of the terms into variouslanguages other than the official IHO languages. In each supplement, the terms will be listed alphabetically inEnglish, with their equivalent in the other language, but without a translation of the definition. Eachsupplement will also contain an alphabetical listing of the terms in the other language, giving the relevantindex number of the term in the English volume of Part I.

The English text in Volume I of this Dictionary has been modified to ensure maximum agreementwith the French text and also to take into account certain more recent sources which have become availablesince the date of the Fourth Edition.

Valuable advice has been received from the Hydrographic Offices of a number of Member Statesand from other users of the Hydrographic Dictionary which sent the Bureau lists of proposed amendments tothe 1990, Fourth Edition.

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I N T R O D U C T I O N

Terms and expressions are listed alphabetically in English, according to standard dictionarypractice, and assigned index numbers to be used as reference for the translation of terms from one language toanother.

The definitions are given in English, one of the two official languages of the IHO.

The work is cross-referenced in the following manner:

a) Most of the terms and expressions appearing in the definitions when defined under anotherheading are printed in capital letters. Terms or expressions consisting of several words may belisted under a heading with a different word order. In some cases reference is only made to thegeneric term.

b) Attention is drawn to related terms by the words 'see', or 'see also' followed by the term (incapital letters).

c) When synonyms exist, they are usually given at the end of a definition and printed in italics.

Abbreviations

adj. adjectiveadv. adverbn. nounv.t. verb transitivev.i. verb intransitivepl. plural

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A

1 abeam. In a line approximately at right angles to the ship's keel; also: the waist or middle part of the ship.

2 aberration of light. In ASTRONOMY, the apparent displacement in position of a HEAVENLY BODY caused by thecombination of the VELOCITY of light and that of an observer on the surface of the EARTH. Aberration of light due tothe ROTATION of the EARTH on its AXIS is termed diurnal aberration. That due to the REVOLUTION of the EARTHaround the SUN is termed annual aberration. In optics, failure of an OPTICAL SYSTEM to bring all LIGHT RAYSreceived from a point object to a single IMAGE POINT or to a prescribed geometric position. Spherical aberration iscaused by RAYS from various zones of a LENS or MIRROR coming to FOCUS at different distances from the LENS orMIRROR. Chromatic aberration (or chromatism) is due to the differences in REFRACTION of the coloured RAYS of theSPECTRUM; those of each colour having a different FOCUS.

3 abrasion. The wearing away or rounding of surfaces by friction.

4 abscissa. See COORDINATES: PLANE RECTANGULAR.

5 absolute accuracy. See ACCURACY: ABSOLUTE.

6 absolute error. See ERROR.

7 absolute orientation. See ORIENTATION.

8 absolute stereoscopic parallax. See PARALLAX.

9 absorption: atmospheric. Transformation of RADIANT ENERGY into thermal, mechanical, electrical, etc. energy, byinteraction with atmospheric constituents.

10 abyssal(adj.). Belonging to the lowest DEPTHS of the OCEAN, generally below 3,700 METRES (2,000 FATHOMS).

11 abyssal gap. A gap in a sill, ridge, or rise that lies between two abyssal plains.

12 abyssal hills. A tract of small elevations on the deep-sea floor.

13 abyssal plain. An extensive, flat, gently sloping or nearly level region at abyssal depths.

14 acceleration. The rate of change of VELOCITY.

15 acceleration: angular. The rate of change of ANGULAR VELOCITY.

16 acceleration of gravity. The ACCELERATION of a freely falling body caused by the force of GRAVITY.

17 accidental error. See ERROR.

18 acclivity. An upward SLOPE of GROUND; as opposed to DECLIVITY.

19 accommodation. The faculty of the human eye to adjust itself to give sharp IMAGES for different object distances. InSTEREOSCOPY, the ability of the eyes to bring two IMAGES into superimposition for stereoscopic viewing.

20 accretion. The gradual building up of LAND over a long period of time, solely by the action of the forces of nature, on aBEACH by deposition of water or air-borne material. Artificial accretion is a similar build-up of LAND by reason of anact of man. Also called aggradation.

21 accuracy. The extent to which a measured or enumerated value agrees with the assumed or accepted value. SeePRECISION.

22 accuracy: absolute. The evaluation of all errors encountered in defining the position of a single feature or point on aGEODETIC DATUM or system.

23 accuracy: relative. The evaluation of the RANDOM ERRORS in determining the positional orientation of one point orfeature with respect to another.

24 accuracy: repeatable. See REPEATABLE ACCURACY.

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25 achromat. See ACHROMATIC LENS.

26 achromatic lens. A compound LENS that has been partly corrected for chromatic aberration. Such a LENS iscustomarily made to bring green and red LIGHT RAYS to approximately the same point focus. Also called achromat. SeeABERRATION OF LIGHT.

27 aclinic line. See EQUATOR: MAGNETIC.

28 acoustic. Science of sound, including its production, transmission and effects.

29 acoustic bearing. See BEARING: SONIC.

30 acoustic line of position. See SONIC LINE OF POSITION.

31 acoustic navigation. See NAVIGATION: SONIC.

32 acoustic sounding. See ECHO SOUNDING.

33 acoustic wave. See WAVE: SOUND.

34 active satellite. 1. An artificial satellite which transmits an electromagnetic signal. A satellite with the capability totransmit, repeat, or retransmit electromagnetic information as contrasted with passive satellite.2. As defined by the International Telecommunication Union (ITU), an earth satellite carrying a station intended totransmit or retransmit radiocommunication signals.

35 active system. Any electromagnetic or sonic position fixing system operating with an interrogation-response procedure.See also PASSIVE SYSTEM.

36 active tracking system. A satellite system which operates by transmission of signals to and receipt of responses from thesatellite.

37 adiabatic process. A thermodynamic change of state of a system in which there is no transfer of heat or mass across theBOUNDARIES of the system. In an adiabatic process, compression always results in warming, expansion in cooling.

38 adiabatic temperature change. RISE or fall of temperature due to compression or expansion without gain or loss of heatto the surroundings.

39 adjacent coasts. The coasts lying either side of the land boundary between two adjoining States.

40 adjusted position. See POSITION.

41 adjustment. The determination and application of CORRECTIONS to OBSERVATIONS or MEASUREMENTS, for thepurpose of reducing ERRORS or removing internal inconsistencies in observed results. Adjustment may either refer tomathematical procedures or to CORRECTIONS applied to instruments.

42 aerial. See ANTENNA.

43 aerial camera. See CAMERA.

44 aerial navigation. See NAVIGATION: AIR.

45 aerial photogrammetry. See PHOTOGRAMMETRY.

46 aerial photograph. See PHOTOGRAPHY.

47 aerial photography. See PHOTOGRAPHY.

48 aerial survey. See SURVEY.

49 aerial triangulation. See PHOTOTRIANGULATION.

50 aerology. Study of the FREE ATMOSPHERE.

51 aeromarine light. See LIGHT.

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52 aeronautical beacon. See BEACON.

53 aeronautical chart. See CHART.

54 aeronautical ground light. See LIGHT.

55 aeronomy. Term proposed for the study of the chemistry and physics of the high ATMOSPHERE.

56 aerotriangulation. See PHOTOTRIANGULATION.

57 affluent. A STREAM flowing into a larger STREAM or LAKE; a TRIBUTARY.

58 afloat. Floating, as opposed to being AGROUND.

59 age of diurnal inequality. The time interval between the maximum semi-monthly north or south declination of the moonand the maximum effect of the declination upon the range of tide or the speed of the tidal current, this effect beingmanifested chiefly by an increase in the height or speed difference between the two high (low) waters or flood (ebb)currents during the day. The tides occurring at this time are called TROPIC TIDES. Also called diurnal age.

60 age of parallax inequality. The time interval between perigee of the moon and the maximum effect of parallax upon theRANGE OF TIDE or the speed of the TIDAL CURRENT.

61 age of phase inequality. The time interval between new or full moon and the maximum effect of these phases upon therange of tide or the speed of the tidal current.

62 age of the moon. The elapsed time since the last new moon; usually expressed in DAYS and fractions of a DAY. SeePHASES OF THE MOON.

63 age of tide. See AGE OF PHASE INEQUALITY.

64 agger. See TIDE: DOUBLE.

65 aggradation. See ACCRETION.

66 agonic line. The line through all points on the EARTH's surface at which the MAGNETIC VARIATION is zero.

67 aground(adj. and adv.). Touching, resting or lodged on the BOTTOM of shallow water. The opposite is AFLOAT.

68 ahead. Bearing approximately 000° relative. The term is often used loosely for dead ahead or bearing exactly 000°relative. The opposite is ASTERN.

69 aid to navigation. BUOYS, BEACONS, FOG SIGNALS, LIGHTS, RADIO BEACONS, LEADING MARKS, radioposition fixing systems, and generally any charted or otherwise published device serving the interests of safeNAVIGATION. See NAVIGATIONAL AID.

70 aiming line. See COLLIMATION: LINE OF.

71 air almanac. See under ALMANAC.

72 air base. In PHOTOGRAMMETRY, the line joining two AIR STATIONS, or the length of this line; also, the distance (atthe SCALE of the STEREOSCOPIC MODEL) between adjacent PERSPECTIVE CENTRES as reconstructed in theplotting instrument. See also PHOTO BASE.

73 air camera. A CAMERA specially designed for use in flying craft.

74 air mass. Ensemble of air particles whose PATHS and physical properties exhibit, in the horizontal, only small andcontinuous differences. The mass may extend over an area of several million square miles and over a depth of severalkilometres.

75 air navigation. See NAVIGATION.

76 air photograph. See PHOTOGRAPH: AERIAL.

77 air photography. See PHOTOGRAPHY: AERIAL.

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78 air sextant. See SEXTANT.

79 air speed. The SPEED of an aircraft relative to the surrounding ATMOSPHERE. As opposed to GROUND SPEED.

80 air station. In PHOTOGRAMMETRY, the point in space occupied by the camera lens at the moment of EXPOSURE.Also called exposure station or camera station.

81 albedo. The ratio of radiant energy reflected to that received by a surface, usually expressed as a percentage.

82 alga. A plant of simple structure which grows chiefly in water, such as the various forms of seaweed.

83 aliasing. An occurence in tidal analysis when the sea level varies with a period that is less than the sampling period. Dueto the varying position of the samples on the curve, a spurious tidal frequency appears in the analysis. It is usually onlypossible when there is a seiche at the observation site or observations are taken at intervals greater than one hour.

84 alidade. Pivoted sight bar that moves over a graduated arc. The upper part of a THEODOLITE. The alidade used intopographic SURVEYING consists of a straight-edge ruler carrying a TELESCOPE or other sighting device and used inrecording a DIRECTION on the plane table sheet. The term is also used to describe a BEARING CIRCLE fitted with aTELESCOPE to facilitate OBSERVATION of BEARINGS. If a TELESCOPE is used, the instrument is often termed atelescopic alidade.

85 alignment. The placing of objects along a straight line. In NAVIGATION, the bringing into line of two or moreconspicuous objects, such as LIGHTS, BEACONS, etc. Also their BEARING as seen by an observer from seaward.

86 alignment correction(tape). See TAPE: ALIGNMENT CORRECTION.

87 alignment of base. The laying out of a BASE LINE in correct ALIGNMENT by means of a THEODOLITE and rangepoles, before the length of the line is measured.

88 alkalinity. The number of milliequivalents of hydrogen ions that are neutralized by 1 KILOGRAM of SEA WATER at20°C when a large excess of acid is added.

89 almanac. A periodical publication of astronomical data useful to a navigator. It contains less information than anephemeris, and values are generally given to less precision. If information is given in a form and to a precision suitable forair navigation, it is called an air almanac.

90 almanac: air. A periodical publication containing various computed astronomical DATA primarily intended for use inAIR NAVIGATION. See also EPHEMERIS.

91 almanac: nautical. An annual publication containing various computed astronomical DATA primarily intended for use inMARINE NAVIGATION. See also EPHEMERIS.

92 almucantar. Any SMALL CIRCLE on the CELESTIAL SPHERE parallel to the HORIZON. Also called parallel ofaltitude.

93 alphanumeric. Contraction of alphabetic-numeric. Characters including letters of the alphabet and numbers.

94 alt-azimuth instrument. An instrument equipped with both horizontal and vertical graduated CIRCLEs, for theOBSERVATION of horizontal and vertical DIRECTIONS or angles.

95 altimeter. An instrument that indicates directly the HEIGHT above a reference surface.

96 altimeter: barometric. A sensitive ANEROID BAROMETER calibrated to read the HEIGHT above a pressure datumaccording to the standard atmospheric pressure-to-height relationship.

97 altimeter: pulse radio. See ALTIMETER: RADIO.

98 altimeter: radar. See ALTIMETER: RADIO.

99 altimeter: radio. Any instrument used for determining an aircraft's flight altitude by the measurement of time intervalsbetween the emission and return of electromagnetic PULSES. Also called pulse radio altimeter or radar altimeter. SeeALTITUDE: FLIGHT.

100 altimetry. The process of determining the difference of ELEVATION by the use of ALTIMETERS.

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101 altitude. The vertical distance of a LEVEL, a point or an object considered as a point, measured from a given datum,usually MEAN SEA LEVEL. See also ELEVATION. In ASTRONOMY, the vertical angle between the plane of theHORIZON and the line to a CELESTIAL BODY. See also ANGLE OF DEPRESSION and ANGLE OF ELEVATION.

102 altitude: apparent. The SEXTANT ALTITUDE of a CELESTIAL BODY corrected for INDEX ERROR, DIP (HEIGHTof EYE), and (for SUN or MOON only) SEMIDIAMETER. Also called rectified altitude.

103 altitude: ex-meridian. An ALTITUDE of a CELESTIAL BODY near the CELESTIAL MERIDIAN of the observer, towhich a CORRECTION is to be applied to determine the MERIDIAN ALTITUDE. Also called circummeridian altitude.

104 altitude: flight. The vertical distance above a given DATUM, usually MEAN SEA LEVEL, or an aircraft in flight.

105 altitude: meridian. The ALTITUDE of a CELESTIAL BODY when it is on the CELESTIAL MERIDIAN of theobserver.

106 altitude: observed. The SEXTANT ALTITUDE of a CELESTIAL BODY after all corrections (see ALTITUDE:APPARENT) have been applied. In American terminology the term is usually employed in the place of TRUEALTITUDE.

107 altitude: rectified. See ALTITUDE: APPARENT.

108 altitude: sextant. The ALTITUDE as indicated by a SEXTANT before CORRECTIONS are applied.

109 altitude: true. The APPARENT ALTITUDE of a CELESTIAL BODY after all corrections have been applied. InAmerican terminology usually referred to as OBSERVED ALTITUDE.

110 altitude difference. See INTERCEPT.

111 altocumulus. See CLOUD GENERA.

112 altostratus. See CLOUD GENERA.

113 aluminium sheet. An aluminium-mounted paper sheet occasionally used for BOAT-SHEETS, SMOOTH SHEETS, orplane table boards.

114 ambient noise. The erratic electromagnetic or sonic background noise emitted by natural or artificial sourcescontaminating the proper signal.

115 ambiguity. Uncertainty of value or meaning because of the possibility of two or more interpretations. The condition whennavigational COORDINATES define more than one POSITION, DIRECTION, LINE OF POSITION, or SURFACE OFPOSITION.

116 Amici prism. A prism which deviates the RAYS of light through 90° and because of its shape inverts the IMAGE. SeeROOF PRISM.

117 amidships. At, near, or toward the middle of a ship.

118 ampere. The unit of density of electric current in the SI SYSTEM.

119 amphidromic point. A no-tide point, from which COTIDAL LINES radiate.

120 amplitude. In ASTRONOMY, the arc of the HORIZON, between a CELESTIAL BODY at rising or setting and true eastor west point. In tide terminology, the semi-range of the HARMONIC CONSTITUENT. In physics, the maximumdeparture of a WAVE or other periodic PHENOMENON from the average or zero position.

121 amplitude modulation. See MODULATION.

122 anaglyph. A STEREOGRAM in which the two views are printed or projected superimposed in complementary colours,usually red and green. By viewing through filter spectacles of corresponding complementary colours a STEREOSCOPICIMAGE is formed.

123 anaglyphic map. See under MAP.

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124 analemma. A SCALE of the SUN's daily DECLINATION drawn from TROPIC OF CANCER to TROPIC OFCAPRICORN on TERRESTRIAL GLOBES. An ORTHOGRAPHICAL PROJECTION of the sphere made on the planeof the MERIDIAN, the eye being supposed to be an infinite distance and in the east or west point of the HORIZON.

125 anallatic lens. A convergent LENS fitted between the OBJECTIVE LENS and the DIAPHRAGM of TELESCOPES ofinstruments for direct measurements of distances between the OPTICAL CENTRE of the instrument and a graduated staffcalled STADIA.

126 analogue. The way of representing information by continuous data.

127 analogue computer. See under COMPUTER.

128 analytical (or analytic) photogrammetry. See under PHOTOGRAMMETRY.

129 anastigmatic(adj.). Free from ASTIGMATISM. Corrected for ASTIGMATISM.

130 anchorage. An area in which vessels anchor or may anchor.

131 anchorage buoy(s). BUOY(S) marking the limits of an ANCHORAGE.

132 anchor buoy. A BUOY marking the position of an anchor.

133 anchor ice. ICE which is attached to the BOTTOM, irrespective of the nature of its formation. Also called bottom ice,depth ice, ground ice, lappened ice and underwater ice.

134 anemometer. Instrument used in the measurement of wind speed or of wind speed and direction. See alsoANEMOGRAPH.

135 anemogram. Record of the ANEMOGRAPH.

136 anemograph. An instrument which records WIND speed and direction. See also ANEMOMETER.

137 aneroid barometer. See BAROMETER.

138 aneroid capsule. Metallic capsule, of thin sides, partially evacuated by a fixed amount, carrying an arrangement to preventits collapsing under ATMOSPHERIC PRESSURE, and whose shape changes in accordance with changes of this pressure.

139 angle: check. See CHECK ANGLE.

140 angle: concluded. In a TRIANGULATION, the third angle of a triangle, not measured, but computed from the other twoangles.

141 angle: conversion. See ARC TO CHORD CORRECTION.

142 angle: crossing. The angle at which two LINES OF POSITION intersect. Also called angle of cut.

143 angle: danger. See DANGER ANGLE.

144 angle: drift. See DRIFT ANGLE.

145 angle: hour. See HOUR ANGLE.

146 angle: masthead. See MASTHEAD ANGLE.

147 angle: parallactic. See PARALLACTIC ANGLE.

148 angle: phase. See PHASE ANGLE.

149 angle: slope. See SLOPE ANGLE.

150 angle: vectorial. See COORDINATES: PLANE POLAR and COORDINATES: POLAR.

151 angle of convergence. See PARALLACTIC ANGLE.

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152 angle of cut. See ANGLE: CROSSING.

153 angle of depression. The angle in the VERTICAL PLANE between the horizontal and the line to an object below theHORIZON. Also called negative altitude.

154 angle of elevation. The angle in the VERTICAL PLANE between the horizontal and the line to an object above theHORIZON. Also called positive altitude.

155 angle of incidence. The angle between the line of motion of a RAY and the perpendicular to a surface at the point ofimpingement.

156 angle of reflection. The angle between the line of motion of a reflected RAY and the perpendicular to a surface at thepoint of REFLECTION.

157 angle of refraction. The angle between a refracted RAY and the perpendicular to the refraction surface.

158 angstroem. A unit of length, used especially in expressing the length of light waves, equal to one ten-thousandth of aMICRON or one hundred-millionth of a centimetre (1 x 10-8 cm).

159 angular acceleration. See ACCELERATION.

160 angular distance. The angular difference between two directions, numerically equal to the angle between two linesextending in the given directions. The arc of the GREAT CIRCLE joining two points, expressed in angular units.

161 angular distortion. See DISTORTION.

162 angular parallax. See PARALLACTIC ANGLE.

163 angular rate. See SPEED: ANGULAR.

164 angular speed. See SPEED.

165 angular velocity. See SPEED: ANGULAR.

166 annotation. Any marking on illustrative material for the purpose of clarification, such as numbers, letters, symbols, andsigns.

167 annual aberration. See ABERRATION OF LIGHT.

168 annual change. See MAGNETIC ANNUAL CHANGE.

169 annual inequality. Seasonal variation in water level or tidal current speed, more or less periodic, due chiefly tometeorological causes.

170 annual parallax. See PARALLAX.

171 annual variation. See MAGNETIC ANNUAL VARIATION.

172 annular eclipse. See ECLIPSE.

173 anomalistic month. See MONTH.

174 anomalistic year. See YEAR.

175 anomaly. Deviation from the normal or natural characteristics of a PHENOMENON. Abnormality. In ASTRONOMY,true anomaly is the angle at the SUN between lines connecting the SUN with a PLANET and with the PLANET'sPERIHELION. Mean anomaly and eccentric anomaly are also considered.

176 anomaly: gravity. See GRAVITY ANOMALY, BOUGUER ANOMALY, FREE AIR ANOMALY, ISOSTATICANOMALY.

177 anomaly: magnetic. See LOCAL MAGNETIC DISTURBANCE.

178 Antarctic circle. The GEOGRAPHIC PARALLEL having a south LATITUDE equal to the complement of theDECLINATION of the WINTER SOLSTICE (lat. = 66o33' S app.).

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179 antenna. A conductor or system of conductors for radiating or receiving RADIO WAVES. Also called aerial.

180 antenna: directional. An ANTENNA for transmitting or receiving SIGNALS in, or from, a given direction.

181 antenna: omnidirectional. An ANTENNA having essentially uniform response in AZIMUTH and a directional pattern inELEVATION.

182 antenna: screened. Any ANTENNA which is screened in some way to counteract disturbances.

183 antenna: unidirectional. An ANTENNA having a single well-defined direction of maximum radiation intensity ormaximum GAIN.

184 anthelion. Pure white, rounded spot, but sometimes iridescent or surrounded by coloured rings or arcs, which appears onvery rare occasions opposite the SUN and at the same HEIGHT above the HORIZON.

185 anticline. An arch of stratified ROCK in which the LAYERS bend downward in opposite directions from the CREST.

186 anticyclone. Region of the ATMOSPHERE where the pressure is high relative to the surrounding region at the sameLEVEL. It is represented on a SYNOPTIC CHART by a system of ISOBARS at a specified LEVEL or of CONTOURS ata specified pressure which enclose relatively high values of pressure or LEVEL. Also called high.

187 antimeridian. The MERIDIAN which is 180o of LONGITUDE from a given MERIDIAN. One MERIDIAN with itsantimeridian constitute a complete GREAT CIRCLE.

188 antipode(s). Place(s) on the surface of the EARTH directly opposite to each other.

189 antiselena. Luminous PHENOMENON analogous to ANTHELION, the luminary being the MOON.

190 anti-trade. Air current with a westerly component which in various subtropical regions of either hemisphere sometimesblows above the TRADE WIND.

191 aperture: relative. For a photographic or telescopic LENS, the ratio of the EQUIVALENT FOCAL LENGTH to thediameter of the ENTRANCE PUPIL. Expressed as f: 4.5; also called f-number, f-stop, or speed of lens.

192 aperture of antenna. The diameter of a reflector antenna, usually expressed in units of WAVE LENGTH.

193 aperture stop. The physical element (such as a DIAPHRAGM, or lens periphery) of an OPTICAL SYSTEM which limitsthe size of the PENCIL of RAYS traversing the system. Also called the stop. Diameter of that part of the LENS actuallyused.

194 apex. The top, PEAK, or highest point of something, as of a MOUNTAIN. See also VERTEX.

195 apex: solar. The point of the CELESTIAL SPHERE towards which the SOLAR SYSTEM as a whole is moving. It lies inthe CONSTELLATION of Hercules.

196 aphelion. That orbital point farthest from the SUN when the SUN is the centre of attraction, as opposed to PERIHELION.

197 aphotic(adj.). Without light.

198 apochromatic lens. A LENS that has been corrected for CHROMATIC ABERRATION for at least three colours.

199 apogean tide(s). See under TIDE.

200 apogee. That orbital point farthest from the EARTH when the EARTH is the centre of attraction, as opposed to PERIGEE.

201 apparent altitude. See ALTITUDE: APPARENT.

202 apparent horizon. See HORIZON.

203 apparent motion. See RELATIVE MOTION.

204 apparent noon. See NOON.

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205 apparent position. See POSITION.

206 apparent precession. Apparent change in the direction of the axis of rotation of a spinning body, as a GYROSCOPE, dueto the ROTATION of the EARTH.

207 apparent shoreline. The seaward limit of marine vegetation, such as mangrove, marsh grass, or trees in water that wouldreasonably appear to the mariner from a distance to be the fast shoreline. The seaward limit of kelp, low grass in water,and other low-lying vegetation normally does not constitute an apparent shoreline.A line drawn on the chart in lieu of the mean high water line or the mean water level line in areas where either may beobscured by marsh, mangrove, cypress, or other type of marine vegetation. This line represents the intersection of theappropriate datum with the outer limits of vegetation and appears to the navigator as shoreline.

208 apparent solar day. See DAY: SOLAR.

209 apparent solar time. See TIME.

210 apparent sun. See SUN.

211 apparent time. See TIME: APPARENT SOLAR.

212 apparent wind. See RELATIVE WIND.

213 approximate position. A position that is considered to be less than third-order accuracy, but is generally considered to bewithin 100 feet (30.5 metres) of its correct geographic location. The method of location may be an indication of therecorded accuracy.

214 apron. 1. A gently dipping featureless surface, underlain primarily by SEDIMENT, at the base of any steeper SLOPE.2. The area of wharf or quay for handling cargo.3. A sloping underwater extension of an iceberg.4. An outwash plain along the front of a glacier.

215 apsis(pl. apsides). See LINE OF APSIDES.

216 arc: astronomical. The apparent arc described above (diurnal arc) or below (nocturnal arc) the HORIZON by the SUNor another CELESTIAL BODY.

217 archipelagic apron. A gentle SLOPE with a generally smooth surface on the SEA FLOOR, particularly found aroundgroups of ISLANDS or SEAMOUNTS.

218 archipelagic baseline(s). See BASELINE.

219 archipelagic sea lane. Sea lanes designated by an ARCHIPELAGIC STATE for the passage of ships and aircraft.

220 archipelagic state. A State constituted wholly by one or more archipelagos, eventually including other islands.

221 archipelagic waters. The waters enclosed by ARCHIPELAGIC BASELINES.

222 archipelago. A group of ISLANDS.

223 arc of triangulation. See TRIANGULATION: ARC OF.

224 arc of visibility. The arc of a LIGHT SECTOR designated by its limiting BEARINGS in which a LIGHT is visible asviewed from a vessel.

225 Arctic circle. The GEOGRAPHICAL PARALLEL having a north latitude equal to the complement of theDECLINATION of the SUMMER SOLSTICE (lat. = 66o33' N app.).

226 arc to chord correction. The quantity used by seamen to correct a GREAT CIRCLE BEARING of a point B from a pointA to a MERCATOR BEARING, and usually referred to as the half convergency. On the MERCATOR PROJECTION,this quantity is equal to half the convergency, on the SPHEROID, of the MERIDIANS through the two points beingconsidered. Also called conversion angle. See CONVERGENCE OF THE MERIDIANS.

227 Area. In United Nations Law of the Sea terminology the sea-bed and ocean floor and subsoil thereof, beyond the limits ofnational jurisdiction.

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228 area clearance. The effective depth, within specified limits, obtained by a WIRE DRAG SURVEY.

229 area to be avoided. A routing measure comprising an area within defined limits in which either navigation is particularlyhazardous or it is exceptionally important to avoid casualties and which should be avoided by all ships, or certain classesof ships.

230 area survey. A method of BOTTOM RELIEF SURVEY consisting of surveying overlapping strips which allowsoundings to be obtained with a specified accuracy at any position of the area under survey.

231 argillaceous(adj.). Applied to all ROCKS or SEDIMENTS composed partly or completely of CLAY.

232 arming. Tallow or other substance placed in the recess at the lower end of a SOUNDING LEAD for obtaining a sample ofthe BOTTOM. See BOTTOM SAMPLE.

233 arm of the sea. A narrow portion of the SEA projecting from the main body. The expression is often shortened to 'arm'.

234 array(s). The order in which equipment (antenna, oceanographic equipment) or mathematical quantities are ordered.

235 articulated beacon. A vertical pipe structure that oscillates around a universal coupling connected to a sinker. Thestructure is kept upright by the buoyancy of a submerged floating chamber. Primarily designed to mark narrow channelswith greater precision than conventional buoys.

236 articulated light. See ARTICULATED BEACON.

237 artificial horizon. See HORIZON.

238 artificial intelligence. 1. The capability of a device to perform functions that are normally associated with humanintelligence, such as reasoning, learning, and self-improvement.2. Research and study in methods for the development of a machine that can improve its own operations. The developmentor capability of a machine that can proceed or perform functions that are normally associated with human intelligence, aslearning, adapting, reasoning, self-correction, automatic improvement.3. The study of computer and related techniques to supplement the intellectual capabilities of man. As man has inventedand used tools to increase his physical powers, he now is beginning to use artificial intelligence to increase his mentalpowers. In a more restricted sense, the study of techniques for more effective use of digital computers by improvedprogramming techniques.

239 artificial installation. See INSTALLATION OFFSHORE.

240 artificial island. See INSTALLATION OFFSHORE.

241 artificial satellite. See SATELLITE.

242 ascending node. See NODE.

243 ashore(adj. and adv.). On the SHORE, on LAND or AGROUND.

244 assumed position. See POSITION.

245 astenosphere. See MANTLE.

246 astern. Bearing approximately 180° relative. The term is often used loosely for DEAD ASTERN, or bearing exactly 180°relative. The opposite is AHEAD.

247 asteroid. One of the many small PLANETS revolving around the SUN, most of the ORBITS being between those of Marsand Jupiter. Also called PLANETOID or MINOR PLANET.

248 astigmatism. An aberration affecting the sharpness of IMAGES for objects off the AXIS, in which the RAYS passingthrough different meridians of the LENS come to FOCUS in different planes.

249 astro compass. See under COMPASS.

250 astrolabe. An instrument formerly used for measuring ALTITUDES of CELESTIAL BODIES. See ASTROLABE:PRISMATIC.

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251 astrolabe: prismatic. An ASTROLABE consisting of a TELESCOPE in a horizontal position, with a prism andARTIFICIAL HORIZON attached at its objective end, used for determining ASTRONOMIC POSITIONS.

252 astrometry. That branch of ASTRONOMY dealing with the determination of POSITIONS and motions of CELESTIALBODIES, including the EARTH.

253 astronautics. The science of locomotion and travel outside the EARTH's ATMOSPHERE, involving the problems ofARTIFICIAL SATELLITES and of interplanetary journeys.

254 astro-navigation. See NAVIGATION: CELESTIAL.

255 astronomic(al) arc. See ARC.

256 astronomic(al) azimuth. See AZIMUTH.

257 astronomic(al) control. See CONTROL.

258 astronomic(al) day. See DAY.

259 astronomic(al) equator. See EQUATOR.

260 astronomic(al) latitude. See LATITUDE.

261 astronomic(al) longitude. See LONGITUDE.

262 astronomic(al) meridian. See MERIDIAN.

263 astronomic(al) observation(s). See OBSERVATION.

264 astronomic(al) parallel. See PARALLEL.

265 astronomic(al) position. See POSITION.

266 astronomic(al) refraction. See REFRACTION.

267 astronomic(al) station. See STATION.

268 astronomic(al) tide. See TIDE.

269 astronomic(al) time. See TIME.

270 astronomic(al) transit. See TRANSIT.

271 astronomic(al) traverse. See TRAVERSE.

272 astronomic(al) triangle. The triangle on the CELESTIAL SPHERE formed by arcs of GREAT CIRCLES connecting theCELESTIAL POLE, the ZENITH and the CELESTIAL BODY. Also called navigational triangle.

273 astronomic(al) twilight. See TWILIGHT.

274 astronomic(al) year. See YEAR: TROPICAL.

275 astronomy. The science which deals with the size, constitution, motions, RELATIVE POSITIONS, etc. of CELESTIALBODIES, including the EARTH. See also ASTROMETRY.

276 astronomy: geodetic. ASTRONOMY as applied to GEODESY.

277 astronomy: nautical (or navigational). ASTRONOMY as applied to NAVIGATION.

278 atlas. A collection of CHARTS or MAPS to be kept (loose or bound) in a volume.

279 atmosphere. Gaseous envelope which surrounds the EARTH. The gaseous envelope surrounding any STAR, PLANET,etc. A unit of pressure, equal to the pressure exerted per square centimetre by a column of mercury 760 mm high at atemperature of 0°C when the ACCELERATION OF GRAVITY is 980.665 centimetres per SECOND2.

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280 atmosphere: free. That part of the ATMOSPHERE above the FRICTION LAYER, in which the air motion is affected toa negligible degree by surface friction.

281 atmosphere: ICAO. STANDARD ATMOSPHERE adopted by the International Civil Aviation Organization (ICAO). Itsmain features are mean sea-level pressure of 1013.25 mb, and a temperature lapse rate of 6.5o C/km from surface to theTROPOPAUSE situated at 11 km.

282 atmosphere: standard. Conventional reference atmosphere, chosen to represent in a simple way the approximate averagecondition in the VERTICAL.

283 atmospheric. ELECTROMAGNETIC WAVE resulting from an electric discharge (LIGHTNING) in theATMOSPHERE.

284 atmospheric drag. A major perturbation of close artificial satellite orbits by the resistance of the atmosphere.

285 atmospheric pressure. See PRESSURE.

286 atmospheric refraction. See REFRACTION.

287 atmospheric window. The region in the radiation spectrum containing wavelengths between 8.5 and 11 micrometers,which are not absorbed to any great extent by atmospheric gases. In the absence of cloud, terrestrial radiation of thiswavelength is lost to space, enabling the cooling of the earth to take place. Other narrower wavebands also do not absorblong-wave radiation, but these are of less importance in this context.

288 atoll. A coral ISLAND consisting of a ring-shaped REEF nearly or entirely surrounding a central LAGOON.

289 atom. The smallest particle of an element which can enter into a chemical combination.

290 atomic clock. See CLOCK.

291 Atomic Time: International. The time reference coordinate established by the Bureau International de l'Heure on thebasis of atomic clocks.

292 attenuation. A lessening in amount, particularly the reduction of the AMPLITUDE of a WAVE or the intensity of light orSOUND with distance from the origin.

293 audiofrequency. A FREQUENCY within the audible RANGE, about 20 to 20.000 CYCLES per SECOND. Also calledsonic frequency.

294 aurora. Luminous PHENOMENON which appears in the high ATMOSPHERE, mainly in high LATITUDES, in theform of RAYS, arcs, bands, draperies, or CORONA. Also called polar aurora.

295 aurora australis. AURORA of the southern hemisphere.

296 aurora borealis. AURORA of the northern hemisphere.

297 autocollimator. See COLLIMATOR.

298 automated cartography. The performance of cartographic functions in a fully automatic mode.

299 automatic floating station. A BUOY equipped with instruments automatically recording oceanographic andmeteorological DATA. Also called automatic buoy.

300 automatic gain control. A circuit which automatically maintains a constant output volume regardless of input signalstrength. Also called automatic volume control.

301 automatic plotter. See PLOTTER.

302 automatic tide gauge. See GAUGE: TIDE.

303 automatic volume control. See AUTOMATIC GAIN CONTROL.

304 automation. The technique of improving human productivity in the processing of materials, energy and information, byutilizing in various degrees elements of automatic CONTROL, and of automatically executed product programming.

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305 autopositive. A material which gives a positive copy from a positive TRANSPARENCY (or a negative from a negative)by direct processing.

306 autumnal equinox. See EQUINOX.

307 avulsion. Rapid EROSION of shoreland by WAVES during a STORM.

308 awash (adj. and adv.). Flush with, or washed by the WAVES.

309 axis. The line, real or imaginary, about which something centres or revolves.

310 axis: horizontal. In a THEODOLITE or TRANSIT, the AXIS about which the TELESCOPE of the instrument rotateswhen moved vertically. Also called trunnion axis, or transit axis.

311 axis: optical. See AXIS OF LENS.

312 axis: polar. In a system of POLAR or SPHERICAL COORDINATES, the primary AXIS of DIRECTION.

313 axis: principal. See AXIS OF LENS.

314 axis: trunnion. See AXIS: HORIZONTAL.

315 axis: vertical. In a THEODOLITE or TRANSIT, the line through the centre of the instrument about which the ALIDADErotates.

316 axis of camera. A line perpendicular to the FOCAL PLANE of the CAMERA and passing through the emergent nodalpoint of the LENS SYSTEM. Also called camera axis. See NODAL POINT.

317 axis of channel. The centre line of a CHANNEL. See also TALWEG.

318 axis of collimation. See COLLIMATION AXIS.

319 axis of homology. In PHOTOGRAMMETRY, the intersection of the plane of the PHOTOGRAPH with the horizontalplane of the MAP or the plane of reference of the GROUND. Corresponding lines in the PHOTOGRAPH and map planesintersect on the axis of homology. Also called the axis of perspective, or map parallel.

320 axis of lens. The straight line which passes through the centres of curvature of the lens surfaces. Also called principalaxis, optical axis or lens axis.

321 axis of optical system. The line formed by the coinciding principal axes of the series of optical elements.

322 axis of perspective. See AXIS OF HOMOLOGY.

323 axis of spirit level. The line tangent to the surface of a spirit level tube against which the bubble forms, at the centre of thegraduated SCALE of the LEVEL, and in the plane of the tube and its centre of curvature. Also called level axis or spiritlevel axis.

324 axis of tilt. In PHOTOGRAMMETRY, a line through the PERSPECTIVE CENTRE perpendicular to the PRINCIPALPLANE.

325 azimuth. A horizontal angle reckoned clockwise from the meridian. The horizontal direction of a celestial point from aterrestrial point, expressed as the angular distance from a reference direction. It is usually measured from 000° at thereference direction clockwise through 360°. An azimuth is often designated as true, magnetic, compass, grid, or relative asthe reference direction is true, magnetic, compass, or grid north, or heading, respectively. Unless otherwise specified, theterm is generally understood to apply to true azimuth which may be further defined as the arc of the horizon, or the angle atthe zenith, between the north part of the celestial meridian or principal vertical circle and a vertical circle, measured from000° at the north part of the principal vertical circle clockwise through 360°. When the angle is measured in eitherdirection from north or south, and labelled accordingly, it is properly called azimuth angle; when measured either directionfrom east or west, and labeled accordingly, it is called AMPLITUDE.

326 azimuth (of a celestial body). The angle between the observer's MERIDIAN and the VERTICAL CIRCLE through thebody, measured in the plane of the HORIZON. There is no universally accepted manner of reckoning the AZIMUTHS.Quadrantal, half-circle, and whole-circle methods are being used. In the latter, the AZIMUTH is measured preferablyclockwise from north. See also AZIMUTH ANGLE.

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327 azimuth: astronomic(al). The angle between the astronomic meridian plane of the observer and the plane containing theobserved point and the VERTICAL of the observer.

328 azimuth: astronomic(al) determination of. The determination of the AZIMUTH of a line or BASE LINE byASTRONOMICAL OBSERVATIONS.

329 azimuth: back. An AZIMUTH 180o from a given AZIMUTH. In GEODESY, the DIRECTION of the line BA asdistinguished from the forward azimuth AB. The two differ by 180° plus the amount of CONVERGENCE OF THEMERIDIANS between points A and B. Also called reverse azimuth. See AZIMUTH: GEODETIC.

330 azimuth: forward. See AZIMUTH: GEODETIC.

331 azimuth: geodetic. For the GEODESIC LINE from A to B, the angle between the tangent to the MERIDIAN at A and thetangent to the GEODESIC LINE at A. This AZIMUTH is called the forward azimuth for the line AB. See AZIMUTH:BACK.

332 azimuth: grid. The angle in the plane of PROJECTION between a straight line and the Y-AXIS of a PLANE-RECTANGULAR COORDINATE system. See BEARING: GRID.

333 azimuth: Laplace. A GEODETIC AZIMUTH derived from an ASTRONOMIC AZIMUTH by means of the LAPLACEEQUATION.

334 azimuth: magnetic. At the point of OBSERVATION, the angle between the VERTICAL PLANE through the observedobject and the VERTICAL PLANE in which a freely suspended symmetrically magnetized NEEDLE, influenced by notransient artificial magnetic disturbance, will come to rest.

335 azimuth: reverse. See AZIMUTH: BACK.

336 azimuth: true. AZIMUTH relative to TRUE NORTH.

337 azimuthal equidistant projection. See PROJECTION.

338 azimuthal orthomorphic projection. See PROJECTION: STEREOGRAPHIC.

339 azimuthal projection. See PROJECTION.

340 azimuth angle. AZIMUTH measured from 0° at the north or south reference direction clockwise or counter-clockwisethrough 90° or 180°. It is labelled with the REFERENCE DIRECTION as a prefix and the DIRECTION of measurementfrom the REFERENCE DIRECTION as a suffix.

341 azimuth circle. See BEARING CIRCLE.

342 azimuth diagram. A DIAGRAM for obtaining graphically rather than by computation the AZIMUTHS of observedCELESTIAL BODIES.

343 azimuth mark. A MARK set at a significant distance from a TRIANGULATION or TRAVERSE STATION to mark theend of a line for which the AZIMUTH has been determined, and to serve as a starting or reference azimuth for later use.

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B

344 back azimuth. See AZIMUTH.

345 backbeach. Also written as two words. See BACKSHORE.

346 backing wind. Counter-clockwise change of wind direction, in either hemisphere.

347 backrush. The seaward return of water following UPRUSH of WAVES on a BEACH.

348 backrush current. See CURRENT: BACKRUSH.

349 backshore. That part of a BEACH which is usually dry, being reached only by the highest TIDES. Also called backbeach.See also FORESHORE.

350 backsight. In NAUTICAL ASTRONOMY, an OBSERVATION of a CELESTIAL BODY made by facing 180° from theAZIMUTH of the body. In LEVELLING, a reading on a LEVELLING ROD held on a point whose ELEVATION hasbeen previously determined and which is not the closing SIGHT of a level circuit. Also written as two words. See alsoFORESIGHT.

351 backwash. Water or WAVES thrown back by an obstruction such as a ship, BREAKWATER, CLIFF, etc.

352 backwater. Water turned back by an obstruction, opposing CURRENT, etc. Water held back from the main FLOW.

353 ballistic camera. See CAMERA.

354 band of colour. See STRIPES (OF COLOUR)

355 band of frequency. The RANGE of FREQUENCY between two specific limits. Also called frequency band.

356 bandwidth. The number of units (HERTZ, kilohertz, etc.) of FREQUENCY required for transmission.

357 bank. An ELEVATION of the SEA FLOOR located on a CONTINENTAL (or an ISLAND) SHELF, over which theDEPTH of water is relatively shallow but sufficient for safe SURFACE NAVIGATION. A shallow area of shiftingSAND, GRAVEL, MUD, etc. as a sand bank, mud bank, etc. The sloping margin of a RIVER or STREAM; theGROUND bordering upon a RIVER. A raised margin of a POND, LAKE, etc. See LEFT BANK and RIGHT BANK.

358 bar. A RIDGE or succession of RIDGES of SAND or other substances extending across the MOUTH of a RIVER orHARBOUR and which may obstruct NAVIGATION. Derived pressure in the CGS SYSTEM. See also MILLIBAR.

359 bar buoy. A BUOY marking the location of a BAR.

360 bar check. An on-site calibration method for echo sounders.

361 bar scale. See SCALE.

362 bar sweeping. A sweeping procedure using a beam suspended horizontally under a vessel. The sweep passing over thebottom may be equipped with rockers or other sensors to record contact with the bottom.

363 baroclinity. The condition and type of motion in which pressure is not constant on surfaces of constant density, e.g.internal waves.

364 barogram. Record made by a BAROGRAPH.

365 barograph. RECORDING BAROMETER.

366 barometer. Instrument for measuring ATMOSPHERIC PRESSURE.

367 barometer: aneroid. BAROMETER whose sensitive element comprises one or a series of ANEROID CAPSULES.

368 barometer: marine. MERCURY BAROMETER used on board ships, characterized by a constriction in the tube in orderto damp out OSCILLATIONS of the mercury caused by the movements of the ship.

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369 barometer: mercurial (or mercury). A BAROMETER in which ATMOSPHERIC PRESSURE is balanced against thepressure exerted by a column of mercury.

370 barometer: recording. BAROMETER which gives a continuous graphic representation of the atmospheric pressurevariations with TIME.

371 barometric altimeter. See ALTIMETER.

372 barometric elevation. See ELEVATION.

373 barometric gradient. See PRESSURE GRADIENT.

374 barometric levelling. See LEVELLING.

375 barometric pressure. See PRESSURE: BAROMETRIC.

376 barotropy. The condition and type of motion in which pressure is constant on surfaces of constant density, e.g. surfacetides.

377 barrel buoy. See BUOY.

378 barrier reef. A CORAL REEF fronting the SHORE, at some distance from it, and separated from it by a LAGOON or anavigable CHANNEL of moderate DEPTH. See also FRINGING REEF.

379 barye. Pressure unit in the CGS SYSTEM.

380 barysphere. See CENTROSPHERE.

381 basal coplane. See COPLANE: BASAL.

382 basal orientation. See ORIENTATION.

383 basal plane. See EPIPOLAR PLANE.

384 base. In a TRIANGULATION, the side of one of a series of connected triangles, the length of which is measured directlyand with prescribed ACCURACY and PRECISION, and from which the lengths of the other triangle sides are obtained bycomputation. Also called base line or triangulation base line. See also BASE TERMINALS.

385 base apparatus. Any apparatus designed for use in measuring with ACCURACY and PRECISION the length of a BASEin TRIANGULATION, or the length of a line in TRAVERSE.

386 base course (of a vessel). The true course between a vessel's point of departure and arrival.

387 base direction. See DIRECTION.

388 base extension triangulation (British terminology). A part of a TRIANGULATION NET starting with a measured BASEand expanding that relatively short distance to a larger distance comparable to the average length of the triangulation sides.Called base net in U.S. terminology.

389 base height ration. In PHOTOGRAMMETRY, the ratio between the AIR BASE and the FLIGHT ALTITUDE of aSTEREOSCOPIC PAIR of PHOTOGRAPHS.

390 base line. The line from which the outer limits of the territorial sea and certain other outer limits are measured. InRADIOLOCATION, the GEODESIC LINE between two stations operating in conjunction for the determination of aLINE OF POSITION. Also written as one word. See also BASE.

391 baseline: archipelagic. Archipelagic baselines are straight lines joining the outermost points of the outermost islands anddrying reefs of an archipelago.

392 baseline: normal. The low-water line along the coast as marked on large-scale charts officially recognized by the coastalState.

393 baseline: straight. Straight baselines are a system of straight lines joining specified or discrete points on the low-waterline, usually known as straight baseline turning points. Straight baselines are used in DELIMITATION.

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394 base line extension. In RADIOLOCATION, the extension of the BASE LINE beyond the TRANSMITTERS.

395 base map. See MAP.

396 base net. See BASE EXTENSION TRIANGULATION.

397 basepoint. A basepoint is any point on a baseline.

398 base tape (or wire). A tape or wire of metal or alloy, so designed and graduated that it is suitable for measuring thelengths of BASES. See also TAPE: INVAR, TAPE: STEEL and WIRE: INVAR.

399 base terminals. The two extremes of a BASE LINE. Usually marked on a MONUMENT or metal stake buried beneath avisible SURFACE MARK. See also REFERENCE MARK.

400 base tilt. See TILT.

401 basic control. See CONTROL.

402 basic survey. A hydrographic survey so complete and thorough that it does not need to be supplemented by other surveys,and is adequate to supersede, for charting purposes, all prior hydrographic surveys of the area.

403 basin. Sheltered body of water available for port operations connecting either with the SEA or with an outer PORT or withanother basin, sometimes by means of a LOCK or PASSAGE. In geography, the tract of country drained by a RIVER andits TRIBUTARIES or which drains into a particular LAKE or SEA. In OCEANOGRAPHY, a DEPRESSION of the SEAFLOOR more or less equidimensional in form and of very variable extent.

404 basin: tidal. A BASIN affected by tidal forces in which water can be kept at a desired LEVEL by means of a gate.

405 bathymeter. An instrument primarily designed for measuring DEPTH of water.

406 bathymetric (adj.). Of or pertaining to BATHYMETRY.

407 bathymetric chart. See CHART.

408 bathymetric map. A topographic map of the ocean floor, or the bed of a lake.A topographic chart of the bed of a body of water, or a part of it. Generally, bathymetric maps show depths by contourlines and gradient tints.

409 bathymetry. The determination of ocean depths. The general configuration of SEA FLOOR as determined by profileanalysis of depth data.

410 bathyscaphe (or bathyscaph). A free, manned vehicle for exploring the deep OCEAN.

411 bathysphere. A diving sphere for deep-sea observations.

412 bathythermogram. The record of temperature versus DEPTH made by a BATHYTHERMOGRAPH. Rarely used.

413 bathythermograph. A device for obtaining a record of temperature against DEPTH in the OCEAN, from a shipunderway.

414 bay. Wide INDENTATION in the COASTLINE generally smaller than a GULF and larger than a COVE.For the purposes of the United Nations Convention on the Law of the Sea, a bay is a well-marked indentation whosepenetration is in such proportion to the width of its mouth as to contain land locked waters and constitute more than a merecurvature of the coast. See also HISTORIC BAY.

415 bay closing line. A line dividing the internal waters and territorial seas in a bay.

416 bay ice. Level FAST ICE of more than one winter's growth, which may be nourished by surface layers of SNOW.Thickness of ICE and SNOW up to about 2 m above SEA LEVEL. When bay ice becomes thicker than this, it is called anICE SHELF.

417 bayou. A minor sluggish WATERWAY or estuarial CREEK, generally tidal or characterized by a slow CURRENT, witha COURSE generally through LOWLANDS or SWAMPS, TRIBUTARY to or connecting with other bodies of water.Various specific meanings have been implied in different parts of the Southern United States. Sometimes called SLOUGH.

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418 beach. On a SHORE, the area on which the WAVES break and over which shore debris, such as SAND, SHINGLE,PEBBLES accumulate. A beach includes BACKSHORE and FORESHORE.

419 beach comber. See COMBER.

420 beacon. A prominent specially constructed object forming a conspicuous MARK as a fixed AID TO NAVIGATION, orfor use in HYDROGRAPHIC SURVEY. Also called day beacon or DAYMARK.

421 beacon: aeronautical. An AERONAUTICAL GROUND LIGHT visible at all AZIMUTHS, either continuously orintermittently, to designate a particular point on the surface of the EARTH. A radiobeacon may be coterminous with theaeronautical beacon.

422 beacon: hazard. A BEACON marking an OBSTRUCTION or hazard.

423 beacon: identification. An AERONAUTICAL and/or RADIO and/or LIGHT BEACON emitting a coded SIGNAL bymeans of which a particular REFERENCE POINT can be identified.

424 beacon: light (or lighted). A BEACON from which a LIGHT is exhibited.

425 beacon: omnidirectional. See OMNIRANGE.

426 beacon: radio. See RADIOBEACON.

427 beacon: responder. See TRANSPONDER BEACON.

428 beacon: transponder. See TRANSPONDER BEACON.

429 beaconage. A system of BEACONS. See BUOYAGE.

430 beam. RADIANT ENERGY confined to a particular shape.

431 beam compass. An instrument consisting of a beam or rod with sliding sockets that hold pencil, pen, or steel points, usedtogether with a SCALE for plotting exact lengths or for drawing large circles.

432 beam sea. WAVES moving in a direction approximately 90° from the HEADING.

433 beam width. The angular measure of the transverse section of a beam (usually in the main lobe) lying within directionscorresponding to specified values of field strength relative to the maximum.

434 beam wind. WIND blowing in a direction approximately 90° from the HEADING.

435 bearing. The horizontal DIRECTION of one terrestrial point from another, expressed as the ANGULAR DISTANCEfrom a REFERENCE DIRECTION. It is usually measured from 0° at the REFERENCE DIRECTION clockwise through360°. The terms bearing and AZIMUTH are sometimes used interchangeably, but in NAVIGATION the formercustomarily applies to terrestrial objects and the latter to the DIRECTION of a point on the CELESTIAL SPHERE from apoint on the EARTH. A bearing is designated as true, MAGNETIC, or compass as the REFERENCE DIRECTION isTRUE, MAGNETIC, or COMPASS NORTH.

436 bearing: acoustic. See BEARING: SONIC.

437 bearing: compass. See BEARING.

438 bearing: danger. The maximum or minimum BEARING of a point for safe passage of an offlying danger.

439 bearing: great circle. The initial DIRECTION of a GREAT CIRCLE through two terrestrial points, expressed asANGULAR DISTANCE from a REFERENCE DIRECTION. BEARINGS obtained by any form of RADIANT ENERGYare great-circle bearings.

440 bearing: grid. BEARING relative to GRID NORTH. See also AZIMUTH: GRID.

441 bearing: magnetic. BEARING relative to MAGNETIC NORTH. COMPASS BEARING corrected for MAGNETICDEVIATION.

442 bearing: Mercatorial (or Mercator). See BEARING: RHUMB.

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443 bearing: polar. In a system of POLAR or SPHERICAL COORDINATES, the angle formed by the intersection of thereference MERIDIONAL PLANE and the MERIDIONAL PLANE containing the point.

444 bearing: radar. A BEARING obtained by RADAR.

445 bearing: radio. The angle between the apparent direction of a definite source of emission of ELECTRO-MAGNETICWAVES and a REFERENCE DIRECTION, as determined at a RADIO DIRECTION-FINDING STATION.

446 bearing: reciprocal. A BEARING differing by 180° from a given BEARING.

447 bearing: relative. BEARING relative to the HEADING of a craft.

448 bearing: rhumb. The direction of a RHUMB LINE through two terrestrial points expressed as ANGULAR DISTANCEfrom a REFERENCE DIRECTION. Also called mercatorial (or mercator) bearing.

449 bearing: sonic. A BEARING determined by measuring the direction from which a SOUND WAVE is coming. Alsocalled acoustic bearing.

450 bearing: transit. A BEARING taken of two objects when in line.

451 bearing: true. See BEARING.

452 bearing circle. A ring designed to fit snugly over a COMPASS or COMPASS REPEATER, and provided with VANESfor observing BEARINGS. A similar ring provided with means for observing AZIMUTH of the SUN is called an azimuthcircle.

453 bearing plate. See PELORUS.

454 beat. The periodic variation that results from the superposition of two OSCILLATIONS whose FREQUENCIES differ bya small amount.

455 Beaufort wind scale. A numerical scale for indicating wind speed, devised by Admiral Sir Francis Beaufort in 1805.Beaufort numbers (or forces) range from force 0 (calm) to force 12 (hurricane).

456 bed. The GROUND upon which a body of water rests.

457 bedrock. Any solid ROCK underlying SOIL or unconsolidated SEDIMENTS.

458 bel. A dimensionless unit for expressing the ratio of two values of power, the number of bels being the logarithm to thebase 10 of the power ratio. In practice decibel is commonly used, the number of decibels being 10 times the logarithm tothe base 10 of the power ratio.

459 bell buoy. See BUOY: SOUND.

460 belt. In ice terminology, a long area of PACK ICE from a few kilometres to more than 100 kilometres in width.

461 bench. See TERRACE.

462 bench mark (benchmarks). A permanent, stable object containing a marked point of known ELEVATION with respectto a DATUM used as a REFERENCE LEVEL for TIDAL OBSERVATIONS or as a control point for LEVELLING.

463 beneaped (or neaped). Said of a vessel left aground following a spring high tide.

464 benthic division. In the classification of the marine ENVIRONMENT and its inhabitants, a primary division of the SEAwhich includes all of the ocean floor. The other primary division of the SEA is the PELAGIC DIVISION.

465 benthos. The category of marine organisms that live on, in, or close to the BOTTOM of the OCEANS.

466 berg. Short for ICEBERG.

467 bergy bit. A massive piece of SEA ICE or disrupted HUMMOCKED ICE; also medium-sized piece of floatingGLACIER ICE. Generally less than 5 m above SEA LEVEL, and not more than about 10 m across.

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468 berm. A narrow, raised path or EMBANKMENT along a STREAM, CANAL, or BEACH. On a BEACH it may beformed by the deposit of material by WAVES and marks the limit of HIGH TIDES.

469 berm line. In CARTOGRAPHY, the outer edge of vegetation shown as SHORELINE on CHARTS in marsh or mangroveareas.

470 berth. Place in which a ship is moored at WHARF.

471 beset(adj.). Of a ship when surrounded so closely by SEA ICE that steering control is lost. The term does not implypressure. See also ICEBOUND, NIPPED.

472 Bessel's spheroid. See REFERENCE SPHEROID.

473 between-the-lens shutter. See SHUTTER.

474 bias. The distortion of a result through negligence of a factor usually introducing a systematic error of unchangingmagnitude and sign throughout a given series of observations.

475 bifurcation. A division into two branches.

476 bifurcation buoy. See BUOY: BIFURCATION.

477 bight. A bend or curve in the COASTLINE, a bend in a RIVER, etc. In ice terminology, an extensive crescent-shapedindentation in the ICE EDGE formed either by WIND or CURRENT.

478 Bilby steel tower. A demountable and transportable TRIANGULATION TOWER for elevating a THEODOLITE andLUMINOUS SIGNALS above the GROUND, designed by J.S. Bilby, USC&GS.

479 billow. A great WAVE.

480 binary. The number system using 2 as the base for number representation. Binary numbers are composed of BITS.Generally any element capable of only two mutually exclusive states is called binary.

481 binnacle. The stand in which a COMPASS is mounted and in which lighting and compensatory units are carried.

482 binocular(s). An optical instrument consisting of a pair of TELESCOPES joined so as to be adaptable to the use of botheyes simultaneously.

483 binocular vision. Simultaneous vision with both eyes.

484 biology: marine. The study of the life, history and ECOLOGY of marine and brackish water plants and animals.

485 bioluminescence. The emission of light by living organisms. See also NOCTILUCA.

486 biosphere. Spherical terrestrial LAYER comprising the lower part of the ATMOSPHERE, the SEAS and the upperLAYERS of the SOIL within which living organisms can exist naturally.

487 bissextile year. See YEAR: CIVIL.

488 bit. Short for "binary digit", the digit in binary number representation. A bit can only assume the binary values "0" and "1".A binary number representation is composed of bits just as a decimal number is represented by decimal digits.A unit of 8 bits is called a BYTE.It also stands for "basic indissoluble information unit", a unit used for measuring the information content in informationtheory.

489 blind rollers. Long, high SWELLS which have increased in HEIGHT, almost to the breaking point, as they pass overSHOALS or run in shoaling water. Called blind seas in some localities.

490 blind seas. See BLIND ROLLERS.

491 blind zone. An area within or from which little or no radio SIGNAL is received. Also called zone of silence.

492 blip. Indication of a SIGNAL on the SCOPE of an electronic instrument, produced by a short sharply-peaked PULSE ofvoltage.

27

493 blizzard. Violent and very cold WIND which is laden with SNOW, some part, at least, of which has been raised fromsnow-covered GROUND. This term originated in North America but has been extended to include similar WINDS in othercountries.

494 block correction. See CHARTLET.

495 blow up. Enlarge photographically.

496 bluff. A CLIFF or HEADLAND with an almost perpendicular face.

497 blunder. See ERROR: GROSS.

498 boat compass. See COMPASS.

499 boat grid. See CAREENING GRID or GRIDIRON.

500 boat sheet (U.S. terminology). The work sheet used by the HYDROGRAPHER in the field for plotting the details of aHYDROGRAPHIC SURVEY as it progresses. See also FIELD BOARD.

501 boat slip. A SLIPWAY designed specifically for boats.

502 bog. Wet spongy ground consisting of decaying vegetation, which retains stagnant water, too soft to bear the weight of anyheavy body.

503 bold coast. A prominent land mass that rises steeply from the SEA.

504 bollard. Small shaped post, mounted on a wharf or dolphin used to secure ship's lines. Also see MOORING.

505 boom. A floating barrier used to protect a river or harbour mouth or to create a sheltered area for storage purposes.

506 boomer. Seismic instrument for shallow penetration work. The boomer TRANSDUCER produces acoustic pulses by themotion of a metal plate in the water.

507 border. The district lying along the edge of a country or territory; a frontier. The boundary line which separates onecountry from another, the frontier line.

508 border break. A cartographic technique to extend cartographic detail of a map or chart beyond the neatline into themargin to show important features. This technique eliminates the necessity of producing an additional CHART. Alsocalled Blister.

509 border of chart. The NEATLINE defining the limits of the area charted.

510 borderland. A region adjacent to a CONTINENT, normally occupied by or bordering a SHELF, that is highly irregularwith DEPTHS well in excess of those typical of a SHELF.

511 border scale. See SCALE.

512 bore. A high breaking WAVE of water, advancing rapidly up an ESTUARY. Bores can occur at the MOUTHS of shallowRIVERS if the TIDE RANGE at the MOUTH is large. They can also be generated in a RIVER when TSUNAMIS entershallow coastal water and propagate up the RIVER. Also called eager, mascaret, or tidal bore.

513 boring. Forcing a vessel under power through ICE.

514 bottom. Any GROUND covered by water.

515 bottom: nature of. The feature of the BOTTOM including the material of which it is composed and its physicalcharacteristics. Also called character (or characteristics) of the bottom, or quality of the bottom.

516 bottom characteristics. Designations used on survey and nautical charts to indicate the consistency, colour, andclassification of the sea bottom. Also called NATURE OF THE BOTTOM, CHARACTER OF THE BOTTOM.

517 bottom contour chart. A chart designed for surface and sub-surface bathymetric navigation seaward of the 100-fathomcontour. Bottom configuration is portrayed by DEPTH CONTOURS and selected soundings.

28

518 bottom friction. The force resulting from the interaction between the ocean bottom and water particle motion over it.

519 bottom ice. See ANCHOR ICE.

520 bottom profiler. An ECHO SOUNDER for precision surveys of the sea bottom surface.

521 bottom relief survey. A survey conducted to obtain information on the spatial characteristics of the sea bottom relief.

522 bottom sample. A portion of the sea bottom material brought to the surface for examination. See also CORE.

523 bottom sampler. Any device for obtaining a BOTTOM SAMPLE. See also CORER, DREDGE, DRIVER ROD, GRABand SNAPPER.

524 bottom sampling. The process of collecting bottom samples.

525 bottom sediments. See SEDIMENTS: BOTTOM.

526 Bouguer anomaly. A difference between an observed value of GRAVITY and a theoretical value at the point ofobservation, which has been corrected for the effect of the TOPOGRAPHY and ELEVATION only, the TOPOGRAPHYbeing considered as resting on a plane of indefinite extent.

527 boulder. A rounded ROCK with a diameter of 256 millimetres (about 10 INCHES) or larger.

528 boulder clay. See TILL.

529 boundary. Anything marking a limit; bound; BORDER.

530 boundary layer. The LAYER of fluid in the immediate vicinity of a bounding surface. It is the LAYER in whichfrictional forces are not negligible.

531 boundary monument. A material object placed on or near a boundary line to preserve and identify the ground location ofthe boundary line. Where it is impracticable to establish a monument on or very close to a boundary line, the position ofthe boundary line on the ground is preserved by means of reference marks. The term monument is sometimes used toinclude both the mark on the boundary and the reference mark.

532 boundary wave. See WAVE: INTERNAL.

533 Bourdon tube. Closed, curved, flexible tube of elliptic cross-section which is deformed, according to type, by variationsof ATMOSPHERIC PRESSURE or temperature and so provides a measurement of the particular element.

534 Bowditch rule. A method of ADJUSTMENT of TRAVERSES.

535 box compass. See DECLINATOIRE.

536 box gauge. See GAUGE: FLOAT.

537 box sextant. A small portable instrument used for approximate measurement of angles.

538 brackish water. Water in which salinity values range from approximately 0.50 to 17.00 parts per thousand.

539 brash ice. Accumulation of FLOATING ICE made up of small fragments not more than 2 metres across; the wreckage ofother forms of ICE.

540 breaker. A WAVE breaking on the SHORE, over a REEF, etc. Breakers may be roughly classified into three kinds,although the categories may overlap: spilling breakers break gradually over a considerable distance; plunging breakerstend to curl over and break with a crash; and surging breakers peak up, but then instead of spilling or plunging they surgeup on the beach face. The French word 'brisant' is also used for the obstacle causing the breaking of the WAVE.

541 breaking of waves. Disintegration of a WAVE which breaks with foam in SHALLOWS whose gradient is more or lessregular. An incomplete breaking can likewise result from a sudden reduction in DEPTH.

29

542 breakwater. A structure protecting a shore area, HARBOUR, ANCHORAGE, or BASIN from WAVES. See alsoFLOATING BREAKWATER.

543 breeze. A WIND of moderate strength. See LAND and SEA (or LAKE) BREEZE.

544 bridge. 1. An elevated structure extending across or over the weather deck of a vessel, or part of such a structure. The termis sometimes modified to indicate the intended use, such as navigating bridge or signal bridge.2. A structure erected over a depression or an obstacle such as a body of water, railroad, etc., to provide a roadway forvehicles or pedestrians.

545 bridge: bascule. A counterpoise bridge rotated in a vertical plane about an axis at one or both ends. Also called balancebridge.

546 bridge: draw. A general name for bridges of which part or the entire span of the bridge may be raised or drawn aside toallow ships to pass through.

547 bridge: lift. A movable bridge which is capable of being lifted vertically to allow vessels to pass beneath.

548 bridge: swing. A movable bridge which rotates in a horizontal plane about a vertical pivot to allow the passage of vessels.

549 bridge clearance. See CLEARANCE.

550 bridging. See STEREOTRIANGULATION.

551 broad on the beam. Bearing 090° relative (broad on the starboard beam) or 270° relative (broad on the port beam). If thebearings are approximate, the expression on the beam or abeam should be used. See also ON THE BEAM.

552 broad on the bow. Bearing 045° relative (broad on the starboard bow) or 315° relative (broad on the port bow). If thebearings are approximate, the expression on the bow should be used. See also ON THE BOW.

553 broad on the quarter. Bearing 135° relative (broad on the starboard quarter or 225° relative (broad on the port quarter). Ifthe bearings are approximate, the expression on the quarter should be used. See also ON THE QUARTER.

554 broadcast. Communication by RADIO intended for reception at any point within a specified area.

555 broadcast ephemeris. Ephemeris transmitted by a satellite which describes its position and orbital parameters.

556 broadside on. Beam on, as to the wind or sea.

557 broken telescope transit. See TRANSIT.

558 broken water. An area of small WAVES and EDDIES occurring in what otherwise is a calm SEA.

559 brook. A small STREAM; a RIVULET. Also called run.

560 bubble: spirit (or sensitive). See LEVEL: SPIRIT.

561 bubble sextant. See SEXTANT.

562 bucket temperature. Temperature of surface sea water trapped and measured in a bucket or similar receptacle.

563 bulkhead. On land, a structure or partition built to retain or prevent sliding of the land. A secondary purpose is to protectthe upland against damage from wave action. Bulkheads are frequently backfilled thereby increasing the utility of theadjacent land area.

564 bull's eye level. See LEVEL: SPIRIT.

30

565 buoy. A floating object moored to the BOTTOM in a particular (charted) place, as an AID TO NAVIGATION or forother specific purposes. Navigational buoys may be classified according to:(a) their shape, appearance, or construction, such as barrel, CAN, cask, CONICAL, cylindrical, DAN, KEG, NUN,PILLAR, SPAR, spherical, or TOPMARK BUOY;(b) their colour, such as black, chequered, green, red buoy;(c) their location, such as BIFURCATION, FAIRWAY, JUNCTION, MID-CHANNEL, MIDDLE-GROUND, orTURNING BUOY;(d) the various kinds of hazards or dangers to NAVIGATION which they mark, such as BAR, ISOLATED DANGER,FISH TRAP, OBSTRUCTION, SPOIL GROUND, TELEGRAPH or WRECK BUOY;(e) their particular purpose or use, such as ANCHOR, ANCHORAGE, COMPASS ADJUSTMENT, DREDGING,FAREWELL (or landfall), MARKER, QUARANTINE, STATION (or watch), or WARPING BUOY.

566 buoy: automatic. See AUTOMATIC FLOATING STATION.

567 buoy: bifurcation. A distinctive lateral BUOY used to indicate both the point at which a channel divides and the channelthat has been designated as preferred channel.

568 buoy: cable. A BUOY used to indicate the run of a SUBMARINE CABLE.

569 buoy: cardinal. A BUOY used to indicate that the best navigable water is to be found in one particular of the quadrantsthat are bounded by the four bearings NW, NE, SE and SW taken from the position of the mark.

570 buoy: fairway. A BUOY intended to be the first to be seen during an approach from the open sea to a FAIRWAY.

571 buoy: lateral. A BUOY used to indicate a lateral limit of NAVIGABLE water.

572 buoy: life. A BUOY intended to keep persons afloat. Also called life jacket.

573 buoy: light (or lighted). A BUOY with a LIGHT having definite characteristics for detection and identification duringdarkness. See CHARACTERISTICS OF LIGHT.

574 buoy: lighted sound. A LIGHT BUOY provided with a SOUND SIGNAL.

575 buoy: mooring. A BUOY secured to the BOTTOM by permanent moorings with means for mooring a vessel by use of itsanchor chain or mooring lines.

576 buoy: radar. A BUOY with a cluster of RADAR REFLECTORS.

577 buoy: radiobeacon. A BUOY equipped with a MARKER RADIOBEACON.

578 buoy: sound. A BUOY equipped with a characteristic SOUND SIGNAL. Sound buoys may be further classifiedaccording to the manner in which the SOUND is produced, such as bell, gong, horn, trumpet, or whistle buoy.

579 buoy: special. A BUOY primarily used to indicate an area or feature referred to in nautical documents rather than to assistnavigation.

580 buoy: super. A very large BUOY designed to carry a signal light of high luminous intensity at a high elevation. See alsoLARGE AUTOMATIC NAVIGATION BUOY (LANBY).

581 buoy: survey. A BUOY used by SURVEY SHIPS to control HYDROGRAPHIC SURVEYS. See also BUOY-CONTROL METHOD.

582 buoyage. A system of, or providing with, BUOYS, serving the purpose of indicating NAVIGABLE waters. SeeBEACONAGE.

583 buoyancy. Force resulting from the upward pressure of water, acting upon the immersed volume of a body, set against thetotal WEIGHT of such body. If the volume immersed is such that the upward pressure counterbalances the WEIGHT ofthe body, the buoyancy is said to be positive and the body floats on the surface; if the volume immersed is insufficient, theweight of the body is greater than the upward pressure, the buoyancy is negative, and the body sinks.

584 buoyancy: centre of. CENTRE OF GRAVITY of liquid displaced by a floating body.

585 buoyant beacon. See ARTICULATED LIGHT.

31

586 buoy-control method. A system of accurately located BUOYS on which THREE-POINT FIXES could be observed, or towhich distances could be measured by radio acoustic methods, for extending HYDROGRAPHIC SURVEYS beyond theVISIBILITY of shore control.

587 butte. An isolated flat-topped HILL, similar to, but smaller than a MESA.

588 byte. A unit consisting of 8 BITS, frequently used for measuring the capacity of storage devices.1 Kbyte = 2 10 byte = 1.024 byte (kilo-byte)1 Mbyte = 2 20 byte = 1.048.576 byte (mega-byte)1 Gbyte = 2 30 byte = 1.073.741.824 byte (giga-byte)

32

C

589 cable. A unit of distance originally equal to the length of a ship's anchor cable, but now generally considered to be about600 FEET. In the British Navy it is 608 FEET, or exactly one-tenth of a NAUTICAL MILE. In the United States Navyit is 720 FEET but is infrequently used. Sometimes called cable length. A chain or very strong fibre or wire rope used toanchor or moor vessels or BUOYS. A stranded conductor or an assembly of two or more electric conductors insulatedfrom each other, but laid up together with a strong, waterproof covering.

590 cable: coaxial. A transmission cable consisting of two concentric conductors insulated from each other.

591 cable: degaussing. CABLE carrying an electric current for degaussing a vessel.

592 cable: leader. CABLE lying on the BOTTOM and carrying an electric current by which ships equipped withappropriate instruments, can be guided.

593 cable: submarine. An insulated, waterproof wire or bundle of wires for carrying an electric current under water.

594 cable buoy. See BUOY.

595 cable length. See CABLE.

596 cable sign. A sign on a post marking the point from which a cable runs under water.

597 C/A Code. The standard (Coarse/Acquisition, or Clear/Access) GPS code; a sequence of 1023 pseudo-random binarybiphase modulations on the GPS carrier at a chip rate of 1.023 MHz, thus having a code repetition period of onemillisecond. See also P-CODE.

598 cadastral map. See MAP.

599 cadastral survey. See SURVEY.

600 cadastre. A public record of the extent, value, and ownership of LAND for purposes of taxation.

601 cairn. A mound of STONES, usually conical or pyramidal, raised as a LANDMARK or to designate a point ofimportance in SURVEYING.

602 caisson. A steel structure used for closing the entrance of LOCKS, WET and DRY DOCKS. See also LOCK GATE.

603 calcareous (adj.). Composed of or containing calcium or calcium carbonate.

604 calculator. A machine for performing arithmetical operations usually more complex than that done by an addingmachine; a calculating machine, which may be mechanical, electrical, electronic or a combination of all three. A personwho calculates.

605 calendar line. See DATE LINE.

606 calibrate (v.t.). To fix, check or correct the GRADUATIONS of a measuring instrument. To determine the calibre of.

607 calibrated focal length. See FOCAL LENGTH.

608 calibration. The act or process of determining certain specific measurements in an instrument or device by comparisonwith a STANDARD, for the purpose of correcting or compensating ERRORS or for purposes of record.

609 calibration: direct base. The calibration of a positioning system with the help of one or more baselines of knownlength, which is determined with an instrument having an accuracy superior to that of the positioning system.

610 calibration: direct simultaneous. The calibration of a positioning system at the same time when the distances used forcalibration are measured using another system or instrument of superior accuracy.

33

611 calibration: indirect position. The calibration of a positioning system using positions of the survey vessel at differentlocations which were determined independently of the positioning system. True distances can be derived from thesepositions and by comparison with the data rendered by the positioning system, calibration values can be deduced.

612 calibration: indirect variation. The calibration of a positioning system by obtaining redundant observations so thatcalibration values can be deduced mathematically from the discrepancies between uncalibrated values at differentlocations.

613 calibration of compass. See SWINGING SHIP.

614 calling-in point. Point at which vessels are required to report to a traffic control center.

615 calling the soundings. Announcement made by LEADSMAN when heaving the LEAD.

616 calm. WIND with a SPEED zero or less than 2 KNOTS (BEAUFORT SCALE wind force 0). The state of sea whenthere are no WAVES.

617 calorie. A unit of quantity of heat, originally defined as the amount of heat required to raise the temperature of oneGRAM of water through one DEGREE C. The 15-degree gram-calorie is the amount of heat required to raise thetemperature of one GRAM of water from 14°.5 C to 15°.5 C, and is equal to 4.1855 JOULES.

618 calving. The breaking away of a mass of ICE from a GLACIER, ICE FRONT, or ICEBERG.

619 camber. A small BASIN, usually with a narrow ENTRANCE, situated inside a HARBOUR.

620 camera. A lightproof chamber or box in which the IMAGES of exterior objects are projected upon a sensitized PLATEor film, through an opening usually equipped with a LENS or LENSES, SHUTTER and variable aperture.

621 camera: aerial. A CAMERA specially designed for use in aircraft.

622 camera: ballistic. A precision terrestrial camera usually employing glass plates, used at night to photograph suchobjects as ROCKETS or SATELLITES against a star background. If the CAMERA is mounted so that it tracks theSTARS or the object, it may be called a tracking camera.

623 camera: continuous-strip. A CAMERA in which a continuous-strip exposure is made by rolling the film continuouslypast a narrow slit opening at a SPEED proportional to the GROUND SPEED of the aircraft.

624 camera: deep sea. A camera system specially designed for obtaining PHOTOGRAPHS of the ocean bottom.

625 camera: multiple lens. A CAMERA with two or more LENSES, with the AXES of the LENSES systematicallyarranged at fixed angles in order to cover a wide field by simultaneous EXPOSURES in all chambers.

626 camera: photogrammetric. A general term applicable to CAMERAS used in any of the several branches ofPHOTOGRAMMETRY.

627 camera: stereometric. A combination of two CAMERAS mounted with parallel OPTICAL AXES on a short rigidbase; used in TERRESTRIAL PHOTOGRAMMETRY for making PHOTOGRAPHS in STEREOSCOPIC PAIRS.

628 camera: surveying (or mapping). A CAMERA specially designed for the production of PHOTOGRAPHS to be usedin SURVEYING. The prefix 'mapping' or 'surveying' indicates that the CAMERA is equipped with means formaintaining and indicating the INTERIOR ORIENTATION of the PHOTOGRAPHS with sufficient ACCURACY forsurveying purposes. A mapping camera may be an aerial mapping camera or a terrestrial mapping camera.

629 camera: terrestrial. A CAMERA designed for use on the GROUND. See PHOTOTHEODOLITE.

630 camera: tracking. See CAMERA: BALLISTIC.

631 camera axis. See AXIS OF CAMERA.

632 camera station. See AIR STATION.

633 canal. An artificial WATERCOURSE. A long narrow ARM OF THE SEA extending inland between ISLANDS, orbetween ISLANDS and the MAINLAND.

34

634 can buoy. A BUOY the above-water part of which is in the shape of a cylinder. Sometimes called cylindrical buoy.

635 candela. The unit of LUMINOUS INTENSITY in the SI SYSTEM.

636 candlepower. LUMINOUS INTENSITY expressed in CANDELAS. Also written as two words.

637 cantilever extension. PHOTOTRIANGULATION from a controlled area to an area of no CONTROL. Also, theconnection by RELATIVE ORIENTATION and scaling of a series of PHOTOGRAPHS in a strip to obtain STRIPCOORDINATES.

638 canyon. A relatively narrow, deep depression with steep sides, the bottom of which generally has a continuous slope.

639 canyon: submarine. A relatively narrow, deep DEPRESSION with steep SLOPES, the BOTTOM of which gradescontinuously downward.

640 cap. A feature with a rounded cap-like top.

641 capacitor. See CONDENSER.

642 cape. A piece of LAND projecting into a body of water.

643 capillary wave. See WAVE.

644 card. See COMPASS CARD.

645 cardanic suspension. See GIMBALS.

646 cardinal points. The four points of the HORIZON at the intersections of the HORIZON with the MERIDIAN and thePRIME VERTICAL; the north, south, east and west points.

647 cardinal system. A buoyage system generally used to indicate dangers where the COAST is flanked by numerousISLANDS, ROCKS and SHOALS as well as to indicate dangers in the open sea. In this system the BEARING (TRUE)of the MARK from the danger is indicated to the nearest CARDINAL POINT. A system of BUOYAGE in which theaids are assigned shape, colour, and number distinction in accordance with location relative to the nearestOBSTRUCTION. The CARDINAL POINTS delineate the sectors for buoy location.

648 careen. Causing a vessel to lie over on one side.

649 careening grid. A timber structure in the intertidal zone serving as a support for vessels at low stages of the tide topermit work on the exposed portion of the vessel's hull.

650 carrier. See WAVE: CARRIER.

651 carrier wave. See WAVE.

652 cartogram. A MAP showing geographical statistics by means of lines, dots, shaded areas, etc.

653 cartographer. Person employed in drawing and constructing CHARTS or MAPS.

654 cartography. The art and science of expressing graphically, by MAPS and CHARTS, the known physical features ofthe EARTH, or of another CELESTIAL BODY. Often includes the works of man and his varied activities.

655 cartouche. A panel on a MAP, often with decoration, enclosing the TITLE or other LEGENDS, the SCALE, etc.

656 Cassini projection. See PROJECTION.

657 cast. In OCEANOGRAPHY, a single lowering of a series of WATER BOTTLES or other oceanographic instruments atan OCEANOGRAPHIC STATION. Also referred to as oceanographic cast.

658 catenary. The curve formed by a uniform CABLE, CHAIN, or TAPE supported only at its ends.

659 catenary correction (tape). See TAPE: SAG CORRECTION.

35

660 cathode-ray tube. A vaccum tube in which electrons emitted by a hot cathode are formed into a narrow BEAM,accelerated at high VELOCITY and directed towards a specially prepared SCREEN which fluoresces at the point wherethe electrons strike.

661 catoptric light. See LIGHT.

662 causeway. A raised way across low or wet GROUND or water.

663 cautionary (or caution) note. Information calling special attention to some fact, usually a DANGER AREA shown ona CHART, or other publication.

664 cavitation. The turbulent formation, generally mechanically induced, including growth and collapse of bubbles in afluid, and occurring when the static pressure at any point in fluid flow is less than fluid vapour pressure.

665 cay (also kay, key). A low, flat ISLAND of SAND, CORAL, etc. awash or drying at LOW WATER; a term originallyapplied to the coral islets around the COAST and ISLANDS of Caribbean Sea.

666 ceiling. See HEIGHT OF CLOUD BASE.

667 celestial body. Any body pertaining to the heavens, constituting a unit for astronomical study, such as the SUN, theMOON, the PLANETS, FIXED STARS, COMETS, etc. Also called heavenly body.

668 celestial concave. See CELESTIAL SPHERE.

669 celestial coordinates. See COORDINATES.

670 celestial equator. See EQUATOR.

671 celestial equator system of coordinates. A set of CELESTIAL COORDINATES based on the CELESTIALEQUATOR as the PRIMARY GREAT CIRCLE. Usually DECLINATION, RIGHT ASCENSION and HOURANGLE, or SIDEREAL HOUR ANGLE. Also called equinoctial system of coordinates.

672 celestial globe. See STAR GLOBE.

673 celestial horizon. See HORIZON.

674 celestial latitude. See LATITUDE.

675 celestial line of position. A LINE OF POSITION determined by means of one CELESTIAL BODY.

676 celestial longitude. See LONGITUDE.

677 celestial meridian. See MERIDIAN.

678 celestial navigation. See NAVIGATION.

679 celestial pole. See POLE.

680 celestial sphere. An imaginary sphere of infinite radius, concentric with the EARTH, on which CELESTIAL BODIESare imagined to be projected. Also called celestial concave.

681 cellular wave. See WAVE.

682 Celsius temperature scale. A thermodynamic scale of temperature (t°C) defined as a function of the KELVINTEMPERATURE SCALE (T°K) by the relationship t°C = T°K 273.15. Name for Anders Celsius (1701-44), Swedishastronomer who first described, in 1742, the centigrade temperature scale.NOTE. According to a resolution of the Ninth International Conference on Weights and Measures (1948), temperatureshould be designated 'degrees Celsius' and the designation 'degrees centigrade' should be discontinued.

683 centering (or centring). The placing of an instrument with its VERTICAL AXIS in the VERTICAL through theground mark. The setting of LENSES of an optical instrument in such a manner that their OPTICAL AXES are in thesame straight line.

684 centigrade temperature scale. See CELSIUS TEMPERATURE SCALE.

36

685 centimetre-gram-second (CGS) system. The system of physical units in which the fundamental units of length, massand TIME are the centimetre, GRAM and SECOND, respectively.

686 central meridian. See MERIDIAN: INITIAL.

687 central processing unit (CPU). In computers in general the unit that comprises the arithmetic logic unit and the controlunit.

688 centre: reduction to. See REDUCTION.

689 centre of buoyancy. See BUOYANCY.

690 centre of gravity. See GRAVITY.

691 centrifugal force. The force with which a body moving under constraint along a curved PATH reacts to the constraint.Equal and opposite to the CENTRIPETAL FORCE.

692 centripetal force. The force directed towards the centre of curvature which constrains a body to move in a curvedPATH. Equal and opposite to the CENTRIFUGAL FORCE.

693 centrosphere. The central CORE of the EARTH, composed of dense material, and making up most of its mass. Alsocalled barysphere or CORE.

694 cesium clock. See CLOCK: CESIUM.

695 cesium magnetometer. See MAGNETOMETER.

696 C-factor (U.S.A.). In PHOTOGRAMMETRY, an empirical value which expresses the vertical measuring capability ofa given stereoscopic system; generally defined as the ratio of the flight height to the smallest CONTOUR INTERVALaccurately plottable. The C-factor is not a fixed constant but varies over a considerable RANGE, according to theelements and conditions of the photogrammetric system. In planning for AERIAL PHOTOGRAPHY, the C-factor isused to determine the flight height required for a specified CONTOUR INTERVAL, CAMERA, and instrument system.

697 CGS system. See CENTIMETRE-GRAM-SECOND (CGS) SYSTEM.

698 chain. A group of TRANSMITTERS broadcasting the same programme. In ELECTRONIC NAVIGATION, severalrelated transmitting stations in geographic proximity (Decca chain, Loran chain, etc.). In DIGITAL data, a sequence ofone or more line segments.

699 chaining. See TAPING.

700 chain adjustment. A mathematical adjustment of a whole series of figures, for example, a CHAIN OFTRIANGULATION.

701 chain of triangulation. See TRIANGULATION: ARC OF.

702 challenger. See INTERROGATOR.

703 change of tide. A reversal of the direction of motion (rising or falling) of a TIDE. The expression is also sometimesapplied to a reversal in the SET of a TIDAL CURRENT. Also called turn of the tide. See also STAND OF TIDE.

704 channel. That part of a body of water (sometimes dredged) deep enough for NAVIGATION through an area otherwisenot navigable. It is usually marked by a single or double line of BUOYS and sometimes by RANGES. The deepest partof a STREAM, BAY or STRAIT through which the main CURRENT flows. See also STRAIT.

705 channel: frequency. A continuous portion of the frequency spectrum appropriate for a transmission utilizing aspecified class of emission.

706 channel: sea. A long, narrow, U-shaped or V-shaped shallow depression of the SEA FLOOR usually occurring on agently sloping PLAIN or FAN. A continuously sloping, elongated DEPRESSION commonly found in CONES orABYSSAL PLAINS and customarily bordered by levees on one or both sides.

37

707 character. As pertains to typography, a graphic symbol such as a letter of the alphabet, figure, punctuation mark or signemployed in recording language.In data processing, an element of a CODE set used for representing information.

708 characteristics of light. All particularities of a LIGHT, such as colour, PERIOD, group number, VISIBILITY,HEIGHT above SEA LEVEL and CHARACTER. Also called light characteristics. See also GROUP.

709 character of light. The distinct character, such as fixed, flashing, or occulting, which is given to each LIGHT to avoidconfusion with neighbouring ones.

710 character (or characteristics) of the bottom. See BOTTOM: NATURE OF.

711 chart. A special-purpose MAP generally designed for NAVIGATION or other particular purposes. See CHART:NAUTICAL, PAPER.

712 chart: aeronautical. A representation of a portion of the EARTH, its CULTURE and RELIEF, specifically designed tomeet the requirements of AIR NAVIGATION.

713 chart: bathymetric. A topographic CHART of the BED of a body of water, or a part of it. Generally, bathymetriccharts show DEPTHS by CONTOUR LINES and GRADIENT TINTS.

714 chart: blank. A chart with a reduced amount of detail serving as cartographic basis for graphical documents of allkinds and for the depiction of reference data.

715 chart: coast (or coastal). A NAUTICAL CHART intended for inshore COASTWISE NAVIGATION.

716 chart: cotidal. A CHART showing COTIDAL LINES.

717 chart: current. A CHART on which current data are graphically depicted.

718 chart: datum. A CHART depicting the undulations of the GEOID.

719 chart: fair. See FAIR CHART.

720 chart: field. A NAUTICAL CHART prepared at the survey site, usually contains limited, and/or unedited information.

721 chart: fishery. A chart with additional details required for the exploitation of marine natural resources.

722 chart: general. A NAUTICAL CHART intended for offshore COASTWISE NAVIGATION.

723 chart: gnomonic. A CHART on the GNOMONIC PROJECTION. Also called great circle chart.

724 chart: geophysical. A chart which primarily shows parameters of geophysical nature.

725 chart: gravity anomaly. A CHART on which current GRAVITY ANOMALIES are graphically depicted.

726 chart: great circle. See CHART: GNOMONIC.

727 chart: grid. A chart only showing a cartographic grid on its inner border.

728 chart: harbour. A NAUTICAL CHART intended for NAVIGATION and ANCHORAGE in HARBOURS and smallWATERWAYS.

729 chart: hydrographic. See CHART: NAUTICAL.

730 chart: index. An outline CHART showing the limits and identifying designation of navigational CHARTS, volumes ofSAILING DIRECTIONS, or LIST OF LIGHTS.

731 chart: layer depth. A chart showing the synoptic distribution of LAYER DEPTHS.

732 chart: magnetic. A CHART showing for an established EPOCH the values of such magnetic elements asVARIATION, DIP, HORIZONTAL and VERTICAL INTENSITY of the EARTH'S MAGNETIC FIELD, and TOTALMAGNETIC INTENSITY.

38

733 chart: marine. See CHART: NAUTICAL.

734 chart: Mercator. A CHART on the MERCATOR PROJECTION. This is the CHART commonly used for MARINENAVIGATION. On a Mercator chart, a RHUMB LINE is a straight line.

735 chart: nautical. A CHART specifically designed to meet the requirements of MARINE NAVIGATION, showingDEPTHS of water, NATURE OF BOTTOM, ELEVATIONS, configuration and characteristics of COAST, dangers andAIDS TO NAVIGATION. Also called marine chart, hydrographic chart, or simply CHART.

736 chart: new. The first publication of a CHART.

737 chart: new edition. See NEW EDITION (OF CHART).

738 chart: original. A CHART which has been drawn up as a result of direct SURVEY.

739 chart: pilot. A CHART containing information required for safe NAVIGATION, including OCEAN CURRENTS, ICEat SEA, WIND ROSES, storm tracks, ISOTHERMS, MAGNETIC VARIATION, great circle routes, limits of TRADEWINDS, etc.

740 chart: polar. A CHART of the polar areas, or one on a polar PROJECTION.

741 chart: provisional. A special CHART, usually for which there is an urgent need. The chart is compiled from approvedsource materials and may be drafted for direct reproduction. All charted information is usually combined on the blackprinting plate and no other colours are shown.

742 chart: radar. A CHART designed primarily for use with RADAR, or one suitable for this purpose.

743 chart: radionavigational. A chart with additional details for the determination of a vessel's position with the aid ofradionavigation systems.

744 chart: route (or routeing). A CHART showing routes between various places, usually with distances indicated.

745 chart: sailing. A small scale nautical chart for OFFSHORE NAVIGATION.

746 chart: sea bottom sediment. A chart primarily showing information on the types and characteristics of sea bottom.

747 chart: star. A representation, on a flat surface, of the CELESTIAL SPHERE or a part of it, showing the POSITIONSof the STARS and sometimes other features of the CELESTIAL SPHERE.

748 chart (or map): synoptic. See CHART (or MAP): WEATHER.

749 chart: tidal current. A CHART showing by arrows and numbers the average direction and SPEED of TIDALCURRENTS at a particular part of the current cycle.

750 chart (or map): weather. Geographical MAP on which meterological conditions or elements are represented byfigures, SYMBOLS, or ISOPLETHS. Also called synoptic chart (or map).

751 chart: working. In HYDROGRAPHIC SURVEY, a working document (CHART or PROJECTION) by means ofwhich a survey programme can be broadly planned.

752 chart amendment patch. See CHARTLET.

753 chart comparison unit. A device which provides simultaneous, superimposed views of a CHART and radar scope.

754 chart compilation. The production of a new or revised map or chart, or portions thereof, from existing maps, aerialphotographs, surveys, new data and other sources or the selection, assembly, and graphic representation of all relevantinformation required for the preparation of a map or chart.

755 chart datum. See DATUM: CHART.

756 charted depth. See DEPTH.

39

757 chart evaluation survey. The Chart Evaluations Surveys (CES) program is designed to:1. Resolve all deficiencies reported or discovered. (A deficiency is defined as charted information that can be made more complete through field examination, or information which should be charted but is not.)2. Evaluate the adequacy/accuracy of hydrographic information on existing charts.3. Verify or revise information published in the appropriate Coast Pilot.4. Conduct user evaluation and public relations efforts to provide an awareness of products and obtain user input.

758 chartlet. A small auxiliary CHART giving new details on a particular area, to be added to the CHART afterpublication. Also called block correction or chart amendment patch. A small CHART, such as one showing thecoverage area of electronic navigational systems, with the distribution of its LINES OF POSITION, CORRECTIONS tobe applied to readings, location and identification of TRANSMITTERS, etc.

759 chart portfolio. A systematic grouping of nautical charts covering a specific geographical area.

760 chart production. All processes in the design, compilation, draughting and reproduction of a chart.

761 chart projection. See PROJECTION.

762 chart reading. Interpretation of the symbols, lines, abbreviations, and terms appearing on charts. May be called mapreading when applied to maps generally.

763 chart scale. See SCALE.

764 chart sounding datum. The tidal datum to which soundings and drying heights on a chart are referred. It is usuallytaken to correspond to a low water stage of the tide. Often shortened to CHART DATUM, especially when it is clearthat reference is not being made to a horizontal datum.

765 chart symbol. A character, letter, or similar graphic representation used on a chart to indicate some object,characteristic, etc. May be called map symbol when applied to any map.

766 check angle. A third angle taken to a fourth point as a check to a THREE-POINT FIX.

767 check lines of sounding. SOUNDING LINES run after completing the main system of SOUNDING LINES for thepurpose of checking them. See also CROSS LINES OF SOUNDING.

768 chlorinity. The total amount in GRAMS of chlorine, bromine and iodine contained in one KILOGRAM of SEAWATER, assuming that the bromine and the iodine had been replaced by chlorine. See SALINITY.

769 chlorophyll. The green pigment contained in the leaves of plants. See PHOTOSYNTHESIS.

770 chlorosity. The property of SEA WATER corresponding to the CHLORINITY expressed as GRAMS per LITRE at20°.

771 choppy (adj.). Of the SEA, having short, abrupt, breaking WAVES dashing against each other.

772 choppy sea. The state of the sea caused by interaction of waves running in different directions.

773 chromatic aberration. See ABERRATION OF LIGHT.

774 chromatism. See ABERRATION OF LIGHT.

775 chromosphere. The reddish LAYER of incandescent gases around the SUN, visible at a TOTAL ECLIPSE. A similarLAYER surrounding a STAR.

776 chronograph. An instrument for measuring and recording TIME or time intervals.

777 chronometer. A portable timekeeper with a compensated balance, capable of showing TIME with extremePRECISION and ACCURACY.

778 chronometer: sidereal. A CHRONOMETER that indicates SIDEREAL TIME at a given place, or that correspondingto a given MERIDIAN.

779 chronometer: standard. The CHRONOMETER with which the others aboard a vessel are compared and to which allOBSERVATIONS involving TIME are referred.

40

780 chronometer correction. The amount to be added algebraically to the TIME of the CHRONOMETER to obtain thecorrect TIME.

781 chronometer rate. The amount gained or lost by a CHRONOMETER in a unit of TIME. It is usually expressed inSECONDS per 24 HOURS, and is sometimes called daily rate.

782 chronometric method. A former method of determining LONGITUDE by transporting CHRONOMETERS betweenSTATIONS whose DIFFERENCE OF LONGITUDE was to be determined.

783 cinders. Fragments formed when MAGMA is blown into the air; larger in size than VOLCANIC ASH. Cinders are aconstituent of certain marine sediments.

784 circle. In a surveying instrument, the graduated disk which is perpendicular to and centered about an AXIS ofROTATION and is calibrated to read the amount of ROTATION.

785 circle: dip. See DIP CIRCLE.

786 circle: fundamental. See PRIMARY GREAT CIRCLE.

787 circle: great. The intersection of a sphere and a plane through its center. See also ORTHODROME.

788 circle: horizontal. In a surveying instrument, the graduated CIRCLE used for measuring horizontal angles.

789 circle: hydrographic. A double-reflecting instrument intended primarily for use in HYDROGRAPHIC SURVEY;enables measurement of angles up to 180°. See also SEXTANT: SOUNDING.

790 circle: small. The intersection of a sphere and a plane which does not pass through its center.

791 circle: vertical. A GREAT CIRCLE on the CELESTIAL SPHERE passing through the ZENITH and NADIR. Verticalcircles are perpendicular to the HORIZON. In a surveying instrument, the graduated CIRCLE used for measuringvertical angles.

792 circle of declination. See HOUR CIRCLE.

793 circle of equal altitude. A SMALL CIRCLE on the surface of the EARTH at every point of which, at the instant ofOBSERVATION, the observed CELESTIAL BODY (SUN, STAR, or PLANET) has the same ALTITUDE and,therefore, the same ZENITH DISTANCE.

794 circle of latitude. A GREAT CIRCLE of the CELESTIAL SPHERE through the ecliptic poles, and henceperpendicular to the plane of the ECLIPTIC.

795 circle of position. The SMALL CIRCLE on the surface of the EARTH on which an OBSERVATION shows theobserver to be. The expression is most frequently used to indicate the CIRCLE OF EQUAL ALTITUDE. Also calledposition circle.

796 circle of right ascension. See HOUR CIRCLE.

797 circle of uncertainty. A circle having as its centre a POSITION, however determined, and as its radius the maximumPROBABLE ERROR of the POSITION; a circle within which a craft is considered to be located.

798 circle of visibility. That circle surrounding an AID TO NAVIGATION and in which the aid is visible. See RANGE OFVISIBILITY.

799 circle sheet (U.S. terminology). A BOAT SHEET or a SMOOTH SHEET on which intersecting systems of curves aredrawn, each curve corresponding to the LOCUS of some angle between two STATIONS. POSITIONS are plotted oncircle sheets at the intersection of the LOCI of two observed angles, each LOCUS being found by INTERPOLATIONbetween the curves drawn on the sheet. Circle sheets are prepared and employed in HYDROGRAPHIC SURVEYS forplotting FIXES without the use of PROTRACTOR. Also called standard circle sheet; formerly called sextant chart.See also FIXED ANGLE PLOT, and LATTICE.

800 circular fix. See FIX.

801 circular level. See LEVEL: SPIRIT.

41

802 circular radiobeacon station. A RADIO STATION which needs not necessarily be manned, the emissions of which,radiated around the HORIZON, enable its BEARING to be determined by means of the RADIO DIRECTION FINDERof a ship.

803 circum-meridian altitude. See ALTITUDE: EX-MERIDIAN.

804 circumpolar (adj.). Revolving about the ELEVATED POLE without setting. A CELESTIAL BODY is circumpolarwhen its POLAR DISTANCE is approximately equal to or less than the LATITUDE of the observer. The actual limit isextended somewhat by the combined effect of REFRACTION, SEMIDIAMETER, PARALLAX, and the HEIGHT ofthe observer's eye above the HORIZON.

805 cirrocumulus. See CLOUD GENERA.

806 cirrostratus. See CLOUD GENERA.

807 cirrus. See CLOUD GENERA.

808 civil day. See DAY.

809 civil time. See TIME.

810 civil twilight. See TWILIGHT.

811 civil year. See YEAR.

812 clamping error. See ERROR.

813 clamp (or clamping) screw. A screw for holding in place a moving part of an instrument as during anOBSERVATION or reading with a THEODOLITE or SEXTANT.

814 clamshell snapper. See SNAPPER.

815 clapotis. The French equivalent for a type of STANDING WAVE.

816 Clarke's spheroid. See REFERENCE SPHEROID.

817 clay. Mineralogically, a hydrous aluminium silicate material with plastic properties and a crystal structure. As a sizeterm, refers to sediment particles ranging from 0.0039 to 0.00024 millimeter, in which case it includes rock flour,calcareous muds, aragonite, etc.

818 clean (adj.). Free from OBSTRUCTIONS, unevenness, imperfections, as a clean ANCHORAGE.

819 clear (v.t.). To make such distance from an object as to have open sea-room.

820 clearance. In NAVIGATION, a safe distance off a danger. In METEOROLOGY: a) decrease of total CLOUDAMOUNT when the latter is large; b) TIME at which this decrease takes place; c) gap in a cloud layer covering theentire sky. Also called clearing.

821 clearance: bridge. Minimum vertical or horizontal space available for passage.

822 clearance: overhead pipeline. Minimum vertical or horizontal space available for passage.

823 clearance: safe overhead. The HEIGHT above the VERTICAL CONTROL DATUM at which the highest points of aship can pass under an overhead power CABLE without risk of electrical discharge from the cable to the ship or withoutmaking contact with a BRIDGE.

824 cleared depth. The effective depth over a feature, obtained by a wire drag survey.

825 clearing. See CLEARANCE.

826 clearing line. A straight line, on a CHART, that marks the BOUNDARY between a safe and a dangerous area; or thatpasses clear of a navigational danger. Sectors of lighthouse lights are usually bounded by them.

827 clearing marks. Selected MARKS, natural or otherwise, used to help vessels keep clear of dangers.

42

828 clear sweeping. A form of BAR SWEEPING. A sweep, set at a predetermined depth, and passed over an area to ensurethat no obstructions have been missed.

829 cliff. LAND rising abruptly for a considerable distance above the water or surrounding LAND.

830 climate. Fluctuating aggregate of atmospheric conditions characterized by the states and developments of theWEATHER of a given area.

831 climatology. Study of CLIMATES (causes, variations, distributions, types, etc.).

832 clinometer. An instrument for indicating the degree of SLOPE or the angle of ROLL or PITCH of a vessel, accordingto the plane in which it is mounted.

833 clock. Instrument for measuring TIME and recording HOURS.

834 clock: atomic. A precision clock that depends for its operation on an electrical OSCILLATOR regulated by the naturalvibrational FREQUENCY of an atomic system (such as a beam). The combination of two (or more) radionuclideswhose activities can be utilized to ascertain time intervals.

835 clock: cesium. An atomic clock using the properties of cesium.

836 clock: crystal. A precision clock essentially consisting of a generator of constant FREQUENCY controlled by aresonator made of quartz crystal with suitable methods for producing continuous ROTATION to operate time-indicatingand related mechanisms. Also called quartz clock.

837 clock: quartz. See CLOCK: CRYSTAL.

838 closed traverse. See TRAVERSE.

839 close pack ice. PACK ICE composed of FLOES mostly in contact, such that NAVIGATION becomes difficult even forspecially constructed vessels. ICE CONCENTRATION 7/10th to 9/10th.

840 closing error. See ERROR OF CLOSURE.

841 closing line. A line that divides the internal waters and territorial seas of a coastal State or the archipelagic waters of anarchipelagic State.

842 closing the horizon. See HORIZON.

843 closure: error of. See ERROR OF CLOSURE.

844 closure of triangle. See ERROR OF CLOSURE OF TRIANGLE.

845 cloud. Visible aggregate of minute particles of liquid water or ICE, or of both together, in suspension in theATMOSPHERE. This aggregate may include larger particles of liquid water or ICE, non-aqueous particles or solidparticles, originating for example from industrial gases, smoke or DUST.

846 cloud amount. The fraction of the sky covered by the CLOUDS of a certain genus, certain species, certain variety,certain layer, or of a certain combination of CLOUDS. Also called 'partial cloud amount'. See CLOUD GENERA.

847 cloud classification. System of distinguishing and grouping CLOUDS according to one or more of the followingcriteria: a) appearance, b) process of formation, c) common HEIGHT, d) particulate composition.

848 cloud genera. Main characteristic mutually exclusive forms of CLOUDS, constituting the basis of the CLOUDCLASSIFICATION included in the International Cloud Atlas, 1956. The cloud genera are ten in number: cirrus,cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus, cumulonimbus.

849 clutter. Confused unwanted ECHOES on a radar display.

850 coast. The edge or margin of the LAND next to the SEA; the SEASHORE. Sometimes defined as the meeting of theLAND and SEA considered as the BOUNDARY of the LAND.

851 coast (v.t.). To sail along or near a COAST.

852 coastal chart. See CHART: COAST.

43

853 coastal current. See CURRENT.

854 coastal refraction. See REFRACTION.

855 coastal survey. See SURVEY.

856 coast chart. See CHART.

857 Coast Guard station. See LIFE-SAVING STATION.

858 coastline. See SHORELINE.

859 coastlining. The process of obtaining DATA from which the COASTLINE can be drawn on a CHART.

860 coast pilot (U.S.A.). A descriptive book for the use of mariners, containing detailed information of coastal waters,harbour facilities, etc., of an area. Such books are prepared by the National Ocean Service for waters of the U.S.A. andits possessions. See also SAILING DIRECTIONS.

861 coastwise navigation. See NAVIGATION.

862 coaxial cable. See CABLE.

863 cobble. A naturally rounded STONE larger than a PEBBLE.

864 cocked hat. Triangle on CHART, formed by three POSITION LINES that do not cross at one point. Also calledtriangle of error.NOTE. This expression is seldom used by American navigators.

865 cocurrent line. A line through places having the same tidal current hour.

866 code. A method of information representation by mapping it into a machine-readable alphabet; e.g. textual informationmay be represented in computer memory by the ASCII or the EBCDIC alphabet.

867 codeclination. 90° minus the DECLINATION. See POLAR DISTANCE.

868 coding delay. An arbitrary time delay in the transmission of pulse signals. In the LORAN system this is insertedbetween the transmission of master and slave signals to prevent zero or small readings, and aid in distinguishingbetween master and slave station signals.

869 coefficient of refraction. See REFRACTION: COEFFICIENT OF.

870 coelenterate. One of a phylum of two-staged (sessile and free-floating) organisms. The sessile stage basically iscylindrical and is called a polyp; the free-swimming stage is disc or bell shaped and is called a medusa or JELLYFISH.

871 coherent electromagnetic wave. See WAVE: ELECTROMAGNETIC.

872 col. A marked DEPRESSION in the summit line of a MOUNTAIN CHAIN, generally affording a PASS from oneSLOPE to the other. In METEOROLOGY, a saddle-backed region of almost uniform pressure which appears betweentwo DEPRESSIONS and two ANTICYCLONES arranged alternately in a cross.

873 colatitude. The complement of the LATITUDE, or 90° minus the LATITUDE.

874 cold front. Any non-occluded FRONT which moves in such a way that colder air replaces warmer air.

875 cold wave. Marked cooling of the air, or the invasion of very cold air, over a large area.

876 collector tracing. A comprehensive record of the work compiled while the HYDROGRAPHIC SURVEY is inprogress.

877 collimate (v.t.). In physics and ASTRONOMY, to render parallel to a certain line or direction; to render parallel, asRAYS of light; to adjust the LINE OF SIGHT of an optical instrument so that it is in the proper position relative to theother parts of the instruments. In PHOTOGRAMMETRY, to adjust the FIDUCIAL MARKS of a CAMERA so thatthey define the PRINCIPAL POINT.

44

878 collimation: line of. The line through the OPTICAL CENTRE of the OBJECTIVE LENS of a TELESCOPE and thecentre of the RETICLE. Also called LINE OF SIGHT, sight line, pointing line or aiming line of the instrument.

879 collimation adjustment. The process of bringing the LINE OF COLLIMATION of a TELESCOPE into closeagreement with the COLLIMATION AXIS.

880 collimation axis. The line through the OPTICAL CENTRE of the OBJECTIVE LENS perpendicular to the AXIS ofROTATION of the TELESCOPE. Also called axis of collimation.

881 collimation error. See ERROR OF COLLIMATION.

882 collimator. An optical device for artificially creating a TARGET at infinite distance (a BEAM of parallel RAYS oflight) used in testing and adjusting certain optical instruments. It usually consists of a converging LENS and a TARGET(a system or arrangement of CROSS HAIRS) placed at the principal FOCUS of the LENS.

883 collimator: auto. A COLLIMATOR provided with a means of illuminating its CROSS HAIRS so that, when areflecting plane is placed normal to the emergent LIGHT BEAM, the reflected IMAGE of the CROSS HAIRS appearsto be coincident with the CROSS HAIRS themselves. This device is used in calibrating optical and mechanicalinstruments.

884 collimator: vertical. A TELESCOPE so mounted that its COLLIMATION AXIS can be made to coincide with theVERTICAL (or direction of PLUMB-LINE). The vertical collimator serves as an optical PLUMB-LINE, and may bedesigned for use in placing a MARK on the GROUND directly under an instrument or in centering an instrumentdirectly over a MARK on the GROUND.

885 colour of sea water. See SEA WATER.

886 colour proof. The final proof for checking before printing utilizing all of the negatives involved to produce a copy ofthe final product with all of the required colours. See also PROOF.

887 colour plate. A general term for a pressplate from which any given colour is printed.

888 colour separation. Either the process of preparing a separate drawing, engraving or negative for each colour required inthe production of a chart or a photographic process or electronic scanning procedure using colour filters to separate amulti-coloured copy into separate images of each of the three primary colours.

889 colure: equinoctial. The HOUR CIRCLE through the EQUINOXES.

890 colure: solsticial. The HOUR CIRCLE through the SOLSTICES.

891 coma. An aberration affecting the sharpness of IMAGES off the AXIS, in which RAYS from a point object off theAXIS passing through a given circular zone of the LENS, come to a FOCUS in a circle rather than a point, and thecircles formed by RAYS through different zones are of different sizes and are located at different distances from theAXIS. Therefore, the IMAGE of a point object is comet-shaped. The foggy envelope surrounding the nucleus of aCOMET.

892 comb: micrometer. See MICROMETER.

893 comber. A deep water WAVE whose crest is pushed forward by a strong WIND and is much larger than aWHITECAP. Also called beachcomber. A long-period SPILLING BREAKER. See also ROLLER.

894 comet. A HEAVENLY BODY having a starlike nucleus with a luminous foggy envelope (see COMA), and usuallydeveloping a long luminous tail when near the SUN.

895 common (or vulgar) establishment. See ESTABLISHMENT.

896 compact pack ice. Pack ice in which the concentration is 10/10 (8/8) and no water is visible.

897 compacted ice edge. A close, clear-cut ice edge compacted by wind or current. It is usually on the windward side of anarea of pack ice.

898 compacting. Pieces of sea ice are said to be compacting when they are subjected to a converging motion, whichincreases ice concentration and/or produces stresses which may result in ice deformations.

45

899 comparator. An instrument for measuring a dimension in terms of a STANDARD. An optical instrument formeasuring RECTANGULAR or POLAR COORDINATES of points on any plane surface, such as a photographicPLATE.

900 comparing watch. See WATCH.

901 compass. An instrument consisting of two legs jointed by a PIVOT used for describing circles or transferringmeasurements. Also referred to as pair of compasses. An instrument for indicating a horizontal REFERENCEDIRECTION relative to the EARTH.

902 compass: astro. An instrument which, when oriented to the horizontal and to the CELESTIAL SPHERE, indicates ahorizontal REFERENCE DIRECTION relative to the EARTH. This COMPASS is designed primarily for theOBSERVATION of CELESTIAL BODIES to determine the ORIENTATION of an aircraft relative to the AZIMUTHof a CELESTIAL BODY.

903 compass: boat. A small portable COMPASS mounted in a box for convenient use in small water craft.

904 compass: box. See DECLINATOIRE.

905 compass: deadbeat. An aperiodic COMPASS; one that, when deflected, returns by one direct movement to its properreading.

906 compass: dipping. See DIP CIRCLE.

907 compass: dry (or dry card). A COMPASS without a liquid-filled bowl, particularly a MAGNETIC COMPASShaving a very light COMPASS CARD. Such a MAGNETIC COMPASS is seldom used in modern practice. SeeCOMPASS: LIQUID.

908 compass: dumb. See PELORUS.

909 compass: gyroscopic. A COMPASS having one or more GYROSCOPES as the directive element, and tending toindicate TRUE NORTH. Also called gyro compass.

910 compass: hanging. A COMPASS having its BINNACLE overhead and its graduated card facing downward. Alsoreferred to as overhead compass, or inverted compass.

911 compass: inverted. See COMPASS: HANGING.

912 compass: liquid. A MAGNETIC COMPASS having a bowl completely filled with liquid in order to damp the swingingof the card. Sometimes called spirit compass, or wet compass.

913 compass: magnetic. A COMPASS depending for its directive force upon the attraction of the MAGNETISM of theEARTH for a MAGNET free to turn in an horizontal direction.

914 compass: master. A COMPASS controlling one or several REPEATERS.

915 compass: master gyro. A GYROSCOPIC COMPASS for controlling one or several remote indicators, called GYROREPEATERS.

916 compass: overhead. See COMPASS: HANGING.

917 compass: pocket. A portable COMPASS (employed in COASTLINING or RECONNAISSANCE SURVEY).

918 compass: sky. A type of ASTRO COMPASS which utilizes the POLARIZATION of sunlight in the sky, designed foruse in the arctic during long periods of TWILIGHT. It operates whenever the ZENITH is clear whether or not the SUNis visible, but when the SUN is more than about 6.5° below or 10° above the HORIZON, readings are uncertain.

919 compass: spirit. See COMPASS: LIQUID.

920 compass: standard. A COMPASS designated as the STANDARD for a vessel. It is located in a favourable positionand is accurately calibrated.

921 compass: steering. The COMPASS placed next to the steering wheel, by which the COURSE is steered. See alsoSTEERING REPEATER.

46

922 compass: sun. A form of ASTRO COMPASS utilizing the shadow of a GNOMON.

923 compass: surveyor's. An instrument for determining a MAGNETIC BEARING of a LINE OF SIGHT by the use of asighting device, a graduated HORIZONTAL CIRCLE and a pivoted NEEDLE.

924 compass: trough. See DECLINATOIRE.

925 compass: tubular. A COMPASS supplied for attachment to certain types of LEVELS and THEODOLITES.

926 compass: wet. See COMPASS: LIQUID.

927 compass adjustment. See COMPENSATION OF MAGNETIC COMPASS.

928 compass adjustment buoy. A BUOY placed at a favourable location to assist a vessel to adjust its COMPASS orswing ship. Also called swinging buoy. See also SWINGING SHIP.

929 compass bearing. See BEARING.

930 compass bowl. That part of a COMPASS in which the COMPASS CARD is mounted. A magnetic compass bowl isusually filled with liquid. The compass bowl turns with the craft while the COMPASS CARD remains essentially in anorth-south direction.

931 compass calibration. See SWINGING SHIP.

932 compass card. That part of a COMPASS on which the direction graduations are placed. Also referred to as card.

933 compass compensation. See COMPENSATION OF MAGNETIC COMPASS.

934 compass corrector. See COMPENSATOR: MAGNETIC.

935 compass course. See COURSE.

936 compasses: pair of. See COMPASS.

937 compass meridian. See MERIDIAN.

938 compass needle(s). Magnetized steel needle(s) of high RETENTIVITY that tends to keep north-south line ofCOMPASS CARD in MAGNETIC MERIDIAN.

939 compass north. See NORTH: COMPASS.

940 compass points. The 32 divisions of a COMPASS, at intervals of 11°1/4.

941 compass repeater. A device for repeating at a distance the indications of the MASTER COMPASS.

942 compass rose. A circle graduated in DEGREES clockwise from 0° (north) to 360° printed on a CHART for use as aPROTRACTOR. Compass roses are also placed at other convenient locations to facilitate measurements of directions.They may be oriented to the TRUE or the MAGNETIC NORTH.

943 compensate (v.t.). To counteract an ERROR, as in an instrument; to counterbalance.

944 compensating coils. The coils placed near a MAGNETIC COMPASS to neutralize the effect of the vessel's degaussingsystem on the COMPASS.

945 compensation depth. See DEPTH.

946 compensation of magnetic compass. The process of neutralizing or reducing to a minimum the magnetic effects thevessel itself exerts on a MAGNETIC COMPASS. It is based on the principle that the magnetic effect of the iron andsteel of the vessel can be counterbalanced by means of MAGNETS and SOFT IRON placed near the COMPASS. Alsocalled compass adjustment, compass compensation, or magnetic compensation. See also COMPASS ADJUSTMENT.

947 compensator: magnetic. A MAGNET, piece of SOFT IRON such as a sphere or iron bar, placed in a BINNACLE orin vicinity of a MAGNETIC COMPASS for reducing MAGNETIC DEVIATION to a minimum. Also called compasscorrector or corrector magnet. See FLINDERS BAR, HEELING MAGNET, QUADRANTAL CORRECTORS.

47

948 compilation. In CARTOGRAPHY, the production of a new or improved MAP or CHART (or portion thereof) fromexisting MAPS or CHARTS, AERIAL PHOTOGRAPHS, SURVEYS, new DATA, and other sources. InPHOTOGRAMMETRY, the production of a MAP (or portion of a MAP) from AERIAL PHOTOGRAPHS andgeodetic control data, by means of photogrammetric instruments. Sometimes called stereocompilation.

949 compilation diagram. See SOURCE DIAGRAM.

950 compiler. A program that translates programs from one programming language (source program) into anotherprogramming language (object program). Typically the compiler translates from a higher, user understandable languageto a lower, machine interpretable language.

951 component. See HARMONIC CONSTITUENT.

952 composite electronic navigational system. One contrived in such a way that the POSITION is determined by lines ofdifferent types, for instance, by a LATTICE of circles and a LATTICE of HYPERBOLAE.

953 composite path. A wave track which lies partly over LAND and partly over water.

954 composite photograph. See PHOTOGRAPH.

955 composite sailing. See SAILING.

956 compound tide. See TIDE.

957 compression (of the earth). See FLATTENING OF THE EARTH.

958 compression ratios. See FLATTENING OF THE EARTH.

959 compressional wave. See WAVE.

960 computer. A person who performs calculations.A device capable of accepting DATA, processing it according to stored instruction that are followed in automatic

sequence, and supplying the results in some form. It usually consists of input and output devices, STORAGE, arithmeticand logical units, and a control unit.

961 computer: analogue. A physical device or system which behaves in a manner analogous to some system under study,simulating the processes and producing results which are measured in terms of physical quantities. Often an electricalsystem.

962 computer: digital. A machine for carrying out mathematical processes by operations based on counting, as distinctfrom an ANALOGUE COMPUTER.

963 computer-assisted (computer-aided) cartography. A system which is used to perform specific cartographic functionby means of computer hardware and software which aids humans to make decisions and/or perform their functions.Because of the graphic nature of cartography, such a system always contains, or makes use of, a computer graphicssystem.

964 computer graphics. All methods and techniques used in computer sciences to represent data in graphical form and toprocess images.

965 concentration: ice. See ICE.

966 concluded angle. See ANGLE.

967 condenser. In optics, a LENS or LENS SYSTEM designed to concentrate the ILLUMINATION from a light source ona limited area. In electricity, a device for receiving and storing an electric charge. Also called a capacitor.

968 condition equation. See EQUATION.

969 conductivity. The ability to transmit, as electricity, heat, sound etc.

970 cone. See FAN.

971 configuration of terrain. The TOPOGRAPHY of a region.

48

972 confluence. Flowing together. The place where two or more RIVERS, STREAMS, etc. unite. In METEOROLOGY,progressive drawing together of the STREAMLINES in the direction of FLOW.

973 conformality. The special property of CONFORMAL PROJECTION, i.e. that of having correct relationship about apoint.

974 conformal projection. See PROJECTION.

975 conic (or conical) projection. See PROJECTION.

976 conical buoy. A BUOY the above water part of which is in the shape of a cone.

977 conical projections. The map projections produced by projecting the geographic meridians and parallels onto a conewhich is tangent to or intersects the surface of a sphere, and then developing the cone into a plane.

978 conjunction. In ASTRONOMY, the situation of two CELESTIAL BODIES having the same CELESTIALLONGITUDE or the same sidereal HOUR ANGLE. For a SUPERIOR PLANET, it occurs when the SUN is betweenthe EARTH and the PLANET.

979 conjunction: inferior. The CONJUNCTION of an INFERIOR PLANET and the SUN when the PLANET is betweenthe EARTH and the SUN.

980 conjunction: superior. The CONJUNCTION of an INFERIOR PLANET and the SUN when the SUN is between theEARTH and the PLANET.

981 connection: geodetic. OBSERVATIONS on one or more STATIONS common to the two different geodetic schemes ofTRIANGULATION to be incorporated.

982 console. The housing of the main operating unit of electronic equipment, in which INDICATORS and generalCONTROLS are located. A portion of a COMPUTER used to control the machine manually.

983 consolidated pack ice. Pack ice in which the concentration is 10/10 (8/8) and the floes are frozen together.

984 conspicuous (adj.). Term applied to an object either natural or artificial which is distinctly and notably visible fromseaward.

985 constant deviation. See DEVIATION.

986 constant error. See ERROR.

987 constellation. Formerly a number of FIXED STARS grouped within an imaginary outline. Now, a region of theCELESTIAL SPHERE marked by arbitrary boundary lines.

988 constituent. See HARMONIC CONSTITUENT.

989 constituent day. See DAY.

990 contact glass. See FOCAL PLANE PLATE.

991 contact plate. See FOCAL PLANE PLATE.

992 contact print. See PRINT.

993 contiguous zone. A zone contiguous to a coastal state's territorial sea, which may not extend beyond 24 nautical milesfrom the baselines from which the breadth of the territorial sea is measured. The coastal state may exercise certaincontrol in this zone subject to the provisions of International Law.

994 continent. One of the main continuous bodies of LAND on the EARTH's surface. The main LAND, as distinguishedfrom outlying ISLANDS; MAINLAND.

995 continental borderland. A region adjacent to a CONTINENT, normally occupied by or bordering a CONTINENTALSHELF, highly irregular, with DEPTHS well in excess of those typical of a CONTINENTAL SHELF.

49

996 continental margin. The zone, generally consisting of SHELF, SLOPE and RISE, separating the CONTINENT fromthe ABYSSAL PLAIN or DEEP OCEAN FLOOR.

997 continental rise. A gentle RISE with a generally smooth surface lying between the CONTINENTAL SLOPE and theDEEP OCEAN FLOOR.

998 continental (or island) shelf. A zone adjacent to a CONTINENT (or around an ISLAND), extending from the LOWWATER LINE to the DEPTH at which there is usually a marked increase of SLOPE to greater DEPTH. See SHELF.

999 continental shelf limit. The limit of the continental shelf or continental margin determined in accordance with theprovisions of the United Nations Convention on the Law of the Sea.

1000 continental (or island) slope. The zone of DECLIVITY from the outer EDGE of a CONTINENTAL (or ISLAND)SHELF into greater DEPTH. See SLOPE.

1001 continuous strip camera. See CAMERA.

1002 continuous wave. See WAVE.

1003 contour (or contour line). A line connecting points of equal ELEVATION or equal DEPTH. See also FORM LINE,HACHURES, ILLUMINATED CONTOURS, INDEX CONTOURS.

1004 contour: depression. A closed CONTOUR inside of which the GROUND is at lower ELEVATION than outside. Seealso FORM LINE.

1005 contour: depth. See DEPTH CURVE.

1006 contouring. The process of establishing lines representing equal values of a quantity on a map or chart.

1007 contour interval. The difference in ELEVATION between adjacent CONTOURS.

1008 contour line. See CONTOUR.

1009 contour map. See MAP.

1010 contour sweeping. A form of BAR SWEEPING. A sweep, set at a predetermined depth, and passed over the bottom atright angles to the depth contours. When contact is made the position is recorded.

1011 contrast. In PHOTOGRAPHY, the actual difference in DENSITY between the highlights and the shadows on aNEGATIVE or POSITIVE. Contrast is not concerned with the magnitude of DENSITY but only with the difference inDENSITIES. Also the rating of a photographic material corresponding to the relative density difference which itexhibits.

1012 control. A system of points with established POSITIONS or ELEVATIONS, or both, which are used as fixedreferences in positioning and correlating map features. Control is generally classified in four orders (with first orderdenoting highest quality) according to the PRECISION of the methods and instruments used in establishing it, and theACCURACY of the resultant POSITIONS and ELEVATIONS. Often called basic control.

Regulation or direction of a machine, electronic equipment, etc.; or the DIAL, knob, switch, etc. for performing thisfunction.

The exercise of directing influence over the movements of a craft or missile, with particular reference to changes indirection and SPEED.

1013 control: astronomical. CONTROL determined from ASTRONOMICAL OBSERVATIONS.

1014 control: basic. See CONTROL.

1015 control: geodetic. A system of CONTROL STATIONS established by geodetic methods. See SURVEY: GEODETIC.

1016 control: ground. CONTROL established by GROUND SURVEYS, as distinguished from CONTROL established byphotogrammetric methods.

1017 control: horizontal. CONTROL that is determined in horizontal POSITION. The POSITION may be referred to anyrecognized geographic or plane coordinate system.

1018 control: minor. See CONTROL: PHOTOGRAMMETRIC.

50

1019 control: photogrammetric. CONTROL established by photogrammetric methods, as distinguished from CONTROLestablished by ground methods. Also called minor control.

1020 control: supplemental. Points established by subordinate SURVEYS, to relate AERIAL PHOTOGRAPHS used inmapping with the system of GEODETIC CONTROL. The points must be positively photoidentified, that is, the pointson the GROUND must be positively correlated with their IMAGES on the PHOTOGRAPHS.

1021 control: vertical. CONTROL determined in ELEVATION relative to a DATUM PLANE.

1022 controlled mosaic. See MOSAIC.

1023 controlling depth. See DEPTH.

1024 control mark. A specific object on the ground whose position and/or elevation has been determined or is to bedetermined.

1025 control point. In PHOTOGRAMMETRY, any STATION in a horizontal and vertical control system that is identifiedon a PHOTOGRAPH and used for correlating the DATA shown on that PHOTOGRAPH. More specific terms arephoto-control point, picture control point and ground control point. A CONTROL STATION.

1026 control station. A point on the GROUND whose POSITION (horizontal and vertical) is used as a base for a dependentSURVEY. Also referred to as CONTROL POINT.

1027 convection. In general, mass motions within a fluid resulting in TRANSPORT and mixing of the properties of thatfluid.

In the ATMOSPHERE, organized internal motions within a LAYER of air, leading to vertical transport of heat,momentum, etc.

1028 convection: forced. In the ATMOSPHERE, CONVECTION caused by mechanical forces, such as those arising fromair motion over a rough or sloping surface.

1029 convection: free. In the ATMOSPHERE, CONVECTION caused by density differences within the air.

1030 convergence. In OCEANOGRAPHY, a situation whereby waters of different origins come together at a point or, morecommonly, along a line known as a convergence line. Along such a line the denser water from one side sinks under thelighter water from the other side. See SINKING.

1031 convergence of the meridians. The angular drawing together of the GEOGRAPHIC MERIDIANS in passing from theEQUATOR to the POLES. The difference between the two angles formed by the intersection of a GREAT CIRCLEwith two MERIDIANS. The difference between the RECIPROCAL BEARINGS of two points. At any point of acartographic PROJECTION, the angle between the MERIDIAN, as represented on the plane of the PROJECTION andGRID in use, and the GRID NORTH line.

1032 convergence zone. In OCEANOGRAPHY, region of CONVERGENCE. The region in the deep OCEAN whereSOUND RAYS, refracted from the DEPTHS, arrive at the surface in successive intervals of 30 to 35 NAUTICALMILES.

1033 conversion. Determination of the rhumb line direction of one point from another when the initial great circle direction isknown, or vice versa.

1034 conversion angle. See ARC TO CHORD CORRECTION.

1035 conversion factor. That factor by which the numerical value made in one system of units must be multiplied to arrive atthe numerical value in another system of units.

1036 conversion scale. A SCALE for the conversion of units of one measurement to equivalent units of anothermeasurement. See NOMOGRAM.

1037 conversion table. A TABLE for the conversion of units of one measurement to equivalent units of anothermeasurement. See NOMOGRAM.

1038 coordinate axes. See COORDINATES: PLANE RECTANGULAR.

1039 coordinated universal time (UTC). See UNIVERSAL TIME COORDINATED.

51

1040 coordinates. Linear or angular quantities which designate the POSITION of a point in relation to a given referencesystem.

1041 coordinates: celestial. Any set of COORDINATES used to define a point on the CELESTIAL SPHERE. TheHORIZON, CELESTIAL EQUATOR and ECLIPTIC SYSTEMS of celestial coordinates are based on theCELESTIAL HORIZON, CELESTIAL EQUATOR and ECLIPTIC respectively, as the PRIMARY GREAT CIRCLE.

1042 coordinates: false. GRID COORDINATES obtained by adding fixed numerical quantities to all grid values. Thesequantities are so large that none of the resultant false coordinates are negative. Such a procedure introduces a falseorigin to the south and west of the true ORIGIN. See COORDINATES: ORIGIN OF.

1043 coordinates: geocentric. COORDINATES that define the POSITION of a point with respect to the centre of theEARTH. See LATITUDE: GEOCENTRIC. The term 'geocentric longitude' is never used, as the quantity which it woulddesignate is the same as GEODETIC LONGITUDE.

1044 coordinates: geodetic. Quantities which define the horizontal POSITION of a point on the SPHEROID OFREFERENCE with respect to the planes of GEODETIC EQUATOR and of a selected GEODETIC MERIDIAN. SeeLATITUDE: GEODETIC and LONGITUDE: GEODETIC.

1045 coordinates: geographic(al). A system of SPHERICAL COORDINATES for defining the POSITIONS of points onthe EARTH.

1046 coordinates: grid. A plane rectangular coordinate system based on, and mathematically adjusted to, a MAPPROJECTION, so that GEOGRAPHIC POSITIONS in terms of LATITUDE and LONGITUDE can be readilytransformed into PLANE COORDINATES, and the computation relating to them made by the ordinary methods ofPLANE SURVEYING.

1047 coordinates: origin of. The point of intersection of the COORDINATE AXES, from which the COORDINATES arereckoned. In mathematical treaties, this ORIGIN is usually given the COORDINATES 0,0; in SURVEYING, however,it is standard practice to give this ORIGIN, COORDINATES having large positive numerical values, thereby avoidingthe use of negative COORDINATES. Also called point of origin. See COORDINATES: FALSE.

1048 coordinates: photograph. A system of COORDINATES, either RECTANGULAR or POLAR, to define thePOSITION of a point on a PHOTOGRAPH.

1049 coordinates: plane. See COORDINATES: PLANE RECTANGULAR.

1050 coordinates: plane polar. The distance and DIRECTION from a central point of reference to a point whose POSITIONis being defined. The point of reference is termed the POLE or ORIGIN; the line (distance) connecting the ORIGINwith the point whose POSITION is being defined is the radius vector; and the angle between the fixed line (POLARAXIS) which the DIRECTION is referred to and the radius vector is the vectorial angle.

1051 coordinates: plane rectangular. A system of COORDINATES in a HORIZONTAL PLANE used to describe thePOSITIONS of points with respect to an arbitrary ORIGIN by means of two distances perpendicular to each other. Thetwo REFERENCE LINES at right angles to each other passing through the ORIGIN are called the coordinate axes. Thedistances parallel to the true (or arbitrarily assigned) north-south axis are called the ordinates, or the y coordinates. Thedistances parallel to the true (or arbitrarily assigned) east-west axis are called the abscissae, or the x coordinates. Thenorth and east directions are usually taken as positive, and the south and west directions are usually taken as negative.Also referred to as plane coordinates.

1052 coordinates: polar. A system of COORDINATES used to describe the POSITION of a point in space with respect toan arbitrarily chosen ORIGIN by means of two DIRECTIONS and one distance; i.e., the vectorial angles and radiusvector magnitude.

1053 coordinates: rectangular. COORDINATES on any system in which the axes of reference intersect at right angles.

1054 coordinates: space. In PHOTOGRAMMETRY the term may refer to any general three-dimensional coordinate systemused to define the POSITION of a point in the object space, as distinguished from the IMAGE of the same point on aPHOTOGRAPH.

1055 coordinates: spherical. A system of POLAR COORDINATES in which the ORIGIN is the centre of a sphere and thepoints all lie on the surface of a sphere. The POLAR AXIS of such a system cuts the sphere at its two POLES.

1056 coordinates: strip. The COORDINATES of any point in a strip, whether on the GROUND or actually an AIRSTATION, referred to the ORIGIN and axes of the air coordinate system of the first OVERLAP.

52

1057 coordinate system. A fixed system of lines used to define the position of a point, line, or plane.

1058 coordinatograph. An instrument used to plot in terms of PLANE RECTANGULAR COORDINATES. It may be anintegral part of a STEREOSCOPIC PLOTTING INSTRUMENT whereby the planimetric motions (x and y) of theFLOATING MARK are plotted directly.

1059 coplane: basal. In PHOTOGRAMMETRY, the condition of EXPOSURE of a pair of PHOTOGRAPHS in which thetwo PHOTOGRAPHS lie in a common plane parallel to the AIR BASE. If the AIR BASE is horizontal, thePHOTOGRAPHS are said to be exposed in horizontal coplane.

1060 copper plate. Polished PLATE on which manuscript drawings were formerly engraved for the printing of CHARTS.

1061 coral. Hard calcareous skeletons of many tribes of marine polyps.

1062 coral head. A massive mushroom or pillar shaped coral growth.

1063 coral reef. A REEF, often of large extent, composed chiefly of CORAL and its derivatives.

1064 co-range line. A line through points of equal TIDE RANGE.

1065 cordillera. A RANGE or system of MOUNTAINS.

1066 core. The central or innermost part of anything. See CENTROSPHERE. In OCEANOGRAPHY, a vertical, cylindricalsample of the BOTTOM SEDIMENTS from which the nature and stratification of the BOTTOM may be determined.Also called core sample or sediment core.

1067 core barrel. The tubular section of a core sampling device. Bottom sediment samples are collected either directly in thecore barrel or in a plastic liner placed inside it. Also called coring (or core) tube.

1068 core catcher. A mechanical device located at the bottom of a CORE BARREL to retain a BOTTOM SAMPLE.

1069 corer. A device for obtaining a CORE; a hollow tube that is driven into the ocean floor. Also called coring instrumentor coring device. See CORER: HYDROPLASTIC, CORER: PISTON, PIGGOT GUN and VACUUM LEAD.

1070 corer: hydroplastic. A deep sea coring device, developed at the U.S. Naval Oceanographic Office, to obtain 3 1/2-inchdiameter cores up to 12 FEET in length. The barrel is constructed of high-impact grade polyvinyl chloride plasticwithout an inner liner. Also called polyvinyl corer.

1071 corer: piston. A CORER equipped with a piston inside the core tube that is connected to the lowering cable. When theCORER penetrates the ocean bottom the stopped cable and piston, in effect, provides a suction which overcomes thefrictional forces acting between the SEDIMENT and the interior wall of the coring tube.

1072 corer: polyvinyl. See CORER: HYDROPLASTIC.

1073 core sample. See CORE.

1074 coring instrument (or device). See CORER.

1075 coring (or core) tube. See CORE BARREL.

1076 Coriolis force. Composite CENTRIFUGAL FORCE, due to the ROTATION of the EARTH, which acts on movingparticles, whose motion is considered relative to that of the EARTH.

1077 corona. PHOTOMETEOR formed by one or more sequences (seldom more than three) of coloured rings of relativelysmall radius, centred on the SUN or MOON.

1078 corrected establishment. See LUNITIDAL INTERVAL.

53

1079 correction. A quantity which is applied to an OBSERVATION or function thereof, to diminish or eliminate the effectsof ERRORS and obtain an improved value of the OBSERVATION or function. It is also applied to reduce anOBSERVATION to some arbitrary STANDARD. The correction corresponding to a given ERROR is of the samemagnitude but of opposite sign. See CHRONOMETER CORRECTION, DIP CORRECTION, INDEX CORRECTION,INSTRUMENTAL CORRECTION, LEAD LINE CORRECTION, PARALLAX CORRECTION, PERSONALCORRECTION, POLARIS CORRECTION, REFRACTION CORRECTION, SEMIDIAMETER CORRECTION,SEXTANT ALTITUDE CORRECTION, SKY WAVE CORRECTION, ORTHOMETRIC CORRECTION, TIDALDIFFERENCE.

1080 correction: atmospheric. The statistical removal of meteorological effects from mean sea level changes.

1081 correction: ionospheric. A correction for errors introduced by ionospheric refraction occuring when signals travelthrough the ionosphere.

1082 correction: relativistic. A correction for errors introduced by relativistic effects of the clock rates of positioningsystems.

1083 correction: sky-wave. The correction to be applied to the signals received via the ionosphere to convert them to theirgroundwave equivalent.

1084 correction(s): tape. See TAPE CORRECTION(S).

1085 correction of soundings. The CORRECTION of the observed DEPTHS for any departure from true DEPTH due to themethod of SOUNDING or to a fault in the measuring apparatus. See also REDUCTION OF SOUNDINGS.

1086 corrector magnet. See COMPENSATOR: MAGNETIC.

1087 correlate equation. See EQUATION: CORRELATE.

1088 correlation. The removal of discrepancies that may exist among survey data, so that all parts are interrelated withoutapparent error.

1089 correspondence. In STEREOSCOPY, the condition that exists when corresponding IMAGES on a pair ofPHOTOGRAPHS lie in the same EPIPOLAR PLANE; the absence of Y-PARALLAX.

1090 cosmic dust. Very fine particles of solid matter in any part of the universe, including METEORIC DUST and zodiacallight particles in the SOLAR SYSTEM, interstellar matter of uncertain origin in the Milky Way Galaxy, andaccumulation of dark matter in other galaxies.

1091 cosmic radiation. RADIATION, of very high energy and great penetrative power, which emanates from cosmicregions.

1092 cotidal chart. See CHART.

1093 cotidal line. A line on a CHART passing through all points where HIGH WATER occurs at the same TIME.

1094 coulomb. The unit of electric charge in the SI SYSTEM.

1095 counter. An indicator on a machine, for keeping count of turns, strokes, etc., of the machine or its parts.

1096 counter current. A secondary CURRENT setting in a direction opposite to that of a main CURRENT.

1097 course. The intended horizontal direction of travel. It is measured from 0° at the REFERENCE DIRECTION clockwisethrough 360°; strictly for MARINE NAVIGATION, the term applies to the direction to be steered, which sometimesdiffers from the direction intended to be made good over the GROUND (see DRIFT ANGLE). The course is designatedas true, magnetic, compass, or grid as the REFERENCE DIRECTION is TRUE, MAGNETIC, COMPASS or GRIDNORTH respectively. See COURSE MADE GOOD and TRACK.

The channel in which water flows. See also WATERCOURSE.

1098 course: compass. See COURSE.

1099 course: great circle. The direction of the GREAT CIRCLE through the point of departure and the destination,expressed as the ANGULAR DISTANCE from a REFERENCE DIRECTION, usually north, to the direction of theGREAT CIRCLE. The angle varies from point to point along the GREAT CIRCLE. At the point of departure it is calledInitial Great-Circle Course; at the destination it is called Final Great-Circle Course.

54

1100 course: grid. See COURSE.

1101 course: magnetic. See COURSE.

1102 course: recommended. See TRACK: RECOMMENDED.

1103 course: true. See COURSE.

1104 course line. A LINE OF POSITION approximately parallel to the COURSE.

1105 course made good. The actual TRACK MADE GOOD over the ground (seabed); the direction of the point of arrivalfrom the point of departure. Course made good is the direction component of the resultant ship's velocity and the watercurrent. Course made good should not be confused with HEADING, or ship's head.

1106 course over ground. The direction of the path over the ground actually followed by a vessel. It is normally a somewhatirregular line. This is a misnomer in that courses are directions steered or intended to be steered through the water withrespect to a reference meridian. Also called track over ground.

1107 course recorder. A device which makes an automatic record of the HEADINGS of a vessel. See DEAD RECKONINGTRACER.

1108 cove. A small BAY.

1109 coverage. The actual area covered (by CHARTS, PHOTOGRAPHS, etc.).The area controlled by a radiolocation system.

1110 covariance. The arithmetic mean or expected value of the product of the deviations of corresponding values of twovariables from their respective mean values.

1111 covers and uncovers (or discovers). Expression intended to indicate an area of a REEF or other projection from theBOTTOM of a body of water which periodically extends above and is submerged below the surface. Also referred to asdries or uncovers.

1112 crab. The condition caused by failure to orient a CAMERA with respect to the TRACK of the airplane. In verticalPHOTOGRAPHY, crab is indicated by the edges of the PHOTOGRAPHS not being parallel to the airbase lines.

1113 crack. Any fracture or RIFT in SEA ICE not sufficiently wide to be described as a LEAD.

1114 crag. A steep or precipitous rugged ROCK.

1115 crater. Bowl-shaped cavity, at the SUMMIT or on the side of a volcano.

1116 creek. A comparatively narrow INLET, of fresh or salt water, which is tidal throughout its whole course. A smallTRIBUTARY.

A small, narrow BAY which extends farther inland than a COVE.

1117 crescent (adj.). See PHASES OF THE MOON.

1118 crest. The highest part of a WAVE, SWELL, RIDGE, etc.

1119 crib. A permanent marine structure usually designed to support or elevate pipelines; especially a structure enclosing ascreening device at the offshore end of a potable water intake pipe. The structure is commonly a heavy timber enclosurethat has been sunken with rocks or other debris.

1120 critical sounding. The least depth in proximity to a known or potential navigational route.

1121 cross bearings. Two or more BEARINGS used as intersecting LINES OF POSITION for fixing the POSITION of acraft.

1122 cross hairs. See CROSS WIRES.

1123 crossing angle. See ANGLE.

1124 crosslines. Sounding lines that cross the main system of lines at either right angles or at an oblique angle to serve as acheck on the accuracy of the work.

55

1125 cross lines of sounding. SOUNDING LINES run approximately at right angles to those of normal direction, frequentlyuseful when examining SHOALS. Cross lines are also run after completing the main system of SOUNDING LINES,with the purpose of checking them.

1126 cross sea. A series of WAVES or SWELL crossing another wave system at an angle.

1127 cross section lines of soundings. SOUNDING LINES run in a SURVEY perpendicular to the FLOW of a STREAMand from BANK to BANK in a RIVER.

1128 cross wind. WIND which blows in a direction perpendicular to that of the motion of a moving object, relative to theEARTH's surface.

1129 cross wires (or hairs). Wires or etched lines, at right angles, held at the FOCAL PLANE of an astronomical or sightingTELESCOPE. See also RETICLE.

1130 cruise line of sounding. See TRACK LINE OF SOUNDING.

1131 cruising radius (or range). The distance a craft can travel at a given SPEED without refueling.

1132 crust. In GEOLOGY, the outer LAYER of the solid EARTH; the LITHOSPHERE. Also called earth's crust.

1133 crystal clock. See CLOCK.

1134 ctd probe. Instrument for measuring electrical CONDUCTIVITY, TEMPERATURE and DEPTH of the SEAWATER. See PROBE.

1135 culmination. The position of a HEAVENLY BODY when at highest APPARENT ALTITUDE. Also, for aHEAVENLY BODY which is continually above the HORIZON, the position of lowest APPARENT ALTITUDE.Culmination occurs when the body transits the LOCAL MERIDIAN. See MERIDIAN TRANSIT.

1136 culture. All features constructed on the surface of the EARTH by man, such as cities, railways, CANALS, etc.

1137 cumulonimbus. See CLOUD GENERA.

1138 cumulus. See CLOUD GENERA.

1139 cupola. A small dome-shaped tower or turret rising from a building.

1140 current. Water or other fluid in essentially horizontal motion.In British terminology, a non-periodical movement of water, generally horizontally, due to many causes such as

different temperatures and prevalent WINDS. Some may be temporary, others permanent.

1141 current: backrush. A GRADIENT CURRENT caused by the rise of water level near shores and in SEA REACHES.

1142 current: coastal. A relatively uniform DRIFT usually flowing parallel to the SHORE in the deeper water adjacent tothe SURF ZONE.

1143 current: density. A GRADIENT CURRENT caused by the horizontal gradient of water density.

1144 current: diurnal. The type of TIDAL CURRENT having only one FLOOD and one EBB period in a TIDAL DAY.

1145 current: drift. A wide, slow-moving OCEAN CURRENT principally caused by WINDS.

1146 current: drift angle. The angle between the true heading and the track of a vessel caused by the action of current.

1147 current: earth. See CURRENT(S): TELLURIC.

1148 current: ebb. See EBB STREAM.

1149 current: flood. See FLOOD STREAM.

1150 current: geostrophic. A current existing when the horizontal pressure gradient and the Coriolis force are in balance.

1151 current: gradient. A current due to the horizontal component of the hydrostatic pressure gradient.

56

1152 current: mixed. The type of TIDAL CURRENT characterized by a conspicuous difference in SPEED and durationbetween the two FLOOD CURRENTS or two EBB CURRENTS occurring each TIDAL DAY.

1153 current: nontidal. Any CURRENT that is caused by other than TIDE PRODUCING FORCES. See also STREAM.

1154 current: ocean/sea. A movement of ocean water characterized by regularity, either of a cyclic nature or as a continuousSTREAM flowing along a definable PATH.

1155 current: periodic. An oscillating CURRENT the values of which recur at somewhat regular intervals.

1156 current: permanent. A CURRENT which continues with relatively little periodic or seasonal change.

1157 current: rectilinear. See CURRENT: REVERSING.

1158 current: residual. The mean CURRENT without periodic components.

1159 current: reversing. A TIDAL CURRENT which flows alternately in approximately opposite directions, with SLACKWATER at each reversal. Such CURRENTS occur principally in areas where motion is largely restricted to relativelynarrow CHANNELS. Also called rectilinear current or rectilinear stream.

1160 current: rip. The return flow of water piled up on SHORE by incoming WAVES and WIND.

1161 current: rotary. A TIDAL CURRENT that flows continually, with the direction of FLOW changing through 360°during a TIDAL CYCLE. Called rotary stream in British terminology.

1162 current: seasonal. A CURRENT which has large changes in SPEED or direction due to seasonal WINDS.

1163 current: semidiurnal. The type of TIDAL CURRENT having two flood and two ebb periods of nearly the sameduration during a TIDAL DAY.

1164 current: stream. A narrow, deep, and fast-moving CURRENT as opposed to a relatively wide and weak DRIFTCURRENT.

1165 current: subsurface. A CURRENT usually flowing below the THERMOCLINE generally at slower SPEEDS andfrequently in a different direction from the CURRENTS near the surface.

1166 current: surface. A CURRENT that does not extend more than a few (2-3) METRES below the surface.

1167 current: surge. The coastal CURRENT influenced by remote waves.

1168 current(s): telluric. Natural electric currents that flow on or near the EARTH's surface in large sheets. Also calledearth current.

1169 current: tidal. The alternating horizontal movement of water associated with the RISE and fall of the TIDE caused byTIDE-PRODUCING FORCES. Also called tidal stream.

1170 current: turbidity. A highly turbid, relatively dense CURRENT carrying large quantities of CLAY, SILT and SANDin suspension which flows down a submarine SLOPE through less dense SEA WATER.

1171 current: wind driven. A CURRENT formed by the force of the WIND.

1172 current chart. See CHART.

1173 current cycle. A complete set of tidal current conditions, as those occurring during a tidal day, lunar month, or Metoniccycle.

1174 current difference. The difference between the TIME of SLACK WATER or STRENGTH OF CURRENT at aSUBORDINATE STATION and at its REFERENCE STATION.

1175 current direction. See DIRECTION OF CURRENT.

1176 current line. See CURRENT POLE.

57

1177 current meter. An instrument for the measurement of either SPEED alone or of both direction and SPEED of aCURRENT. See CURRENT POLE, EKMAN CURRENT METER, PRICE-GURLEY CURRENT METER,ROBERTS RADIO CURRENT METER, SWALLOW FLOAT, SAVONIUS ROTOR CURRENT METER,WOLLASTON CURRENT METER.

1178 current pole. A pole used to determine the direction and SPEED of a SURFACE CURRENT. The direction isdetermined by the direction of motion of the pole, and the SPEED by the amount of an attached graduated line (calledcurrent line or log line) paid out in a specified TIME.

1179 current station. The geographic location at which current observations are conducted. Also, the facilities used to makecurrent observations. These may include a buoy, ground tackle, current meters, recording mechanism, and radiotransmitter. See CONTROL current STATION and SUBORDINATE current STATION.

1180 current tables. TABLES listing predictions of the TIMES and SPEEDS of TIDAL CURRENTS at various places, andother pertinent information. Also called tidal current tables.

1181 cursor. A device used with an instrument, to provide a movable reference, as the runner of a SLIDE RULE, or therotatable bearing indicator on a radar scope.In computer systems, a highlighted point on the DISPLAY which indicates the actual working position. For graphicalediting special cursors (e.g. crosses, cross-hairs) can be selected. On output the cursor is moved automatically by thecomputer; on input it is under user control by means of a manual controlling device.A cursor may also be a manually positionable device, sometimes magnifying lens and a reticula to be used in connectionwith a DIGITIZER.

1182 curvature correction. In ASTRONOMY, a CORRECTION applied to the mean of a series of OBSERVATIONS on aSTAR or PLANET to take account of the divergence of the apparent PATH of the STAR or PLANET from a straightline.

In GEODESY, the CORRECTION applied in some geodetic work to take account of the divergence of the surface ofthe EARTH (SPHEROID) from a plane.

1183 curve: depth. See DEPTH CURVE.

1184 customhouse. The office, especially in harbours, at which customs are collected.

1185 Cust protractor. See PROTRACTOR.

1186 cut: angle of. See ANGLE: CROSSING.

1187 cutter. A boat belonging to a warship.

1188 cycle. One complete series of events or PHENOMENA that recur regularly in the same sequence. See alsoFREQUENCY, HERTZ, PERIOD.

1189 cycle per second. A unit of FREQUENCY more and more superseded by the HERTZ.

1190 cycloidal wave. See WAVE.

1191 cyclone. See DEPRESSION.

1192 cylindrical buoy. See CAN BUOY.

1193 cylindrical projection. See PROJECTION.

58

D

1194 daily rate. See CHRONOMETER RATE.

1195 daily retardation. The amount of time by which corresponding tidal PHASES grow later day by day (averagesapproximately 50 MINUTES).

1196 dam. A barrier to check or confine anything in motion; particularly one constructed to hold back water and raise itslevel to form a RESERVOIR, or to prevent flooding.

1197 damped wave(s). See WAVE.

1198 damping. The dissipation of energy with TIME or distance.The progressive reduction of AMPLITUDE of OSCILLATIONS, WAVES, etc.

1199 dan buoy. A BUOY consisting of a ballasted FLOAT carrying a staff which supports a flag or LIGHT.

1200 danger angle. The maximum (or minimum) angle between two points, as observed from a craft, indicating the limit ofsafe approach to an offlying danger.

1201 danger area. An area designated by a proper authority, in which a danger to craft exists. Also called danger zone.

1202 danger bearing. See BEARING.

1203 danger buoy. See ISOLATED DANGER MARK.

1204 danger line. A line drawn on a CHART, to indicate the limits of safe NAVIGATION for a vessel of specific DRAFT.A dotted line on a CHART to emphasize the presence of an OBSTRUCTION.

1205 dangerous rock. A SUNKEN ROCK of a small area (PINNACLE), at such a DEPTH as to be considered dangerous toSURFACE NAVIGATION.

1206 dangerous semicircle. Region of a TROPICAL CYCLONE, situated to the right of its PATH in the northernhemisphere and to the left of its PATH in the southern hemisphere, in which the SEAS are higher and the WINDS moreviolent and tend also to carry shipping forward on to the CYCLONE's PATH. The other half of the CYCLONE istermed the navigable semicircle.

1207 dangerous wreck. A WRECK submerged at such a DEPTH as to be considered dangerous to SURFACENAVIGATION.

1208 danger sounding. See SOUNDING.

1209 danger to navigation. Any feature or condition that might hinder, obstruct, endanger or otherwise constitute danger tosafe navigation.

1210 danger zone. See DANGER AREA.

1211 dark slide. See SLIDE: DARK.

1212 data. A representation of facts, concepts or instructions in a formalised manner suitable for communication,interpretation or processing.

1213 data bank. A collection of DATA in a common location, relating to a given set of subjects. Usually comprised ofseveral DATA BASES considered as an entity.

1214 data base. An organised collection of DATA stored so as to be capable of use by relevant applications with the DATAbeing accessed by different logical paths. It should be application independent.

1215 data base management system. A set of software for creating, maintaining and accessing the DATA of a DATABASE.

1216 data processing. Any operation carried out with data usually with the help of a computer.

59

1217 data retrieval. The extraction of data which meet certain criteria from a FILE or DATA BANK.

1218 data set. A logical entity of data consisting of several elements (fields) grouped under one criterion.

1219 data telemetry. Measuring a quantity, and transmitting the measured value to a distant station where further processingtakes place.

1220 date line. The line coinciding approximately with the 180th MERIDIAN, at which each calendar day first begins, theBOUNDARY between the -12 and +12 TIME ZONES. The date on each side of this line differs by one DAY, but theTIME is the same in these two ZONES. Sometimes called calendar line or international date line.

1221 datum. Any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for otherquantities. In geodesy a datum is defined by a set of parameters specifying the reference surface or the referencecoordinate system. Therefore two types of datums are required: a horizontal datum which forms the basis forcomputations of horizontal control surveys in which the curvature of the earth is considered, and a vertical datum towhich elevations are referred.

1222 datum: chart. A permanently established surface from which SOUNDINGS or tide heights are referenced, usuallyLOW WATER. Also called DATUM, datum level, DATUM PLANE, hydrographic datum, reference level, referenceplane. See DATUM: TIDAL.

1223 datum: geodetic. A set of parameters specifying the reference surface or the reference coordinate system used forgeodetic control in the calculation of coordinates of points on the earth. Commonly datums are defined as horizontal andvertical datums separately.

For a local geodetic datum the reference surface is defined by five parameters: the LATITUDE and LONGITUDEof an INITIAL POINT, the AZIMUTH of a line from this point, and the parameters of the REFERENCE SPHEROID.

Absolute datums specify the INITIAL POINT of the REFERENCE ELLIPSOID to be (ideally) located at the earth'scentre of mass. For modern reference systems using datum information given by satellites additional parameters aredefined, e.g. gravity models.See also HORIZONTAL CONTROL DATUM.

1224 datum: levelling. See DATUM: VERTICAL CONTROL.

1225 datum: sounding. The horizontal plane or TIDAL DATUM to which the SOUNDINGS on a HYDROGRAPHICSURVEY are reduced. Also called datum for sounding reduction.

1226 datum: tidal. A LEVEL of the SEA defined by some PHASE of the TIDE, from which water depths and HEIGHTS OFTIDE are reckoned.

1227 datum: vertical control. Any LEVEL SURFACE (e.g., MEAN SEA LEVEL) taken as a surface of reference fromwhich to reckon ELEVATIONS. Also called datum level, reference level, reference plane, levelling datum, datum forheights. See also DATUM PLANE.

1228 datum for heights. See DATUM: VERTICAL CONTROL.

1229 datum for sounding. See DATUM: SOUNDING.

1230 datum for sounding reduction. See DATUM: SOUNDING.

1231 datum level. See DATUM: VERTICAL CONTROL and DATUM: CHART.

1232 datum line. See REFERENCE LINE.

1233 datum of tide predictions. The LEVEL from which the HEIGHTS OF TIDE are referenced in the TIDE TABLES. Seealso DATUM: CHART.

1234 datum plane. A VERTICAL CONTROL DATUM. Although a LEVEL SURFACE is not a plane, the VERTICALCONTROL DATUM is frequently referred to as the datum plane.

1235 datum point. See REFERENCE POINT.

60

1236 day. The duration of one ROTATION of the EARTH, or occasionally another CELESTIAL BODY, on its AXIS. It ismeasured by successive TRANSITS of a reference point on the CELESTIAL SPHERE over the MERIDIAN, and eachtype takes its name from the reference used.

The period of daylight, as distinguished from night.

1237 day: apparent solar. See DAY: SOLAR.

1238 day: astronomical. A MEAN SOLAR DAY beginning at NOON.

1239 day: civil. A MEAN SOLAR DAY beginning at MIDNIGHT.

1240 day: constituent. The duration of one ROTATION of the EARTH on its AXIS, with respect to a fictitious STARrepresenting one of the periodic elements in the tidal forces. It approximates the length of a LUNAR or SOLAR DAY.The expression is not applicable to a long-period constituent.

1241 day: lunar. The duration of one ROTATION of the EARTH on its AXIS, with respect to the MOON. Its average lengthis about 24h 50m of MEAN SOLAR TIME. Also called TIDAL DAY.

The duration of one ROTATION of the MOON on its AXIS, with respect to the SUN.

1242 day: mean solar. See DAY: SOLAR.

1243 day: sidereal. The duration of one ROTATION of the EARTH on its AXIS, with respect to the VERNAL EQUINOXover the UPPER BRANCH of a MERIDIAN. Because of the PRECESSION OF THE EQUINOXES, the sidereal daythus defined is slightly less than the PERIOD of ROTATION with respect to the STARS, but the difference is less than0.01 SECOND. The length of the sidereal day is 24 HOURS of SIDEREAL TIME or 23h 56m 04.091s of MEANSOLAR TIME.

1244 day: solar. The duration of one ROTATION of the EARTH on its AXIS, with respect to the SUN. This may be either amean solar day, or an apparent solar day, as the reference is the MEAN or APPARENT SUN, respectively.

The duration of one apparent ROTATION of the SUN.

1245 day: tidal. See DAY: LUNAR and TIDAL DAY.

1246 daybeacon. An unlighted BEACON. A daybeacon is identified by its colour, shape, and number of its daymark. Thesimplest form of daybeacon consists of a single pile with a daymark affixed at or near its top.

1247 daylight saving time. See TIME.

1248 daymark. 1. The identifying characteristics of an aid to navigation which serve to facilitate its recognition against adaylight viewing background. On those structures that do not by themselves present an adequate viewing area to be seenat the required distance, the aid is made more visible by affixing a daymark to the structure. A daymark so affixed has adistinctive colour and shape depending upon the purpose of the aid.2. An unlighted navigational mark.

1249 day number. The sequentially numbered day of the year, usually starting with day number 1 on 1st January.

1250 dead ahead. Bearing 000° relative. If the bearing is approximate, the term ahead should be used.

1251 dead astern. Bearing 180° relative. If the bearing is approximate, the term astern should be used. Also called rightastern.

1252 deadbeat (adj.). Aperiodic, or without a PERIOD.

1253 deadbeat compass. See COMPASS.

1254 deadhead. 1. A block of wood used as anchor buoy.2. A BOLLARD.

1255 dead reckoning. A method of NAVIGATION utilizing only the SPEED and HEADING of the craft, without referenceto external aids.

In AIR NAVIGATION, the best estimate of COURSE and SPEED over the GROUND is used, i.e., dead reckoningincorporates wind vector.

1256 dead reckoning position. See POSITION.

61

1257 dead reckoning tracer. A device that automatically provides a graphical record of the DEAD RECKONING TRACK.See COURSE RECORDER.

1258 dead reckoning track. See TRACK.

1259 dead water. The PHENOMENON which occurs when a ship of low propulsive power negotiates water which has athin LAYER of fresher water over a deeper LAYER of more saline water. As the ship moves, part of its energy goesinto generation of an INTERNAL WAVE which causes a noticeable drop in efficiency of propulsion.

1260 debris. See DETRITUS.

1261 deceleration. Negative ACCELERATION.

1262 decibar. One-tenth of a BAR.

1263 decibel. See BEL.

1264 decimetre. One-tenth of a METRE.

1265 declination. In ASTRONOMY, the angle at the centre of the CELESTIAL SPHERE between the radius passingthrough a CELESTIAL BODY and the plane of the CELESTIAL EQUATOR. Declination is measured by the arc of theHOUR CIRCLE between the CELESTIAL BODY and the CELESTIAL EQUATOR. It is plus when the body is northof the EQUATOR, and minus when south of it.

1266 declination: grid. See GRID.

1267 declination: magnetic. See VARIATION.

1268 declinational inequality. See DIURNAL INEQUALITY.

1269 declination circle. See HOUR CIRCLE.

1270 declinatoire. A combined MAGNETIC COMPASS and STRAIGHT EDGE, suitable for use on a PLANE TABLE tomark the MAGNETIC MERIDIAN. Also known as a box compass or trough compass. See also COMPASS: TUBULAR.

1271 declinometer. A magnetic instrument similar to a SURVEYOR'S COMPASS, but arranged so that the LINE OFSIGHT can be rotated to conform with the NEEDLE or to any desired setting on the HORIZONTAL CIRCLE. Used indetermining the magnetic declination. See VARIATION.

1272 declivity. A downward SLOPE or sloping of a HILL; as opposed to ACCLIVITY.

1273 deep. In OCEANOGRAPHY, an obsolete term which was generally restricted to DEPTHS greater than 6,000 m.An unmarked fathom point on a LEAD LINE.

1274 deep-draught route. A route which is primarily selected for use by ships which, because of their deep DRAUGHT,may not be able to navigate safely outside such route.

1275 deep ocean floor. The surface lying at the bottom of the deep ocean.

1276 deep scattering layer (DSL). See PHANTOM BOTTOM.

1277 deep-sea camera. See CAMERA.

1278 deep-sea lead. See LEAD.

1279 deep-sea thermometer. See THERMOMETER.

1280 deep water route. A route in a designated area within defined limits which has been accurately surveyed for clearanceof sea bottom and submerged obstacles to a minimum indicated depth of water.

1281 defile. A narrow PASS or GORGE between MOUNTAINS.

1282 definition. In PHOTOGRAPHY, and electronic equipment, the degree of clarity and sharpness of an IMAGE. SeeRESOLUTION and RESOLVING POWER.

62

1283 deflection of the plumb line. See DEFLECTION OF THE VERTICAL.

1284 deflection of the vertical. The angle at a point on the EARTH (GEOID) between the VERTICAL (direction of thePLUMB LINE) and the direction of the NORMAL to the SPHEROID OF REFERENCE through the point. Also calleddeflection of the plumb line, deviation of the vertical, or station error.

1285 deflector. An instrument for measuring the directive force acting on a MAGNETIC COMPASS on differentHEADINGS, for use in COMPASS ADJUSTMENT.

1286 degaussing. Neutralization of the strength of the MAGNETIC FIELD of a vessel, by means of suitably arrangedelectric coils permanently installed in the vessel. See also CABLE: DEGAUSSING.

1287 degaussing cable. See CABLE.

1288 degree. A unit of angular or circular-arc measurement, equal to 1/360 of a circle.A unit of measurement of temperature in any SCALE.

1289 Delambre's spheroid. See REFERENCE SPHEROID.

1290 delimitation. The process of establishing the LINE(S) OF DELIMITATION in the context of the United Nations' Lawof the Sea Convention.

1291 delineation. In CARTOGRAPHY, the visual selection and distinguishing of mapworthy features on various possiblesource materials by outlining the features on the source material, or on a map manuscript (as when operating aSTEREOSCOPIC PLOTTING INSTRUMENT); also a preliminary step in COMPILATION.

1292 delta. An area of alluvial DEPOSIT, usually triangular in outline, near the MOUTH of a RIVER.

1293 demagnetize (v.t.). To remove MAGNETISM. The opposite is MAGNETIZE.

1294 densitometer. An electric instrument designed to accurately measure optical DENSITY or tone values and used inplace of the human eye for such purposes.

1295 density. In PHOTOGRAPHY, a measure of the degree of blackening of an exposed film, PLATE or paper afterdevelopment, or the direct IMAGE (in the case of a print-out material). It is defined strictly as the logarithm of theoptical OPACITY.

In OCEANOGRAPHY, density is equivalent to SPECIFIC GRAVITY and represents the ratio, at ATMOSPHERICPRESSURE, of the WEIGHT of a given volume of SEA WATER to that of an equal volume of distilled water at 4.0°C.

1296 density layer. A LAYER of water in which DENSITY increases with DEPTH enough to increase the BUOYANCY ofa submarine.

1297 density of soundings. Intervals between LINES OF SOUNDING and SOUNDINGS in the same line. Density ofsoundings mostly depends on the SCALE and nature of the SURVEY. Also called frequency of soundings.

1298 departure. The distance between two MERIDIANS at any given PARALLEL OF LATITUDE expressed in linearunits, usually NAUTICAL MILES. The distance to the east or west made good by a vessel in proceeding from one pointto another.

1299 departure: point of. The point from which the initial COURSE to reach the point of destination begins.

1300 deposit. Accumulations of solid material (of any type or from any source) on the sea bottom which eventually maybecome compacted and consolidated and form sedimentary ROCK.

1301 depressed pole. See POLE.

1302 depressor. A device to maintain a towfish at a given depth.

1303 depression. Any HOLLOW or relatively sunken area.In METEOROLOGY, a region of the ATMOSPHERE in which the pressure is low relative to the surrounding

region at the same level. It is represented on a WEATHER CHART by a system of ISOBARS at a specified level or ofCONTOURS at a specified pressure which enclose relatively low values of pressure or level. Also called low orcyclone.

63

1304 depression: angle of. See ANGLE OF DEPRESSION.

1305 depression contour. See CONTOUR.

1306 depression of the horizon. See DIP.

1307 depth. The vertical distance from a given water level to the BOTTOM.

1308 depth: charted. The vertical distance from the CHART DATUM to the BOTTOM.

1309 depth: compensation. The DEPTH at which the light intensity is just sufficient to bring about a balance between theoxygen produced by ALGAE through PHOTOSYNTHESIS and that consumed by them through respiration.

1310 depth: controlling. The least DEPTH in the approach or CHANNEL to an area, such as a PORT or ANCHORAGE,governing the maximum DRAFT of vessels that can enter.

1311 depth: standard. A DEPTH below the sea surface at which water properties should be measured and reported,according to the proposal by the International Association of Physical Oceanography in 1936.

1312 depth: thermometric. The DEPTH, in METRES, at which paired protected and unprotected THERMOMETERSattached to a NANSEN BOTTLE are reversed. The difference between the corrected readings of the 2THERMOMETERS represents the effect of the HYDROSTATIC PRESSURE at the DEPTH of reversal. This DEPTHmay then be determined by formula or from a DEPTH ANOMALY GRAPH. DEPTHS obtained by this means are ofgreatest value when a WIRE ANGLE occurs.

1313 depth anomaly graph. A graph constructed to determine the difference between the computed or THERMOMETRICDEPTH and the ideal or assumed DEPTH of reversal of THERMOMETERS attached to a NANSEN BOTTLE.

1314 depth contour. See DEPTH CURVE.

1315 depth curve. A depth curve is a line connecting points of equal water depth which is sometimes significantly displacedoutside of soundings, symbols, and other chart detail for clarity as well as generalization. Depth curves therefore oftenrepresent an approximate location of the line of equal depth as related to the surveyed line delineated on the source.

1316 depth differences. Those differences in DEPTH which may be encountered in HYDROGRAPHIC SURVEYS atcrossing of CROSS LINES and principal LINES OF SOUNDINGS.

1317 depth finder. An instrument for the measurement of the DEPTH of water, particularly an ECHO SOUNDER.

1318 depth gauge. See GAUGE.

1319 depth ice. See ANCHOR ICE.

1320 depth of the sea. The vertical distance from the water surface to the sea bottom.

1321 derelict. Any property abandoned at SEA, often of sufficient size as to constitute a menace to NAVIGATION;especially an abandoned vessel. See WRECK.

1322 derrick. A tall pyramid of latticed steel mounted over a borehole.

1323 descending node. See NODE.

1324 detached position. See POSITION.

1325 detached sounding. See SOUNDING.

1326 detail(s). In CARTOGRAPHY, the items or particulars of information (shown on a MAP by lines, SYMBOLS, andlettering) which, when considered as a whole, furnish the comprehensive representation of the physical and culturalfeatures of the EARTH's surface.

1327 detection threshold. The lowest or highest level of a parameter above or below which sensing equipment can detect theparameter.

64

1328 detritus. In GEOLOGY, material removed by disintegration and other processes from the surface of ROCKS. There is ageneral tendence to use debris.

In MARINE BIOLOGY, suspended matter of organic origin permanently incapable of reproduction organic detritusmay often 'collect' a considerable amount of inorganic material.

1329 deviation. See DEVIATION: MAGNETIC.

1330 deviation: constant. DEVIATION which is the same on any HEADING, which may result from certain arrangementsof asymmetrical horizontal SOFT IRON.

1331 deviation: magnetic. The angle between the COMPASS NEEDLE and the MAGNETIC MERIDIAN, expressed indegrees east or west of MAGNETIC MERIDIAN. Also called deviation.

1332 deviation: quadrantal. DEVIATION which changes its sign (E or W) approximately each 90° change of HEADING.It is caused by INDUCED MAGNETISM in horizontal SOFT IRON.

1333 deviation: residual. DEVIATION of a MAGNETIC COMPASS after COMPENSATION.

1334 deviation: semicircular. DEVIATION which changes sign (E or W) approximately each 180° change of HEADING.

1335 deviation: standard. See ERROR: STANDARD.

1336 deviation of the vertical. See DEFLECTION OF THE VERTICAL.

1337 deviation table. A TABLE of the DEVIATION of a MAGNETIC COMPASS on various HEADINGS, magnetic orcompass.

1338 dew. Deposit of water drops on objects at or near the GROUND, produced by the condensation of water vapour fromthe surrounding clear air.

1339 dew-point. The temperature to which air must be cooled at constant pressure and constant water vapour content to reachsaturation.

1340 diagenesis. The chemical and physical changes that SEDIMENTS undergo after their deposition, by LITHIFICATION.

1341 diagram. A graphic representation of certain DATA.

1342 dial. The face of a WATCH or CLOCK. The clockwise face of an instrument for indicating, as by a moving pointer, theamount of something; face of a COMPASS, GAUGE or METER.

A SUNDIAL.

1343 diaphone. A device operated by compressed air for producing a distinctive FOG SIGNAL.

1344 diaphragm. In a TELESCOPE, a thin glass disk on which etched lines forming a RETICLE are placed. See alsoRETICLE, and APERTURE STOP.

1345 diapositive. A positive PHOTOGRAPH on a transparent medium. One which has a right reading image when viewedthrough the base support.

1346 difference: tidal. See TIDAL DIFFERENCE.

1347 difference of latitude. The ANGULAR DISTANCE between the PARALLELS of two places measured on aMERIDIAN.

1348 difference of longitude. The angle at the POLE or intercepted arc of EQUATOR between the MERIDIANS of twoplaces.

1349 differential GPS. Differential GPS is implemented by placing a GPS monitor receiver at a precisely known location.Instead of computing a navigation fix, the monitor determines the range error to every GPS satellite it can track. Theseranging errors are then transmitted to local users where they are applied as corrections before computing the navigationresult.

1350 differential levelling. See LEVELLING.

65

1351 differential mode. A method of accurate positioning; transmissions of an electromagnetic position fixing system arereceived at known positions ashore and the offsets between absolute known positions and system positions aretransmitted to the ship in the vicinity. The offsets are applied to the measurements of the ship's receiver.

1352 differential shrinkage. In mapping, the difference in unit contraction along the grain structure of the material ascompared to the unit contraction across the grain structure; frequently refers to photographic film and papers and to mapmaterials in general.

1353 diffraction. The bending of the RAYS of RADIANT ENERGY around the edges of an obstacle, or when passing nearthe edges of an opening, or through a small hole or slit, resulting in the formation of a SPECTRUM, or arrangement byWAVE LENGTH of component WAVES, producing, for visible WAVE LENGTHS, the chromatic SPECTRUM.

The bending of a WAVE as it passes an obstruction.

1354 digital. A method of representing information by combinations of discrete and discontinuous data.

1355 digital chart. See ELECTRONIC CHART.

1356 digital computer. See COMPUTER.

1357 digital imagery processing system. A system which processes images which are stored in digital form.

1358 digital terrain model (DTM). A model representing the terrain surface by a collection of digital data, mainlycoordinates in three dimensions, x, y, and z, and a digitally defined method to interpolate arbitrary terrain heights inbetween the stored values. Not to be confused with digital cartographic model which may also contain digital datarepresenting terrain, but in a form suitable for the drawing of a particular kind of map.

1359 digitization (or digitisation). The process of converting analog data to digital data.

1360 digitizer. A computer aided device equipped with a digitizing table and a CURSOR for digitally capturing graphicaldata (pictures, charts), usually based on vector-techniques (as opposed to a SCANNER). The digitizing table consists ofa gridded electrical network under the table's surface which allows an automatic computation of the planar x/ycoordinates of a cursor's position. Lines are sequentially scanned by moving the cursor along the position andidentifying the appropriate symbol on a menu.

1361 dike (or dyke). An artificial ditch to hold, or control the FLOW of water.An artificial EMBANKMENT to contain or hold back water.In GEOLOGY, a tabular body of IGNEOUS ROCK molded from molten ROCK within a fissure.

1362 dilution of precision (DOP). A parameter relating the precision of the position (and/or an unknown auxiliary) providedby a positioning system to that of the "observed quantities" directly measured by the system. DOP is a measure of theinfluence of the geometry of loci on the precision of position fixing. DOP is frequently used with a qualifying term suchas geometric (GDOP), horizontal (HDOP), vertical (VDOP) etc. to indicate that DOP is related to all or some unknownquantities.

1363 diopter. A unit of refractive power of an optical system in the SI system.

1364 dioptric light. See LIGHT.

1365 dip. The vertical angle, at the eye of an observer between the horizontal and the LINE OF SIGHT to the APPARENTHORIZON. Also called dip of the horizon or depression of the horizon.

The angle between the horizontal and the LINES OF FORCES of the EARTH's MAGNETIC FIELD at any point.Also called magnetic dip or magnetic inclination.

1366 dip circle. An instrument for measuring MAGNETIC DIP. It consists essentially of a DIP NEEDLE or MAGNETICNEEDLE, suspended in such manner as to be free to rotate about a horizontal AXIS. Also called INCLINOMETER, ordipping compass. See also EARTH INDUCTOR.

1367 dip correction. That CORRECTION to SEXTANT ALTITUDE due to the DIP OF THE HORIZON. Also calledheight of eye correction.

1368 dip (or dipping) needle. A MAGNETIC NEEDLE suspended in such a manner as to be free to rotate about ahorizontal AXIS. See DIP CIRCLE.

66

1369 dip of a sea horizon. The vertical angle, at the eye of an observer, between the horizontal and the line of sight to the seahorizon.

1370 dip of the horizon. See DIP.

1371 dipping compass. See DIP CIRCLE.

1372 dipsey lead. See LEAD: DEEP SEA.

1373 direction. In SURVEYING and mapping, the angle between a line or plane and an arbitrarily chosen REFERENCELINE or PLANE.

1374 direction: base. In PHOTOGRAMMETRY, the DIRECTION of the vertical plane containing the AIR BASE whichmight be expressed as a BEARING or an AZIMUTH.

1375 direction: grid. Horizontal DIRECTION expressed as ANGULAR DISTANCE from GRID NORTH.

1376 directional antenna. See ANTENNA.

1377 directional gyroscope. See GYROSCOPE.

1378 directional (or directed) radiobeacon station. A special type of radiobeacon station the emissions of which areintended to provide a definite TRACK for GUIDANCE.

1379 direction finder. See RADIO DIRECTION FINDER.

1380 direction (or directional) light. See LIGHT.

1381 direction method of observation. A method of observing angular relationships wherein the graduated CIRCLE is heldin a fixed position, and the DIRECTIONS of the various SIGNALS are observed around the HORIZON. Thus,DIRECTIONS are pointings whereby angles are found by the differences in DIRECTION.

1382 direction of current. The direction toward which a CURRENT is flowing, called the SET of the CURRENT. Alsocalled current direction.

1383 direction of gravity. See GRAVITY.

1384 direction of movement of the waves. Direction from which the WAVES arrive at a fixed point.

1385 direction of relative movement. See RELATIVE MOTION.

1386 direction of the force of gravity. See GRAVITY: DIRECTION OF.

1387 direction of tilt. The DIRECTION (AZIMUTH) of the PRINCIPAL PLANE of a PHOTOGRAPH. Also, theDIRECTION of the PRINCIPAL LINE on a PHOTOGRAPH.

1388 direction of traffic flow: established. A traffic flow pattern indicating the directional movement of traffic asestablished within a traffic separation scheme.

1389 direction of traffic flow: recommended. A traffic flow pattern indicating a recommended directional movement oftraffic where it is impractical or unnecessary to adopt an established direction of traffic flow.

1390 direction of wind. Direction from which the WIND blows. Also called wind direction.

1391 direction theodolite. See THEODOLITE.

1392 direct levelling. See LEVELLING.

1393 direct wave. See WAVE.

1394 disc (U.S. disk). Rotating storage medium composed of one or two usable surfaces or a vertical assembly of such discswith a common axis. Typically discs combine high storage volumes and short access time allowing fast access to datawhich is distributed randomly over the medium.

67

1395 discoloured water. Unnatural coloured areas in the SEA due to the existence of SHOALS.SEA WATER having a colour other than the blues and greens normally seen. Variations of the colours red, yellow,

green and brown, as well as black and white, have been reported. Discolorations may appear in patches, streaks, or largeareas and may be caused by concentrations of inorganic or organic particles or PLANKTON. See RED TIDE.

1396 discontinuity layer. See THERMOCLINE.

1397 discrepancy. A difference between results of duplicate or comparable measures of a quantity. The difference incomputed values of a quantity obtained by different processes using DATA from the same SURVEY.

1398 discrepancy in position. The difference between a reckoned and an observed position at the same time.

1399 discriminator. A radar circuit that produces a RESPONSE which depends upon the FREQUENCY of the input signal.A transponder beacon circuit which accepts PULSES of proper duration and rejects others.That part of a frequency modulation receiver which converts the modulated SIGNALS directly into audiofrequency

signals.

1400 dish aerial. A parabolic aerial for the receipt and transmission of high frequency radio signals.

1401 dispersion. The separation of RADIANT ENERGY into its various components.In optics, the separation of light into its component colours by its passage through a diffraction grating or by

REFRACTION such as that provided by a prism.In OCEANOGRAPHY, the separation of a complex surface gravity wave disturbance into its component parts.

1402 displacement. In RADIOLOCATION, the half of the band of uncertainty having as its centre line an ELECTRONICLINE OF POSITION.

In PHOTOGRAMMETRY, any shift in the position of an IMAGE on a PHOTOGRAPH which does not alter theperspective characteristics of the PHOTOGRAPH (i.e. shift due to TILT of the PHOTOGRAPH, scale change in thePHOTOGRAPH, and RELIEF of the objects photographed).

1403 displacement drafts. See DRAFT.

1404 display. A visual presentation of data.

1405 distance finding station. A RADIOBEACON equipped with a synchronized SOUND SIGNAL to provide a means ofdetermining distances from the source of the SOUND, by measuring the difference in the time of reception of theSIGNALS.

1406 distance measuring equipment (DME). See ELECTRONIC DISTANCE MEASURING EQUIPMENT.

1407 distance reduction. See REDUCTION TO SEA LEVEL.

1408 distortion. An undesired change in WAVEFORM.In optics, an aberration affecting the position of the IMAGES off the AXIS, in which objects at different

ANGULAR DISTANCES from the AXIS undergo different MAGNIFICATIONS. Also called lens distortion.In PHOTOGRAPHY, any shift in the position of an IMAGE on a PHOTOGRAPH which alters the perspective

characteristics of the PHOTOGRAPH. Causes of image distortion include lens aberration, DIFFERENTIALSHRINKAGE of film or paper, and motion of the film or CAMERA.

1409 distortion: angular. In CARTOGRAPHY, DISTORTION in a PROJECTION because of non-conformality. SeeCONFORMALITY.

1410 distortion: lens. See DISTORTION.

1411 distortion: radial. A lens aberration that is corrected on the PHOTOGRAPH by displacing the IMAGE along a radialline from the PRINCIPAL POINT.

1412 distortion of medium. Changes in the dimensions of SURVEYS or CHARTS due to expansion or contraction of thepaper on which the SURVEY or CHART is drawn or printed. See also DIFFERENTIAL SHRINKAGE.

1413 distributary. A river branch which flows away from the main STREAM without returning to it, as in a DELTA.

1414 diurnal aberration. See ABERRATION OF LIGHT.

68

1415 diurnal arc. See ARC: ASTRONOMICAL.

1416 diurnal current. See CURRENT.

1417 diurnal inequality. The difference between the HEIGHTS of the two HIGH TIDES (high water inequality) or the twoLOW TIDES (low water inequality) during a TIDAL DAY, or the difference in SPEED between the two FLOOD or thetwo EBB CURRENTS during a TIDAL DAY. The average diurnal inequality is called tropic high water inequality andtropic low water inequality when referred to the HIGH WATERS and LOW WATERS, respectively, of TROPICTIDES. Mean diurnal high water inequality is half the average difference between the HEIGHTS of the two HIGHWATERS of each TIDAL DAY over a 19-year period. Mean diurnal low water inequality is half the average differencebetween the HEIGHTS of the two LOW WATERS for a similar period. Also called declinational inequality.

1418 diurnal motion. The apparent daily motion of a celestial body.

1419 diurnal parallax. See PARALLAX.

1420 diurnal range. See RANGE.

1421 diurnal tide. See TIDE.

1422 diver. A person skilled in the practice of diving.

1423 divergence. In OCEANOGRAPHY, a horizontal FLOW of water, in different directions, from a common centre orzone; often associated with UPWELLING.

1424 divider(s). Measuring COMPASSES having both legs pointed, used principally for measuring distances andCOORDINATES.

1425 dock. The space between two PIERS. Also called slip.

1426 dock: dry. An artificial BASIN fitted with a gate or CAISSON, into which vessels can be floated and the water pumpedout to expose the vessel's bottom. Also called graving dock.

1427 dock: floating. A form of DRY DOCK consisting of a floating structure of one or more sections which can be partlysubmerged by controlled flooding to receive a vessel, then raised by pumping out the water so that the vessel's bottomcan be exposed.

1428 dock: graving. See DOCK: DRY.

1429 dock: wet. A DOCK in which water can be maintained at any level by closing a gate when the water is at the desiredlevel.

1430 dock sill. The foundation at the bottom of the entrance to a dry dock or lock against which the caisson or gates close.The depth of water controlling the use of the dock or lock is measured from the sill to the surface.

1431 dog leg. A LEG which does not lead directly to the destination or way point.

1432 doldrums. Zone of calm or light variable WINDS, in the lower atmospheric LAYERS, situated near the THERMALEQUATOR; the zone follows, with slight time lag, the annual meridional fluctuation of the THERMAL EQUATOR.

1433 dolphin. A post or group of posts, used for mooring or warping a vessel, or as an AID TO NAVIGATION. The dolphinmay be in the water, on a WHARF or on the BEACH.

1434 dolphin beacon. A minor light structure consisting of a number of piles driven into the bottom in a geometric or randompattern and drawn together, with a light mounted at the top.

1435 Doppler effect. The apparent change in WAVE LENGTH, and therefore in FREQUENCY of RADIANT ENERGY,when the distance between the source and the observer or RECEIVER is changing. This effect is exploited inunderwater acoustics and satellite positioning.

1436 Doppler navigation. See NAVIGATION.

1437 double high water. See HIGH WATER.

69

1438 double low water. See LOW WATER.

1439 double meridian distance. See MERIDIAN DISTANCE.

1440 double tide. See TIDE.

1441 double zenith distance. See ZENITH DISTANCE.

1442 doubtful sounding. See SOUNDING.

1443 Dove prism. A prism which reverts the IMAGE but does not deviate nor displace the BEAM. A given angularROTATION of the prism about its longitudinal AXIS causes the IMAGE to rotate through twice the angle. Also calleda rotating prism.

1444 down. An area of high, treeless GROUND, usually undulating and covered with grass. See also DUNE.

1445 downstream (adj. and adv.). In the direction of FLOW of a CURRENT or STREAM. The opposite is UPSTREAM.

1446 downwind(adj. and adv.). In the direction toward which the WIND is blowing. The opposite is UPWIND.

1447 dracontic month. See MONTH: NODICAL.

1448 draft (or draught). The vertical distance, at any section of a vessel from the surface of the water to the bottom of thekeel. When measured at or near the bow, it is referred to as draft forward and when measured at or near the stern asdraft aft. The mean draft is the mean of the drafts forward and aft. These drafts are more specifically described asdisplacement drafts as opposed to navigational drafts which are measured to the lowest appendage to the hull asopposed to the keel.

1449 draft aft. See DRAFT.

1450 drafted original. A scribed or drafted CHART or portion of a CHART on stable base material, which may beprocessed for final REPRODUCTION.

1451 draft forward. See DRAFT.

1452 drafting. The art of drawing from given specifications.

1453 draft marks. Numerals at the bow and stern of a vessel that indicate the depth to which the vessel is submerged.

1454 draftsman (or draughtsman). A person who makes drawings.

1455 drag (v.t.). To tow a line or object below the surface to determine the least DEPTH in an area or to ensure that a givenarea is free from navigational dangers to a certain DEPTH. See also SWEEP.

To pull along the BOTTOM, as an anchor.

1456 drag (of theodolite). A slight movement of the graduated CIRCLE produced by the rotation of the ALIDADE.

1457 draught. See DRAFT.

1458 draughtsman. See DRAFTSMAN.

1459 drawing. The graphic representation of data on a non-volatile medium.(U.S. terminology). An impression following the printing of a NAUTICAL CHART of either its black or its magenta

detail on matte finish transparent plastic, used in revising subsequent printings of the CHART.

1460 dredge. An apparatus for bringing up solid material from the bottom of a water area, gathering deep water organisms,etc.

A DREDGER.

1461 dredge (v.t.). To remove solid matter from the BOTTOM of a water area.

1462 dredged area. An area of the BOTTOM of a body of water which has been deepened by dredging.

70

1463 dredger. A vessel or floating structure equipped with machinery for excavating ditches or CANALS, deepeningCHANNELS and HARBOURS, and removing solid matter from the BOTTOM of a water area. Also called aDREDGE.

1464 dredging buoy. A BUOY marking the limit of an area where dredging is being performed.

1465 'dries'. See COVERS and UNCOVERS.

1466 drift. The SPEED of the water due to OCEAN or TIDAL CURRENTS.A wide, slow-moving CURRENT principally caused by WINDS.The distance a craft is moved by CURRENT and WIND.DRIFT ANGLE or LEEWAY.The term is also applied to any superficial deposit caused by a CURRENT of water or air.

1467 drift (v.i.). To move by action of WIND or CURRENT without control.

1468 drift angle. The difference between COURSE steered and COURSE MADE GOOD when due to action of CURRENTand WIND.

The angle between the tangent to the turning circle and the centerline of the ship.

1469 drift bottle. An identifiable sealed bottle which is released into the SEA for use in studying CURRENTS.

1470 drift buoy. A DATA collection BUOY which is not moored to the sea bottom, but drifting freely in response tocurrents, tides, and wind.

1471 drift current. See CURRENT.

1472 drift ice foot. See RAMP.

1473 drifting logship. A LOGSHIP employed for the measurement of TIDAL STREAMS and CURRENTS. It is allowed todrift freely while its POSITIONS are fixed, during the PERIOD of OBSERVATION, by an accompanying boat, or fromSTATIONS ashore.

1474 drift lead. See LEAD.

1475 drift sounding. See SOUNDING.

1476 drift station. A scientific station established on the ICE of the Arctic Ocean.A term sometimes used in shoran operations to designate the ground station about which the aircraft flies. The

second station is then referred to as rate station.

1477 driver rod. A device employed together with a SINKER to obtain BOTTOM SAMPLES when WIRE SOUNDING. Itconsists, essentially, of a galvanized iron tube the lower part of which contains a flap valve for retaining a bottomspecimen. The tube releases the SINKER when it strikes the BOTTOM. Also called driver tube.

1478 driver tube. See DRIVER ROD.

1479 drizzle. Fairly uniform PRECIPITATION composed exclusively of fine drops of water (diameter less than 0.5 mm),very close to one another.

1480 drogue. A current measuring assembly consisting of a weighted parachute and an attached surface buoy. Also calledparachute drogue.

1481 drum. A rotating cylinder on which is set a registering card or paper for recording purposes.Cylinder around which CABLE is wound in a winch.

1482 drum plotter. A plotter which uses a rotating drum or cylinder to move the drawing base in the direction of onecoordinate axis while the plotting head only moves along the other coordinate axis.

1483 dry bulb thermometer. See THERMOMETER.

1484 dry (or dry card) compass. See COMPASS.

1485 dry dock. See DOCK.

71

1486 drying heights. HEIGHTS above SOUNDING DATUM, of any areas (BANKS, FORESHORES, ROCKS, etc.) whichdry at LOW WATER.

1487 drying line. The line marking the transit from water to land; usually the low-water line.

1488 drying reef. A reef or part thereof which dries at low water.

1489 dry proof. An impression of a CHART pulled from the engraved PLATE onto a dry sheet of paper. This impression isfainter than a damp-pulled copy and unsuitable for ordinary use but is free from DISTORTION.

1490 duck blinds. Nonfloating structures used for concealing waterfowl hunters. They usually consist of a woodenframework covered with brush. They are essentially unreported to any charting authority when built. They are unlightedand often constructed in navigable water without regard to the possible hazard they pose, especially to the small craftoperator. Many are substantial structures built on piles. Even after they are eventually reduced to ruins the pilings maypersist for years.

1491 duct. A LAYER in the OCEAN or ATMOSPHERE where REFRACTION and, probably, REFLECTION result in thetrapping and propagation within the LAYER of SOUND WAVES or ELECTROMAGNETIC WAVES.

1492 due publicity. Notification of a given action for general information through appropriate authorities within a reasonableamount of time in a suitable manner. Used in the context of the United Nations' Law of the Sea Convention.

1493 dumb compass. See PELORUS.

1494 dumping ground. See SPOIL GROUND.

1495 dumpy level. See LEVELLING INSTRUMENT: DUMPY LEVEL.

1496 dune. A mound, RIDGE or HILL of drifted SAND on the sea coast or in a desert. See SAND DUNE and DOWN.

1497 duplex. A method in which TELECOMMUNICATION between two STATIONS can take place in both directionssimultaneously.

1498 duplexer. A switching device used to connect a TRANSMITTER and a RECEIVER to the same ANTENNA.

1499 duration of fall. See DURATION OF RISE AND FALL.

1500 duration of flood and duration of ebb. Duration of flood is the interval of time in which a tidal current is flooding, andthe duration of ebb is the interval in which it is ebbing; these intervals being reckoned from the middle of the interveningslack waters or minimum currents. Together they cover, on an average, a period of 12.42 hours for a semidiurnal tidalcurrent or a period of 24.84 hours for a diurnal current. In a normal semidiurnal tidal current, the duration of flood andduration of ebb will each be approximately equal to 6.21 hours, but the times may be modified greatly by the presenceof a non-tidal flow. In a river the duration of ebb is usually longer than the duration of flood because of the fresh waterdischarge, especially during the spring months when snow and ice melt are the predominant influences.

1501 duration of rise and duration of fall. Duration of rise is the interval from low water to high water, and duration of fallis the interval from high water to low water. Together they cover, on an average, a period of 12.42 hours for asemidiurnal tide or a period of 24.84 hours for a diurnal tide. In a normal semidiurnal tide, the duration of rise andduration of fall will each be approximately equal to 6.21 hours, but in shallow waters and in rivers there is a tendencyfor a decrease in the duration of rise and a corresponding increase in the duration of fall.

1502 duration of tide. The time interval between LOW WATER and the following HIGH WATER or between HIGHWATER and the following LOW WATER.

1503 dust. Any substance comminuted or pulverized, especially EARTH or other solid matter in a minute and fine state ofsubdivision, so that the particles are small and light enough to be easily raised and carried in a CLOUD by the WIND.

1504 duststorm. Ensemble of particles of DUST or SAND energetically lifted to great HEIGHTS by a strong and turbulentWIND. Also called sandstorm. Also written as two words.

1505 Dutchman's log. See LOG.

72

1506 DX90-Format. Format for supply or interchange of digital cartographic data. DX90 provides two modes: thesequential, and the chain mode.

1507 dyke. See DIKE.

1508 dynameter. An instrument for measuring the MAGNIFYING POWER of a TELESCOPE.

1509 dynamic positioning. Manoeuvring of a vessel to keep a position relative to a seabed feature; normally underwateracoustic position fixing systems are used for this purpose.

1510 dynamometer. An apparatus for measuring force or energy.

1511 dyne. The unit of force in the CGS SYSTEM.

73

E

1512 eagre (or eager). See BORE.

1513 earth. The PLANET which we inhabit.The solid matter of the GLOBE in distinction from water and air. The GROUND.

1514 earth current. See CURRENT(S): TELLURIC.

1515 earth inductor. An electric instrument used to measure the INCLINATION of the EARTH's MAGNETIC FIELD.

1516 earth light. Feeble light of the dark part of the MOON's disc produced by the solar light reflected by the EARTHincluding the ATMOSPHERE. Also written as one word. Also called Earthshine.

1517 earthquake. A shaking or trembling of the CRUST of the EARTH caused by underground volcanic forces or bybreaking and shifting of ROCK beneath the surface. Also written as one word.

1518 earth's crust. See CRUST.

1519 earth's magnetic field. See GEOMAGNETIC FIELD.

1520 earth's mantle. The relatively plastic region between the CRUST and core of the EARTH. Also called Asthenosphere.

1521 earthshine. See EARTH LIGHT. Also written as two words.

1522 earth tide. See TIDE.

1523 easting. The distance a craft makes good to the east. The opposite is WESTING.In a cartesian reference system the value of one coordinate, usually expressed in metres, measured from the

CENTRAL MERIDIAN in east-west direction; used e.g. in the UTM system.

1524 ebb. The EBB STREAM. Sometimes the term 'ebb' is also used with reference to vertical tidal movement. The oppositeis FLOOD.

1525 ebb current. See EBB STREAM.

1526 ebb interval. Short for strength of ebb interval. The interval between the transit of the moon over the meridian of aplace and the time of the following strength of ebb.

1527 ebb stream. The horizontal movement of water associated with FALLING TIDE. Ebb streams generally set seaward, orin the opposite direction to the tide progression. Also called EBB, ebb current or outgoing stream.

1528 ebb strength. Phase of the ebb tidal current at the time of maximum velocity. Also, the velocity at this time. Also calledstrength of ebb.

1529 ebb tide. See TIDE: FALLING.

1530 eccentric angle. See LATITUDE: PARAMETRIC.

1531 eccentricity. The amount of deviation from a centre.

1532 eccentricity of alidade. The distance between the centre of figure of the index points of an ALIDADE and the centre offigure of the graduated CIRCLE.

1533 eccentricity of circle. The distance between the centre of figure of a graduated CIRCLE of an instrument and its centreof ROTATION.

1534 eccentricity of ellipse. The ratio of the distance between the centre and a focus of an ellipse to the length of its semi-major axis.

1535 eccentricity of instrument. The combination of ECCENTRICITY OF CIRCLE and ECCENTRICITY OF ALIDADE.The effect of eccentricity of instruments on an observed DIRECTION is eliminated by having the VERNIERS orMICROMETER MICROSCOPES, with which the CIRCLE is read, spaced at equal distances around the CIRCLE.

74

1536 eccentric reduction. In TRIANGULATION, the CORRECTION which must be applied to a DIRECTION observed byan instrument with either the instrument or SIGNAL, or both, eccentric, to reduce the observed value to what it wouldhave been if there had been no ECCENTRICITY.

1537 eccentric signal. A SIGNAL which is not in the same vertical line with the STATION it represents.

1538 eccentric station. A survey point over which an instrument is centred and OBSERVATIONS are made, which is not inthe same vertical line with the STATION it represents, and to which the OBSERVATIONS will be reduced beforebeing combined with OBSERVATIONS at other STATIONS. Also called false station or satellite station. SeeECCENTRIC REDUCTION.

1539 echo. Reflected RADIANT ENERGY.

1540 echo(es): false. In ECHO SOUNDING, ECHOES caused by a foreign matter such as KELP or fish, or by LAYERS ofwater differentiated by sudden changes of temperature or SALINITY (or both). False echoes are occasionally recordedby ECHO SOUNDERS, and might be interpreted erroneously as the correct DEPTH. Also called side echo(es) or falsetarget. See PHANTOM BOTTOM.

1541 echo(es): multiple. In ECHO SOUNDING, any of the ECHOES received subsequent to the first one and due to amultiplicity of REFLECTIONS back and forth between the BOTTOM and surface. These REFLECTIONS oftenregister on the DIAL of a usual indicator, or on the FATHOGRAM of a graphic recorder, at multiplies of the trueDEPTH. Such ECHOES are sometimes mistaken for the first ECHO.

1542 echo(es): side. See ECHO(ES): FALSE.

1543 echogram. A graphic record of depth measurements obtained by an ECHO SOUNDER. See FATHOGRAM.

1544 echometer (France). An ultrasonic ECHO SOUNDER with a visual depth-indicating device. The SIGNALS areproduced and received by quartz-crystal transmitting and receiving units. Its RANGE is from 6 to 660 METRES.

1545 echo ranging. The determination of distance by measuring the time interval between transmission of a radiant energysignal, usually SOUND, and the return of its ECHO.

1546 echoscope (France). An ultrasonic ECHO SOUNDER similar to ECHOMETER. The echoscope is portable, intendedfor shoal-water sounding from small boats. Its RANGE is from 1 to 60 METRES.

1547 echo sounder. An instrument for determining the DEPTH of water by measuring the time interval between the emissionof a sonic or ultrasonic SIGNAL and the return of its ECHO from the BOTTOM. Also called echo sounding instrument(or apparatus), sonic depth finder, or ultrasonic depth finder, as appropriate. See also SUB-BOTTOM PROFILER,SWATH(E) SOUNDING SYSTEM.

1548 echo sounding. Determination of the DEPTH of water by measuring the time interval between emission of a sonic orultrasonic SIGNAL and return of its ECHO from the BOTTOM. Also called acoustic sounding. See also ECHOSOUNDER.

1549 echo sounding instrument (or apparatus). See ECHO SOUNDER.

1550 eclipse. The obscuration of a source of light by the intervention of an object.Whole or partial obscuration of a HEAVENLY BODY by the interposition of another, or by the passing of one

luminous HEAVENLY BODY into the shadow of another.

1551 eclipse: annular. An ECLIPSE in which a thin ring of the source of light appears around the obscuring body. AnnularSOLAR ECLIPSES occur, but never annular LUNAR ECLIPSES.

1552 eclipse: lunar. An ECLIPSE of the MOON. When the MOON enters the shadow of the EARTH, it appears eclipsed toan observer on the EARTH.

1553 eclipse: partial. An ECLIPSE in which only part of the source of light is obscured.

1554 eclipse: solar. An ECLIPSE of the SUN. When the MOON passes between the SUN and the EARTH, the SUN appearseclipsed to an observer in the MOON's shadow.

1555 eclipse: total. An ECLIPSE in which the entire source of light is obscured.

75

1556 ecliptic. The GREAT CIRCLE formed by the intersection of the plane of the EARTH's ORBIT around the SUN (orapparent ORBIT of the SUN around the EARTH) and the CELESTIAL SPHERE.

1557 ecliptic latitude. See LATITUDE: CELESTIAL.

1558 ecliptic system of coordinates. A set of CELESTIAL COORDINATES based on the ECLIPTIC as the PRIMARYGREAT CIRCLE; CELESTIAL LATITUDE and CELESTIAL LONGITUDE.

1559 ecology. The science of the economy of animals and plants dealing with the relations of living organism to theirsurroundings.

1560 eddy. A circular movement of water usually formed, where CURRENTS pass OBSTRUCTIONS, between twoadjacent CURRENTS flowing counter to each other, or along the edge of a PERMANENT CURRENT.

1561 eddy viscosity. The turbulent transfer of momentum by EDDIES giving rise to an internal fluid friction, in a manneranalogous to the action of molecular viscosity in LAMINAR FLOW, but taking place on a much larger scale.

The value of the coefficient of eddy viscosity is of the order 104 square centimetres per SECOND.

1562 edge. In TOPOGRAPHY, the CREST of a sharply pointed RIDGE.

1563 edge wave. See WAVE.

1564 eelgrass. See SEAGRASS.

1565 Ekman bottle. A REVERSING WATER BOTTLE which can be closed watertight by means of plates seated in rubber.When at the desired DEPTH, the reversing mechanism is tripped by a MESSENGER.

1566 Ekman current meter. A widely used instrument for measuring the SPEED and direction of the CURRENT at anyDEPTH. It is designed for use from a ship or boat at anchor when the STREAM does not attain a rate of more than 3 to3 1/2 KNOTS.

1567 Ekman spiral. Spiral, polar DIAGRAM of the extremities of the wind vectors at different HEIGHTS, representing thevariation of the WIND with HEIGHT in the FRICTION LAYER on the basis of various simplifying assumptions.

The Ekman spiral is also applied to the variation of wind-driven OCEAN CURRENTS with DEPTH below thesurface.

1568 elbow. A sudden turn in a CHANNEL, RIVER, or SHORELINE.

1569 electric log. See LOG.

1570 electric submerged log. See LOG.

1571 electromagnetic distance measurement (EDM). Any process or technique of distance measurement which dependson a comparison of SIGNALS by electromagnetic means.

1572 electromagnetic log. See LOG.

1573 electromagnetic position fixing. Any process or technique of fixing which employs the properties of electromagneticwaves.

1574 electromagnetic wave. See WAVE.

1575 electrometeor. Visible or audible manifestation of atmospheric electricity, either corresponding to discontinuouselectrical discharges (LIGHTNING, THUNDER) or occurring as more or less continuous PHENOMENA (SAINTELMO'S FIRE, POLAR AURORA).

1576 electronic chart. An integrated, interactive, navigation information system, with which the user can display thehydrographic and positional information that is required to conduct the safe navigation of his vessel. It compriseshydrographic and cartographic databases containing information useful for navigation.

1577 Electronic chart display and information system (ECDIS). The navigation information system which is consideredthe legal equivalent of the nautical chart, displaying selected information from an electronic navigation chart (ENC)integrated with data from positional and, optionally, other sensors.

76

1578 electronic data processing. In a COMPUTER, data processing performed largely or exclusively on electronicequipment.

1579 electronic distance-measuring equipment (EDM). Devices that measure the phase difference between transmittedand returned (i.e. reflected or retransmitted) ELECTROMAGNETIC WAVES, of known FREQUENCY and SPEED,or the round-trip transit time of a pulsed SIGNAL, from which distance is computed. A wide range of such equipment isavailable for surveying and navigational use. Also called distance measuring equipment (DME).

1580 electronic line of position. A LINE OF POSITION established by means of electronic equipment.

1581 electronic navigation. See NAVIGATION.

1582 electronic navigation station. See STATION.

1583 electronic navigational chart (ENC). 1. An electronic navigational chart is a subset of the electronic chart data baseheld on the vessel. It contains information on features useful for navigation such as coastline, obstructions, beacons etc.

2. The digital data base that is intended to be used in conjunction with ECDIS. Standardized as to content, structureand format the ENC may contain supplementary nautical information useful for safe navigation. The ENC is a subset ofthe master data base of chart information.

1584 electronic plotting sheet. See PLOTTING SHEET.

1585 electronic telemeter. See TELEMETER.

1586 electro-optical distance measurement (EODM). Any method of electromagnetic distance measurement based on acomparison of light signals.

1587 electrostriction. Term applied to the creation of elastic tension and changes of shape and volume of an unchargedisolator by an electrical field.

1588 electrotape. A trade name for a precise electronic distance measuring device operating on the same principle as theTellurometer. Originally designated as micro-dist.

1589 elevated pole. See POLE.

1590 elevation. The vertical distance of a point or a LEVEL, on or affixed to the surface of the EARTH, measured fromMEAN SEA LEVEL. The term elevation is sometimes used synonymously with ALTITUDE which in modern userefers particularly to the distance of points or objects above the EARTH's surface.

An area higher than its surroundings, as a HILL.

1591 elevation: angle of. See ANGLE OF ELEVATION.

1592 elevation: barometric. An ELEVATION determined with a BAROMETER.

1593 elevation of sight. Also called height of eye. The visibility distances are relevant with the elevation of the observer'ssight. When measuring a long distance by optics a line of sight should be maintained over the convex curvature of theearth and all obstacles between the measuring points.

1594 elevation tints. See HYPSOMETRIC TINTS.

1595 ellipse. A plane curve constituting the locus of all points the sum of whose distances from two fixed points, called foci,is constant. Also used to characterize errors in positions and measurements.

1596 ellipsoid of reference. See REFERENCE SPHEROID.

1597 ellipsoid of revolution (or rotation). A simple mathematical surface which best approximates the shape of theEARTH. It is the surface generated by an ellipse revolving about one of its AXES. See also SPHEROID: OBLATE,SPHEROID: PROLATE and REFERENCE SPHEROID.

1598 ellipticity of an ellipse. The ratio between the difference in length of the semi-axes of an ellipse and its semi-majoraxis.

1599 ellipticity of the spheroid. See FLATTENING OF THE EARTH.

77

1600 elongation. The ANGULAR DISTANCE of a body of the SOLAR SYSTEM from the SUN. The angle at the EARTHbetween lines to the SUN and another CELESTIAL BODY of the SOLAR SYSTEM. The term is usually used inconnection with INFERIOR PLANETS. The greatest elongation of such a body is its maximum ANGULARDISTANCE from the SUN, as observed from the EARTH.

The position of a STAR when its AZIMUTH east or west of the MERIDIAN is greatest.

1601 embankment. An artificial ELEVATION composed of EARTH, STONE, etc. to hold back water.

1602 embayment. An INDENTATION in a SHORELINE forming an open BAY.

1603 emergent nodal point. See NODAL POINT.

1604 emulsion. In PHOTOGRAPHY, a suspension of a salt of silver in gelatine or collodion, used to coat PLATES, films, orpapers.

1605 enamelled plate. In CARTOGRAPHY, enamelled working surface (normally white), on aluminium, zinc or otherdimensionally stable medium. Used to prepare colour separation originals.

1606 enclosed sea. A gulf, basin, or sea surrounded by two or more States and connected to another sea or the ocean.

1607 end lap. See OVERLAP.

1608 energy: radiant. Energy transmitted by RADIATION.

1609 engraving. The act, process or art of cutting or etching designs or letters on metal plates, wooden blocks, etc. forprinting.

Any printed impression made from an engraved surface.

1610 entrance. The SEAWARD end of a CHANNEL, HARBOUR, DOCK, etc.

1611 entrance pupil. The IMAGE of the APERTURE STOP formed by all the LENS ELEMENTS on the object side of theAPERTURE STOP.

1612 entrance window. The IMAGE of the FIELD STOP formed by all the LENS ELEMENTS on the object side of theFIELD STOP.

1613 envelope. In electronics, a graph defining the variations in AMPLITUDE of successive OSCILLATIONS in anamplitude-modulated wave.

1614 envelope edge. The fore part of an ENVELOPE.

1615 environment. The sum total of all external conditions which may affect an organism, community, material, or energy ifbrought under the influence of these external conditions.

1616 Eotvos balance. See TORSION BALANCE.

1617 Eotvos correction. A correction applied to gravity measurements to balance the effect of the vertical force experiencedby a body moving in an east-west direction on the rotating earth.

1618 Eotvos effect. The east-west component of the movement of a ship, including the effect of marine currents, modifies thecentrifugal force of the earth's rotation. It is a vertical force experienced by a body moving in an east-west direction onthe rotating earth.

1619 epact (of the year). The AGE OF THE MOON at 00 HOUR on January 1.

1620 ephemeris. A statement presenting POSITIONS and related DATA for a CELESTIAL BODY for given EPOCHS(dates) at uniform intervals of time. Also a publication containing such DATA for a number of CELESTIAL BODIES.See also ALMANAC: AIR and ALMANAC: NAUTICAL.

1621 epicentre (or epicenter, epicentrum). The point of the EARTH's surface directly above the FOCUS of anEARTHQUAKE.

1622 epipolar plane. Any plane which contains the EPIPOLES; therefore any plane containing the AIR BASE. Also calledbasal plane.

78

1623 epipolar ray. The line on the plane of a PHOTOGRAPH joining the EPIPOLE and the IMAGE of an object. Alsoexpressed as the trace of an EPIPOLAR PLANE on a PHOTOGRAPH.

1624 epipoles. In the perspective set-up of two PHOTOGRAPHS, (two perspective projections), the points on the planes ofthe PHOTOGRAPHS where they are cut by the AIR BASE.

1625 epoch. A given period of time during which a series of related acts or events takes place.In ASTRONOMY, a particular instant for which certain DATA are given or at which an OBSERVATION is made,

and which then is usable as a REFERENCE POINT.

1626 equal altitudes. Two ALTITUDES of the same CELESTIAL BODY observed east and then west of the MERIDIAN,when it has reached the same value. The expression also applies to the practice, essentially obsolete, of determining theinstant of local APPARENT NOON by observing equal ALTITUDES of the SUN.

1627 equal area projection.See PROJECTION.

1628 equation: condition. An equation which expresses exactly certain relationships that must exist among relatedquantities, which are not independent of one another, exist a priori, and are separate from relationships demanded byOBSERVATION.

1629 equation: correlate. An equation derived from an OBSERVATION or CONDITION EQUATION, usingundetermined multipliers, and expressing the condition that the sum of the squares of the RESIDUAL ERRORSresulting from the application of these multipliers to the OBSERVATION or CONDITION EQUATIONS shall be aminimum.

1630 equation: error. The probability equation which expresses the laws of the occurrence of ACCIDENTAL ERRORS.The error equation is the basis of the method of LEAST SQUARES, used in the ADJUSTMENT of OBSERVATIONSfor determining the most probable value of a result from those OBSERVATIONS.

1631 equation: Laplace. An equation which expresses the relationships between ASTRONOMIC AZIMUTH, GEODETICAZIMUTH, and the ASTRONOMIC LONGITUDE and GEODETIC LONGITUDE.

1632 equation: normal. One of a set of simultaneous equations derived from OBSERVATION, CONDITION orCORRELATE EQUATIONS. In a least-squares adjustment, values obtained from the solution of normal equations(either directly or through the CORRELATE EQUATIONS) are applied to the OBSERVATION or CONDITIONEQUATIONS to obtain the desired CORRECTIONS.

1633 equation: observation. A CONDITION EQUATION which connects interrelated unknowns by means of an observedfunction.

1634 equation: personal. See PERSONAL EQUATION.

1635 equation method. In tidal terminology, an elaborate non-harmonic method for the prediction of TIDES.

1636 equation of time. The CORRECTION to be applied to 12 hours plus UNIVERSAL TIME to obtain the GreenwichHOUR ANGLE of the SUN, or more generally the CORRECTION to be applied to 12 hours plus LOCAL MEANTIME to obtain the LOCAL HOUR ANGLE of the SUN.

1637 equator. The PRIMARY GREAT CIRCLE of a sphere or SPHEROID such as the EARTH, perpendicular to the polarAXIS.

1638 equator: astronomical. A line connecting points having 0° ASTRONOMICAL LATITUDE.

1639 equator: celestial. The GREAT CIRCLE on the CELESTIAL SPHERE whose plane is perpendicular to the AXIS ofthe EARTH. Also called equinoctial.

1640 equator: geodetic. The line on the SPHEROID connecting points of 0° GEODETIC LATITUDE.

1641 equator: heat. See EQUATOR: THERMAL.

1642 equator: magnetic. That line on the surface of the EARTH connecting all points at which the magnetic DIP is zero.Also called aclinic line.

1643 equator: terrestrial. A GREAT CIRCLE of the EARTH in the plane of the CELESTIAL EQUATOR, and equidistantfrom the two POLES.

79

1644 equator: thermal. Line which circumscribes the EARTH and connects all points of highest mean annual temperatureon each MERIDIAN. Also called heat equator.

1645 equatorial calms. Zone of calm or light variable WINDS encountered, in certain regions, between the TRADE WINDSof the two hemispheres.

1646 equatorial horizontal parallax. See PARALLAX.

1647 equatorial tide(s). See TIDE.

1648 equidistance line. See MEDIAN LINE.

1649 equilibrium. A state of balance between forces.

1650 equilibrium argument. The theoretical PHASE of a CONSTITUENT of the EQUILIBRIUM TIDE.

1651 equilibrium tide. See TIDE.

1652 equinoctial. See EQUATOR: CELESTIAL.

1653 equinoctial colure. See COLURE.

1654 equinoctial point. See EQUINOX.

1655 equinoctial spring tide(s). See TIDE.

1656 equinoctial system of coordinates. See CELESTIAL EQUATOR SYSTEM OF COORDINATES.

1657 equinoctial year. See YEAR: TROPICAL.

1658 equinox. One of the two points of intersection of the ECLIPTIC and the CELESTIAL EQUATOR, occupied by theSUN when its DECLINATION is zero. That point occupied on or about March 21, when the SUN's DECLINATIONchanges from south to north, is called vernal equinox or first point of Aries; that point occupied on or about September23, when the DECLINATION changes from north to south, is called autumnal equinox or first point of Libra. Alsocalled equinoctial point.

1659 equinoxes: precession of. See PRECESSION OF THE EQUINOXES.

1660 equiphase zone. See ZONE.

1661 equipotential surface. A surface having the same potential of GRAVITY at every point.

1662 equisignal zone. See ZONE.

1663 equivalent focal length. See FOCAL LENGTH.

1664 equivalent projection. See PROJECTION: EQUAL AREA.

1665 equivalent scale. See SCALE.

1666 era. In GEOLOGY, any of the main divisions of GEOLOGICAL TIME.

1667 erect image. See IMAGE.

1668 erecting eyepiece. See EYEPIECE.

1669 erg. The unit of energy or work in the CGS SYSTEM.

1670 erosion. The general process of the wearing away of ROCKS and SOIL at the EARTH's surface by natural agencies.

1671 error. The difference between an observed or computed value of a quantity and the ideal or true value of that quantity.

1672 error: absolute. Absolute deviation (the value taken without regard to sign) from the corresponding true value.

80

1673 error: accidental. An ERROR whose occurrence depends on the law of chance only. Also called random error orirregular error.

1674 error: clamping. A SYSTEMATIC ERROR in OBSERVATIONS made with a REPEATING THEODOLITE causedby strains set up by the clamping devices of the instrument.

1675 error: closing. See ERROR OF CLOSURE.

1676 error: constant. A SYSTEMATIC ERROR which is the same in both magnitude and sign through a given series ofOBSERVATIONS.

1677 error: external. See ERROR: THEORETICAL.

1678 error: Gaussin. DEVIATION of a MAGNETIC COMPASS due to TRANSIENT MAGNETISM which remains in avessel's structure for short periods after the inducing force has been removed. This ERROR usually appears after avessel has been on the same HEADING for a considerable time.

1679 error: gross. The result of carelessness or a mistake; may be detected through repetition of the measurement. Alsocalled blunder.

1680 error: heeling. The change in the DEVIATION of a MAGNETIC COMPASS when a craft heels, due to the change inthe position of the magnetic influences of the craft relative to the EARTH's MAGNETIC FIELD and to the COMPASS.

1681 error: index. The INSTRUMENTAL ERROR which is constant and attributable to displacement of a VERNIER orsome analogous effect.

1682 error: instrumental (or instrument). A SYSTEMATIC ERROR resulting from imperfections in, or faultyADJUSTMENT of, instruments or devices used.

1683 error: irregular. See ERROR: ACCIDENTAL.

1684 error: mean square. The quantity whose square is equal to the sum of the squares of the individual ERRORS dividedby the number of those ERRORS.

1685 error: parallactic. An ERROR caused by personal or instrumental PARALLAX.

1686 error: periodic. An ERROR whose AMPLITUDE and direction vary systematically with time.

1687 error: personal. The result of the inability of an observer to perceive dimensional values exactly. Personal errors maybe either random or systematic in behaviour. See also PERSONAL EQUATION.

1688 error: polarization. ERROR in a RADIO BEARING or the COURSE indicated by a RADIOBEACON, due tohorizontally-polarized components of the electric field under certain transmission conditions. The terms night error ornight effect have become obsolete.

1689 error: probable. An ERROR (or deviation from the mean) of such magnitude that the likelihood of its being exceededin a set of OBSERVATIONS is equal to the likelihood of its not being exceeded; its value is that of the STANDARDERROR multiplied by 0.6745. The use of STANDARD ERROR is sometimes preferred in statistical studies.

1690 error: random. See ERROR: ACCIDENTAL.

1691 error: regular. See ERROR: SYSTEMATIC.

1692 error: residual. The difference between any value of a quantity in a series of OBSERVATIONS, corrected for knownSYSTEMATIC ERRORS, and the value of the quantity obtained from the ADJUSTMENT of that series. Sometimestermed as residual.

1693 error: root mean square. See ERROR: STANDARD.

1694 error: sextant. The ERROR in the reading of a SEXTANT, due either to lack of proper ADJUSTMENT or toimperfection of manufacture.

81

1695 error: standard. The square root of the arithmetic mean of squared deviations from the mean. Also called standarddeviation, when the deviations do not represent ERRORS, or root mean square error.

1696 error: station. See DEFLECTION OF THE VERTICAL.

1697 error: swirl. The additional ERROR in the reading of a MAGNETIC COMPASS during a turn, due to friction in thecompass liquid.

1698 error: systematic. An ERROR whose magnitude changes in proportion to known changes in observational conditions.Also called regular error.

1699 error: theoretical. A SYSTEMATIC ERROR arising from natural physical conditions, beyond the control of theobserver. Also called external error.

1700 error equation. See EQUATION.

1701 error of closure. The amount by which a quantity obtained by a series of related measurements differs from the true orfixed value of the same quantity. Also called closing error.

1702 error of closure of horizon. The amount by which the sum of a series of adjacent measured horizontal angels around apoint fails to equal exactly 360°.

1703 error of closure of triangle. The amount by which the sum of the three observed angles of a triangle fails to equalexactly 180° plus the SPHERICAL EXCESS of the triangle. Also referred to as closure of triangle.

1704 error of collimation. The angle between the LINE OF COLLIMATION (LINE OF SIGHT) of a TELESCOPE and itsCOLLIMATION AXIS; i.e., the angle by which the LINE OF SIGHT of an optical instrument differs from what itshould be. Also called collimation error.

1705 error of graduation. Inaccuracy in the GRADUATIONS of the SCALE of an instrument. Also called graduationerror.

1706 error of magnetic compass. The angle by which a compass direction differs from the true direction; the algebraic sumof VARIATION and DEVIATION.

1707 error of run (micrometer). The difference in SECONDS of arc, between the intended value of one turn of theMICROMETER SCREW and its actual value as determined by measuring the space between two adjacent graduationmarks of the CIRCLE with the MICROMETER. Also called run of micrometer.

1708 escarpment. A general term for a continuous scarp face.An elongated and comparatively steep SLOPE of the SEA FLOOR, separating flat or gently sloping areas. Also

called sea scarp.

1709 establishment. The interval of time between the transit (upper or lower) of the moon and the next high water at a place.The average establishment at the time of full or new moon is called vulgar or common establishment, or high water fulland change. Also called high water lunitidal interval, or establishment of the port. See LUNITIDAL INTERVAL.

1710 establishment of the port. See ESTABLISHMENT.

1711 estimated position (EP). See POSITION.

1712 estuary. That portion of a STREAM influenced by the TIDE of the body of water into which it flows.A BAY, as the MOUTH of a RIVER, where the TIDE meets the river current.

1713 estimated time of arrival (ETA). The predicted time of reaching a destination or waypoint.

1714 estimated time of departure (ETD). The predicted time of leaving a place.

1715 etch (v.t.). To form an IMAGE within the surface of a printing or drawing material by the use of solvents.

1716 Eulerian current observations. A method to monitor the current at a fixed point by means of current metres.

1717 European Datum. Constituted by the International Spheroid located at the Potsdam origin.

82

1718 evaluation report. A report which summarizes pertinent facts relating to a given hydrographic survey. Included in thereport are sections which address the quality of the hydrographic survey and detailed comparisons made with priorhydrographic surveys and the appropriate nautical chart. Specific evaluations and recommendations regarding theadequacy of the survey to supersede prior survey data and charted information are made.

1719 evaporimeter. Instrument for measuring the amount of water evaporated into the ATMOSPHERE during a given timeinterval. Also called atmometer, atmidometer.

1720 evection. A perturbation of the MOON in its ORBIT due to the attraction of the SUN. See also LUNARINEQUALITY.

1721 Everest spheroid. See REFERENCE SPHEROID.

1722 exercise area. An area shown on CHARTS within which naval, military or aerial exercises are carried out. Also calledmilitary practice area.

1723 exclusive economic zone. The exclusive economic zone is an area, not exceeding 200 nautical miles from the baselinesfrom which the breadth of the territorial sea is measured, subject to a specific legal regime established in the UnitedNations Convention on the Law of the Sea under which the coastal state has certain rights and jurisdiction.

1724 existence doubtful. Of uncertain existence. The expression is used principally on CHARTS to indicate the possibleexistence of a ROCK, SHOAL, etc., the actual existence of which has not been established. Usually shown by theabbreviation 'ED'.

1725 exit pupil. The IMAGE of APERTURE STOP formed by all the LENS ELEMENTS on the IMAGE side of theAPERTURE STOP.

1726 exit window. The IMAGE of the FIELD STOP formed by all the LENS ELEMENTS on the image side of the FIELDSTOP.

1727 ex-meridian altitude. See ALTITUDE.

1728 expansion factor. In RADIOLOCATION, factor representing the degree to which the hyperbolae belonging to a givengroup of HYPERBOLIC LINES OF POSITION move apart as the distance from the BASE LINE increases.

1729 expert system. A knowledge-based computer system which utilizes artificial intelligence to do some of the inferentialcomputation/decision making.

1730 exploration drilling. Drilling operations aimed at the exploration of resources.

1731 exploratory survey. See SURVEY.

1732 explosive anchorage area. An area set apart for anchored ships discharging or receiving explosives.

1733 explosive fog signal. See FOG SIGNAL.

1734 exposure. In PHOTOGRAPHY, the act of exposing a light-sensitive material to a light source.The total quantity of light received per unit area on a sensitized PLATE or film; may be expressed as the product of

the light intensity and the EXPOSURE TIME.

1735 exposure interval. The time interval between the taking of successive PHOTOGRAPHS.

1736 exposure station. See AIR STATION.

1737 exposure time. The time during which a light-sensitive material is subjected to the action of light.

1738 exterior orientation. See ORIENTATION.

1739 exterior perspective centre. See PERSPECTIVE CENTRE.

1740 external error. See ERROR: THEORETICAL.

1741 extinction coefficient. A measure of the space rate of diminution or extinction, of any transmitted light.In OCEANOGRAPHY, the measure of ATTENUATION of downward directed light radiation in the SEA.

83

1742 extrapolation. The process of estimating the value of a quantity beyond the limits of known values by assuming that therate or system of change between the last few known values continues.

1743 extremely high frequency (EHF). See FREQUENCY: RADIO.

1744 extrusion. In CARTOGRAPHY, the extension of detail outside the NEAT LINE.

1745 eye base. See INTERPUPILLARY DISTANCE.

1746 eye of the storm. See TROPICAL CYCLONE.

1747 eyepiece. In an optical device, the lens group which is nearest the eye and with which the IMAGE formed by thepreceding elements is viewed.

1748 eyepiece: erecting. An EYEPIECE containing an auxiliary re-imagining system of one or two LENSES which invertsthe primary IMAGE before it reaches the EYEPIECE proper. The device is now obsolescent, the inversion beingperformed by prism systems. Also called inverting eyepiece.

1749 eyepiece micrometer. See MICROMETER: OCULAR.

84

F

1750 face(s) of theodolite. Either of the two positions, 'face left' (F.L.) and 'face right' (F.R.) of the THEODOLITE, whenOBSERVATIONS are made with the VERTICAL CIRCLE on the left or on the right of the TELESCOPE respectively.

1751 facility (navigational). See AID TO NAVIGATION.

1752 facility (port). See HARBOUR WORKS.

1753 facsimile. An exact reproduction or copy.

1754 fading. Loss of strength in received SIGNALS due to temporary variations in the conditions of propagation.

1755 Fahrenheit temperature scale. A temperature scale defined by the relationship t°F = 1.8 t°C + 32; developed byGabriel D. Fahrenheit (1686-1736), German physicist.

1756 fair chart (British terminology). The final, carefully made plot of a HYDROGRAPHIC SURVEY. In contrast to theFIELD BOARD (BOAT SHEET in U.S. terminology) which is a work sheet plotted during field operations frompreliminary field data, the fair chart is plotted from corrected DATA and represents the official permanent record of thatparticular SURVEY. Also called fair sheet. It is called smooth sheet in U.S. terminology.

1757 fair sheet. See FAIR CHART.

1758 fair tracing (British terminology). A paper tracing of moderate size for transferring the work from the FIELDBOARDS to the FAIR CHARTS.

1759 fairway. The main travelled part of a WATERWAY.

1760 fairway buoy. A BUOY marking a FAIRWAY, with safe water on both sides.

1761 falling tide. See TIDE.

1762 false bottom. See PHANTOM BOTTOM.

1763 false coordinates. See COORDINATES.

1764 false echo. See ECHO.

1765 false horizon. See HORIZON.

1766 false origin. See COORDINATES: FALSE.

1767 false station. See ECCENTRIC STATION.

1768 false target. See ECHO: FALSE.

1769 fan. A relatively smooth, fan-like depositional feature normally sloping away from the outer termination of aCANYON.

1770 farad. The unit of electrical capacitance, in the SI SYSTEM.

1771 farewell buoy. The outermost BUOY marking the ENTRANCE to a CHANNEL or HARBOUR. Also called sea buoy.Known as landfall buoy in British terminology.

1772 fast ice. SEA ICE which remains fast, generally in the position where originally formed, and which may attain aconsiderable thickness. It is found along COASTS, where it is attached to the SHORE, or over SHOALS, where it maybe held in position by ISLANDS, grounded ICEBERGS or grounded polar ice.

1773 fata morgana. The name originally given to a multiple mirage phenomenon often observed over the Straits of Messina,and supposed to be the work of the faity (in Italian: fata) Morgana; later, it was applied to any impressive multipleMIRAGE.

85

1774 fathogram. A graphic record of depth measurements obtained by a fathometer (erroneously applied to anyECHOGRAM).

1775 fathom. A unit of length equal to 6 FEET or 1.83 METRES.

1776 fathom curve. See FATHOM LINE, DEPTH CURVE.

1777 fathom line. A DEPTH CURVE with DEPTH expressed in FATHOMS. Also called fathom curve, isobath.

1778 fault. In GEOLOGY, a break of shear in the EARTH's CRUST with an observable displacement between the two sidesof the break, and parallel to the plane of the break.

1779 Faye anomaly. See FREE-AIR ANOMALY.

1780 feature code. An alphanumeric code (label) attached to all point, line or area data describing a cartographic feature ofparticular quality.

1781 Federation of Astronomical and Geophysical Services (FAGS). Facilitates and promotes the collection ofobservations, information and statistics in the field of astronomical and geophysical sciences. The International TimeBureau is one of the services.

1782 fender. A device let down between the side of a ship or a WHARF or other ship to protect from chafing when ships arelying alongside or to take the shock of a bump when going alongside.

1783 fetch. An area of the sea surface over which SEAS are generated by a WIND having a constant direction and SPEED.The length of the generating area, measured in the direction of the WIND in which the SEAS are generated.

1784 fiducial axes. In PHOTOGRAMMETRY, the lines joining opposite FIDUCIAL MARKS on a PHOTOGRAPH. Alsocalled photograph axes.

1785 fiducial marks. In PHOTOGRAMMETRY, INDEX MARKS, usually four, which are rigidly connected with thecamera lens through the camera body and which form IMAGES on the NEGATIVE and usually define the PRINCIPALPOINT of the PHOTOGRAPH.

Also marks, usually four in number, in any instrument, which define the axes whose intersection fixes thePRINCIPAL POINT of a PHOTOGRAPH and fulfills the requirements of INTERIOR ORIENTATION.

In SURVEYING, an index line or point. A line or point used as a basis of reference.

1786 field board (British terminology). Wooden board, or zinc or plastic sheet, used by HYDROGRAPHERS and landsurveyors for plotting and recording the details of the SURVEY in the field. Separate boards are usually prepared forTOPOGRAPHY and SOUNDING respectively. The former are referred to as topography boards; the latter as soundingboards. See also BOAT SHEET, and PLANE TABLE.

1787 field chart. See CHART.

1788 field examination. A special purpose hydrographic, wire drag, or side scan sonar survey of very limited area; i.e.,usually an investigation of one or more individual and scattered items.

1789 field inspection. In PHOTOGRAMMETRY, the process of comparing AERIAL PHOTOGRAPHS with conditions asthey exist on the GROUND, and of obtaining information to supplement or clarify that not readily discernible on thePHOTOGRAPHS themselves.

1790 field position. See POSITION.

1791 field sheet. The hydrographer's or topographer's work sheet; it presents a graphic display of all surface and subsurfacefeatures in the area being surveyed. See also BOAT SHEET.

1792 field stop. The physical element (such as an APERTURE STOP, DIAPHRAGM or lens periphery) of an OPTICALSYSTEM which limits the field of view covered by the system.

1793 field work. All activities in the field required for a hydrographic survey.

1794 figure of the earth. The defining elements of the mathematical surface which approximates the surface of the GEOID.The figure of the earth has been proved to be approximately an OBLATE SPHEROID. See also GEOID andSPHEROID.

86

1795 file. An entity of data for a specific task or purpose stored on a mass storage device.

1796 filter. In optics, any transparent material which, by absorption, selectively modifies the light transmitted through anOPTICAL SYSTEM.

In ocean-wave forecasting, a set of formulas that define the particular wave frequencies and directions in the fetcharea which are of significance at the point of forecast.

1797 filtering. The process of selecting specific data from a specific source in accordance with certain rules, formulae, oralgorithms.

1798 final great circle course. See COURSE: GREAT CIRCLE.

1799 fiord (or fjord). A long narrow ARM OF THE SEA, running up between high BANKS or CLIFFS, as on the COASTof Norway. Often has a relatively shallow SILL across its ENTRANCE.

1800 firn. Old SNOW which has become granular and dense under the action of various processes of melting and refreezing,also including sublimation.

1801 first nodal point. See NODAL POINT.

1802 first order triangulation. See TRIANGULATION CLASSIFICATION.

1803 first point of Aries. See EQUINOX.

1804 first point of Cancer. See SOLSTICE.

1805 first point of Capricorn. See SOLSTICE.

1806 first point of Libra. See EQUINOX.

1807 first quarter. See PHASES OF THE MOON.

1808 firth. A Scottish word: an ARM OF THE SEA; an ESTUARY of a RIVER.

1809 fish aggregating device (FAD). See FISH HAVEN.

1810 fishery conservation zone. See FISHING ZONE.

1811 fish farm. An assemblage of cages, nets, rafts and floats or posts where fish, including shellfish, are artificiallycultivated.

1812 fish haven. Areas established by private interests, usually sport fishermen, to simulate natural reefs and wrecks thatattract fish. The reefs are constructed by dumping assorted junk in areas which may be of very small extent or maystretch a considerable distance along a depth contour. Fish havens are outlined and labeled on charts. Also called fisheryreefs.

1813 fishing area. See FISHING GROUND.

1814 fishing ground. A water area in which fishing is frequently carried on. Also called fishing area or fishing zone.

1815 fishing reef. See FISH HAVEN.

1816 fishing zone. The offshore zone in which exclusive fishing rights and management are held by the coastal nation.

1817 fish lead. See LEAD.

1818 fish (or fishing) stakes. Poles or stakes placed in SHALLOW WATER to outline FISHING GROUNDS or to catchfish.

1819 fish trap (or weir). A structure for catching fish. See also TUNNY NET.

1820 fish trap buoy. A BUOY marking the limits of a fish trap area.

1821 fish weir. See FISH TRAP.

87

1822 fix. In NAVIGATION, a relatively accurate POSITION determined without reference to any former POSITION. It maybe classed as visual, celestial, electronic, etc., depending upon the means of establishing it.

1823 fix (v.t.). In hydrographic SURVEYING, to determine, at regular intervals, the POSITION of ships or boats, whilesailing along a LINE OF SOUNDING. The usual method of fixing HYDROGRAPHIC SURVEYS within sight ofLAND is the THREE-POINT FIX METHOD.

1824 fix: circular. A useless FIX resulting from the OBSERVATIONS of two angles establishing as LOCI three coincidingCIRCLES OF POSITION. Also called revolver, or swinger.

1825 fix: running. A POSITION determined by crossing LINES OF POSITION obtained at different TIMES and advancedor retired to a common TIME, the expression being applied to a POSITION determined by advancing and retiring a lineover a considerable period of time.

1826 fix: split. A FIX obtained by measuring two angles to four objects whose POSITIONS are known, i.e. with no commoncentre object.

1827 fix: three-point. A FIX obtained by simultaneously observing, with SEXTANTS, the two adjacent angles betweenthree suitably disposed objects whose POSITIONS are known, and plotting the POSITION by a STATION POINTER.See also RESECTION and THREE-POINT PROBLEM.

1828 fixed angle plot (British terminology). Plot prepared in advance for use in normal SOUNDING in lieu of a STATIONPOINTER. The arcs of equal angle subtended by selected MARKS are drawn on the SOUNDING BOARD atconvenient intervals depending on the SCALE. The observed sextant angles can then be plotted in the field by visualinspection. See also CIRCLE SHEET, and SOUNDING: FIXED ANGLE.

1829 fixed angle sounding. See SOUNDING.

1830 fixed platform. See FIXED STATION.

1831 fixed star. A STAR. The expression is used particularly to distinguish STARS from other HEAVENLY BODIES; socalled by ancients to distinguish STARS from the wandering PLANETS.

1832 fixed station. A radiolocation station which is situated in one particular spot throughout the period of its operation. Asopposed to MOBILE STATION. Also called shore station.

In OCEANOGRAPHY, a platform from which repeated OBSERVATIONS are taken. These include coastal andisland stations, LIGHT VESSELS, WEATHER SHIPS, AUTOMATIC FLOATING STATIONS and ICE ISLANDS.Also called fixed platform.

1833 fixing. The process of rendering a developed photographic image permanent by removing the unaffected light-sensitivematerial.

1834 fixing interval. The time or distance elapsed between two subsequent fixes.

1835 fjord. See FIORD.

1836 flare stack. A tall structure used for burning-off waste oil or gas.

1837 flare triangulation. See TRIANGULATION.

1838 flat. A level tract of LAND, as the BED of a dry LAKE or an area frequently uncovered at LOW TIDE. Usually inplural.

1839 flat-bed plotter. A plotter which uses a flat surface to mount the drawing base.

1840 flattening of the earth. The ratio of the difference between the equatorial and polar radii of the EARTH (major andminor semi-axes of the SPHEROID) and its equatorial radius (major semi-axis). The flattening of the earth is the ellip-ticity of the spheroid. Also called compression.

1841 flight altitude. See ALTITUDE.

1842 flight strip. A succession of overlapping AERIAL PHOTOGRAPHS taken along a single COURSE.

1843 Flinders bar. A bar of soft unmagnetized iron placed vertically near a MAGNETIC COMPASS to counteractDEVIATION caused by MAGNETIC INDUCTION in vertical SOFT IRON of the craft.

88

1844 float. Any buoyant object attached to nets or fishing lines: term also used for the small buoys used for currentobservations.

1845 float gauge. See GAUGE.

1846 floating breakwater. A contrivance consisting of a series of square frames of timber connected by mooring chains orCABLES attached to anchors or stone blocks in such a manner as to form a BASIN within which vessels riding toanchor may be protected from the violence of the WAVES.

1847 floating dock. See DOCK.

1848 floating ice. Any form of ICE found floating in water.

1849 floating mark. In PHOTOGRAMMETRY, a mark seen as occupying a position in the three-dimensional space formedby the STEREOSCOPIC FUSION of a pair of PHOTOGRAPHS and used as a reference mark in examining or measu-ring the STEREOSCOPIC MODEL.

1850 floating point. In computer systems the type of numeric data approximately representing real numbers. A floating pointnumber is composed of an exponent and a mantissa specifying the numerical value relative to the exponent. Owing tothe limited length of a computer WORD, the floating point representation may cause a loss of precision.

1851 floating triangulation. See TRIANGULATION.

1852 float-off (British terminology). The paper of a FIELD BOARD when detached from the board on completion of thework for which the FIELD BOARD was prepared.

1853 float well. A vertical pipe or box with a relatively small opening in the bottom, used to enclose the float of a tide gaugeso the float will be little affected by nontidal motions of the water. Synonymous with stilling well.

1854 floe (or ice floe). A piece of SEA ICE other than FAST ICE. Floes over 10 km across are described as vast; thosebetween 1 and 10 km across as big; those between 200 and 1,000 m across as medium; those between 10 and 200 macross as small. Floes less than 10 m across are called ICE CAKES.

1855 flood. The FLOOD STREAM. Sometimes the term 'flood' is also used with reference to vertical tidal movement. Theopposite is EBB.

An overflowing of water on LAND usually dry; inundation.

1856 flood current. See FLOOD STREAM.

1857 flood stream. The horizontal movement of water associated with the RISING TIDE. Flood streams generally set towardthe SHORE, or in the direction of the tide progression. Also called FLOOD, flood current or ingoing stream.

1858 flood tide. See TIDE: RISING.

1859 flotsam. See JETTISON.

1860 flow. The motion characteristic of a fluid.

1861 flume. An inclined CHANNEL for conveying water from a distance to be utilized for power, transportation, irrigation,etc.

1862 fluorescence. Emission of light or other RADIANT ENERGY as a result of and only during absorption ofRADIATION from some other source. See LUMINESCENCE and PHOSPHORESCENCE.

1863 flux: luminous. Quantity, characteristic of RADIANT FLUX which expresses its capacity to produce a luminoussensation.

1864 flux: radiant. Power emitted, transferred or received in the form of RADIATION.

1865 flux-gate magnetometer. See MAGNETOMETER.

1866 f-number. See APERTURE: RELATIVE.

1867 foam line. The front of a WAVE as it advances shoreward, after it has broken.

89

1868 focal distance. See FOCAL LENGTH.

1869 focal length. The distance from the rear NODAL POINT of a LENS to the FOCAL PLANE for an object at infinity.The distance from the surface of a MIRROR to its FOCUS. Also called focal distance.

1870 focal length: calibrated. An adjusted value of the EQUIVALENT FOCAL LENGTH computed to distribute the effectof lens DISTORTION over the entire field used in an AERIAL CAMERA.

1871 focal length: equivalent. The distance measured along the AXIS OF LENS from the rear NODAL POINT to the planeof best average DEFINITION over the entire field used in an AERIAL CAMERA.

1872 focal plane. A plane parallel to the plane of a LENS or MIRROR and passing through the FOCUS. In PHOTOGRA-PHY, the plane (perpendicular to the axis of the LENS) in which IMAGES of points in the object field of the LENS arefocused.

1873 focal-plane plate. A glass plate set in the CAMERA so that the surface away from the LENS coincides with theFOCAL PLANE. Its purpose is to position the EMULSION of the film in the FOCAL PLANE when the film is physi-cally pressed into contact with the glass plate. Also called contact glass or contact plate.

1874 focal-plane shutter. See SHUTTER.

1875 focal point. See FOCUS.

1876 focus (pl. foci). That point at which parallel RAYS of light meet after being refracted by a LENS or reflected by aMIRROR. Also called focal point.

In SEISMOLOGY, the source of a given set of elastic WAVES. The true centre of an EARTHQUAKE, withinwhich the strain energy is first converted to elastic wave energy.

1877 focus (v.t.). The process of adjusting an optical instrument, PROJECTOR, CATHODE-RAY TUBE, etc., to produce aclear and well-defined IMAGE.

1878 fog. Suspension of very small water droplets in the air, generally reducing the horizontal VISIBILITY at the EARTH'ssurface to less than 1 kilometre.

1879 fog: advection. FOG which forms in the lower part of a moist AIR MASS which moves over a colder surface.

1880 fog: ice. Suspension of numerous minute ice crystals in the air, reducing the VISIBILITY at the EARTH's surface.

1881 fog: sea. ADVECTION FOG which forms over the SEA.

1882 fog: wet. FOG formed of droplets sufficiently large to deposit water on objects.

1883 fog bank. FOG, generally caused by local conditions, which extends over a small area some hundreds of METRESwide.

1884 fog bell. A bell used as a FOG SIGNAL.

1885 fog detector light. A light used to automatically determine conditions of visibility which warrant the turning on or offof a sound signal.

1886 fog gong. A gong used as a FOG SIGNAL.

1887 fog gun. A gun used as a FOG SIGNAL.

1888 fog horn. A HORN used as a FOG SIGNAL.

1889 fog patches. FOG in banks irregularly distributed.

1890 fog signal. A warning SIGNAL transmitted by a vessel or AID TO NAVIGATION during period of low VISIBILITY.Also the device producing such a SIGNAL.

1891 fog signal: explosive. Detonation of an explosive used as a navigational warning during low VISIBILITY.

1892 fog signal: radio. A regularly recurring distinctive SIGNAL sent out by a RADIOBEACON during thick WEATHER.

90

1893 fog siren. A siren used as a FOG SIGNAL.

1894 fog trumpet. A trumpet used as a FOG SIGNAL.

1895 fog whistle. A whistle used as a FOG SIGNAL.

1896 following sea. A SEA in which the WAVES move in the general direction of the HEADING.

1897 following wind. WIND blowing in the general direction of a vessel's COURSE.

1898 font. Complete assortment of all the different characters of a particular size and style of type.

1899 Food and Agricultural Organization (FAO). Aims at the improvement of the standard of living, especially of therural population of third world countries, by increasing the production and improving the distribution of agriculturalproducts.

1900 foot (pl. feet). A unit of length equal to 12 INCHES, 1/6 of a FATHOM, or 30.480 centimetres.

1901 foot of the continental slope. The point of maximum change of gradient at the base of the CONTINENTAL SLOPE.

1902 footscrew. See LEVELLING SCREW.

1903 forced convection. See CONVECTION.

1904 forced wave. See WAVE.

1905 foreland. A CAPE or PROMONTORY.

1906 Forel scale. A colour scale employed as a means of determining the COLOUR OF SEA WATER.

1907 foreshore. That part of SHORE which lies between HIGH and LOW WATER MARK at ordinary TIDE.

1908 foresight. In LEVELLING, a reading on a LEVELLING ROD held on a point whose ELEVATION is to be deter-mined.

See also BACKSIGHT.

1909 format. The general plan or arrangement of a product.Specification of succession and type of representation of elements of information (numbers, text) on a medium. A

fixed format is equivalent to a table, while free formats use delimiters to separate individual elements or "tags" (basedon a directory) to identify elements. A set of formats or a complex format determines the structure of how information isrepresented on a medium.

1910 formation sounding. See SOUNDING.

1911 form lines. Lines drawn to represent the shape of TERRAIN; unlike CONTOUR LINES, these are drawn withoutregard to a true vertical DATUM or regular vertical interval.

1912 forward azimuth. See AZIMUTH: GEODETIC.

1913 forward lap. See OVERLAP.

1914 foul. 1. To entangle or become entangled; e.g. as a propeller becoming entangled in cables, nets, or seaweed.2. To attach or come to lie on the surface of submerged objects, usually in large numbers or amounts as barnacles on

the hull of a ship.

1915 foul area. An area of numerous uncharted dangers to navigation. The area charted serves as a warning to the marinerthat all dangers are not charted individually and that navigation through the area may be hazardous.

The term "foul" should not be applied to a soft continuum with indefinite boundaries such as mud or sand; to areascongested with marine vegetation such as kelp or grass in water; or to materials not likely to cause damage to a vessel.

1916 foul berth. A berth in which a vessel cannot swing to her anchor or moorings without fouling another vessel or strikingan obstruction.

91

1917 foul bottom. A hard, uneven, rocky or obstructed BOTTOM having poor holding qualities for anchors, or one havingROCKS or WRECKAGE that would endanger an anchored vessel.

1918 foul ground. An area where the holding qualities for an anchor are poor, or where danger of striking or fouling theGROUND or other OBSTRUCTIONS exists.

1919 fouling. The mass of living and non-living bodies and particles attached to or lying on the surface of a submerged man-made or introduced object; more commonly considered to be only the living or attached bodies.

1920 fractional scale. See SCALE: NATURAL.

1921 fracture zone. An extensive linear zone of unusually irregular TOPOGRAPHY of the SEA FLOOR, characterized bylarge SEAMOUNTS, steep-sided or asymmetrical RIDGES, TROUGHS, or ESCARPMENTS.

1922 frazil ice. Fine spicules or plates of ICE in suspension in water.

1923 free-air anomaly. The difference between observed GRAVITY and theoretical GRAVITY which has been computedfor LATITUDE and corrected for ELEVATION of the STATION above or below the GEOID, by application of thenormal rate of change of GRAVITY for change of ELEVATION, as in free air. Also called Faye anomaly.

1924 free atmosphere. See ATMOSPHERE.

1925 free convection. See CONVECTION.

1926 free gyroscope. See GYRO: FREE.

1927 free port. A port where certain import and export duties are waived (unless goods pass into the country) to facilitate re-shipment to other countries.

1928 free wave. See WAVE.

1929 freezing. See FROST.

1930 freezing point. Temperature of solidification of a liquid in given conditions.

1931 frequency. The number of VIBRATIONS or CYCLES in unit time.

1932 frequency: extremely high (EHF). See FREQUENCY: RADIO.

1933 frequency: high (HF). See FREQUENCY: RADIO.

1934 frequency: low (LF). See FREQUENCY: RADIO.

1935 frequency: medium (MF). See FREQUENCY: RADIO.

1936 frequency: natural. The lowest resonant FREQUENCY of a body or system.

1937 frequency: radio. Any FREQUENCY at which electromagnetic RADIATION of energy is useful for communication.Radio frequencies are usually classed as very low, below 30 kilohertz; low, 30-300 kilohertz; medium, 300-3,000kilohertz; high, 3-30 MEGAHERTZ: very high, 30-300 MEGAHERTZ; ultra high, 300-3,000 MEGAHERTZ; superhigh, 3,000-30,000 MEGAHERTZ; extremely high, 30,000-300,000 MEGAHERTZ.

1938 frequency: resonant. Any FREQUENCY at which a body or system vibrates more readily.

1939 frequency: super high (SHF). See FREQUENCY: RADIO.

1940 frequency: ultra high (UHF). See FREQUENCY: RADIO.

1941 frequency: very high (VHF). See FREQUENCY: RADIO.

1942 frequency: very low (VLF). See FREQUENCY: RADIO.

1943 frequency band. See BAND OF FREQUENCY.

1944 frequency channel. See CHANNEL.

92

1945 frequency modulation. See MODULATION.

1946 frequency of soundings. See DENSITY OF SOUNDINGS.

1947 freshet. An area of comparatively fresh water at or near the MOUTH of a STREAM flowing into the SEA.A FLOOD or overflowing of a RIVER caused by heavy RAINS or melted SNOW.

1948 friction layer. Atmospheric LAYER extending from the EARTH's surface and of depth about 600 to 800 m, withinwhich air motion is affected significantly by surface friction. Above this LAYER lies the 'FREE ATMOSPHERE'.

1949 fringing reef. A REEF closely attached to a SHORE, as contrasted with a BARRIER REEF which is separated fromthe SHORE by a LAGOON.

1950 front. In METEOROLOGY: a) surface of separation of two AIR MASSES (frontal surface); b) line of intersection ofthe surface of separation of two AIR MASSES with another surface or with the GROUND.

1951 frontal surface. See FRONT.

1952 front light. See LIGHT(S): RANGE.

1953 front nodal point. See NODAL POINT.

1954 frost (or freezing). Lowering of air temperature to a value equal to or less than the FREEZING POINT of water (0°C).

1955 frost point. The temperature to which atmospheric moisture must be cooled to reach the point of saturation with respectto ice.

1956 frost smoke. Fog-like CLOUDS, due to the contact of cold air with relatively warm SEA WATER, which appear overnewly formed LEADS and POOLS, or leeward, of the ICE EDGE, and which may persist while SLUSH or SLUDGEand YOUNG ICE are forming.

1957 f-stop. See APERTURE: RELATIVE.

1958 fucus. See ROCKWEED.

1959 full moon. See PHASES OF THE MOON.

1960 function key. A special key on a computer which performs a preprogrammed function.

1961 fundamental circle. See PRIMARY GREAT CIRCLE.

1962 furrow. On the sea floor, a closed, linear, narrow, shallow depression.

1963 fusion: stereoscopic. See STEREOSCOPIC FUSION.

93

G

1964 gage. See GAUGE: TIDE.

1965 gain. The ration of output voltage, current or power to input voltage, current, or power in electronic instruments.

1966 gal (derived from Galileo). The unit of ACCELERATION in the CGS SYSTEM. See MILLIGAL.

1967 gale. WIND with a SPEED between 32 and 37 KNOTS (Beaufort scale wind force 8).

1968 gale warning. See WARNING.

1969 Galton's sun signal. See HELIOSTAT.

1970 galvanometer. An instrument for detecting and determining the intensity and direction of an electric current.

1971 galvanoplasty (or galvanoplastics). The process of coating things with metal by means of electrolysis.

1972 gamma. Photographic term for negative contrast resulting from development, and not the contrast of the subject itself; anumerical measure of CONTRAST in the development of a NEGATIVE.

A small unit of MAGNETIC FIELD INTENSITY generally used in describing the EARTH's MAGNETIC FIELD.It is defined as being equal to 10-5 OERSTED.

1973 gamma ray. See X-RAYS.

1974 gap. In nomenclature of undersea features, a break in a RIDGE or RISE.Any space where survey data fail to meet minimum coverage requirements. This may be a space not covered by any

data or a space where the minimum specified OVERLAP was not obtained.A NOTCH.

1975 gas chromatography. An analysis method used to determine dissolved oxygen and nitrogen content of SEA WATER.

1976 gas field. An area in which natural gas occurs in quantities worthy of exploitation.

1977 gat. A natural or artificial PASSAGE or CHANNEL through SHOALS or steep BANKS, or across a line of BANKSlying between two CHANNELS.

1978 gate: hydraulic. A form of valve used to regulate the FLOW of a liquid in a pipeline, RIVER or DAM.

1979 gauge. An instrument for measuring the size or state of anything.

1980 gauge: depth. A general term for an instrument made to indicate DEPTH of water, especially one in which sea depthsare indicated by pressure of water against a spring, or by degree of compression of air in a tube.

1981 gauge: float. A TIDE GAUGE that is operated by a float in a long, vertical stationary box to which the TIDE isadmitted through an opening in the bottom. Also called box gauge.

1982 gauge: pressure. An instrument for measuring pressure.A TIDE GAUGE operated by the change in pressure at the BOTTOM of a body of water, due to the RISE and fall of

the TIDE.

1983 gauge: rain. Instrument for measuring the depth of water from PRECIPITATION supposed distributed over a hori-zontal impervious surface and not subject to evaporation. Also written as one word.

1984 gauge: tide. A device for measuring the HEIGHT OF TIDE. A graduated staff in a sheltered area where visualobservations can be made; or it may consist of an elaborate recording instrument making a continuous graphic record oftide height against time. Such an instrument is usually actuated by a float in a pipe communicating with the seathrough a small hole which filters out shorter waves.

1985 gauss. The CGS unit of MAGNETIC INDUCTION.

1986 Gaussin error. See ERROR.

94

1987 gelbstoff. Yellowish organic material suspended in sea water which causes greenish colour of the sea water in coastalareas; usually transported into the sea by rivers.

1988 General Bathymetric Chart of the Oceans (GEBCO). A global series of small scale bathymetric charts. Editedthrough international cooperation by IHO and IOC.

1989 general chart. See CHART.

1990 general notices. All information and instructions that hydrographic offices may wish to bring to the attention ofmariners but the nature of which is such that they may not properly refer to any specific nautical document.

1991 generalisation. The omission of less important detail when compiling a chart. Its purpose is to avoid overloading chartswhere space is limited.

1992 geocentric coordinates. See COORDINATES.

1993 geocentric latitude. See LATITUDE.

1994 geocentric parallax. See PARALLAX: DIURNAL.

1995 geodesic. See GEODESIC LINE.

1996 geodesic line. The shortest line on a mathematically derived surface, between two points on that surface. A geodesicline on a REFERENCE SPHEROID is called a GEODETIC LINE. Also termed a geodesic.

1997 geodesy. The science which treats of the determination of the size and figure of the EARTH (GEOID) by such directmeasurements as TRIANGULATION, LEVELLING and gravimetric OBSERVATIONS.

1998 geodesy: satellite. The discipline which employs OBSERVATIONS of an earth satellite to extract geodeticinformation.

1999 geodetic azimuth. See AZIMUTH.

2000 geodetic connection. See CONNECTION.

2001 geodetic control. See CONTROL.

2002 geodetic coordinates. See COORDINATES.

2003 geodetic data. Information concerning points established by a geodetic survey, such as descriptions for recovery,coordinate values, height above sea level, and orientation.

2004 geodetic datum. See DATUM.

2005 geodetic equator. See EQUATOR.

2006 geodetic latitude. See LATITUDE.

2007 geodetic levelling. See LEVELLING.

2008 geodetic line. See GEODESIC LINE.

2009 geodetic longitude. See LONGITUDE.

2010 geodetic meridian. See MERIDIAN.

2011 geodetic parallel. See PARALLEL.

2012 geodetic position. See POSITION.

2013 geodetic survey. See SURVEY.

2014 geographic(al) coordinates. See COORDINATES.

95

2015 geographic information system. A computer based system for handling and integrating data from a variety of sourceswhich is directly or indirectly spatially referenced to Earth.

2016 geographic(al) latitude. See LATITUDE.

2017 geographic(al) longitude. See LONGITUDE.

2018 geographic(al) meridian. See MERIDIAN.

2019 geographic(al) parallel. See PARALLEL.

2020 geographic(al) pole. See POLE.

2021 geographic(al) position. See POSITION.

2022 geographic(al) range. See RANGE.

2023 geoid. The FIGURE OF THE EARTH considered as a mean sea level surface extended continuously through theCONTINENTS. The actual geoid is an EQUIPOTENTIAL SURFACE to which, at every point, the PLUMB LINE(direction in which GRAVITY acts) is perpendicular. It is the geoid which is obtained from observed DEFLECTIONSOF THE VERTICAL and is the surface of reference for ASTRONOMICAL OBSERVATIONS and for GEODETICLEVELLING. See REFERENCE SPHEROID.

2024 geologic(al) survey. See SURVEY.

2025 geologic(al) time. See TIME.

2026 geology. The science which has for its object the investigation of the EARTH's CRUST, of the STRATA which enterinto its composition with their mutual relations, and of the successive changes to which their present condition andposition are due.

2027 geology: submarine. A field of study which applies the science of GEOLOGY to the earth beneath the SEA.

2028 geomagnetic field. The MAGNETIC FIELD of the EARTH. Also called terrestrial magnetic field or earth's magneticfield.

2029 geomagnetism. The MAGNETISM of the EARTH. Also called terrestrial magnetism.

2030 geomatics. The science and technology of spatial information management, including the acquisition, storage, analysisand processing, display and dissemination of geo-referenced information.

2031 geometric dilution of precision. See DILUTION OF PRECISION.

2032 geometric latitude. See LATITUDE: PARAMETRIC.

2033 geometric projection. See PROJECTION: PERSPECTIVE.

2034 geomorphology. A branch of both geography and geology that deals with the form of the earth, the generalconfiguration of its surface, and the changes that take place in the evolution of land forms.

2035 geophysics. The study of the physical characteristics and properties of the EARTH.

2036 geosphere. The solid and liquid portions of the EARTH; the LITHOSPHERE plus the HYDROSPHERE.

2037 geostationary satellite. An earth satellite moving eastward in an equatorial, circular orbit at an altitude (approximately35,900 kilometers) such that its period of revolution is exactly equal to (synchronous with) the rotational period of theearth. Such a satellite will remain fixed over a point on the earth's equator. Although geostationary satellites are fre-quently called geosynchronous or synchronous satellites, the orbit of an eastward moving synchronous satellite must beequatorial if the satellite is to remain fixed over a point on the equator. Otherwise, the satellite moves daily in a figure-eight pattern relative to the earth. Also called fixed satellite.

2038 geostrophic wind. That horizontal wind velocity for which the Coriolis force exactly balances the horizontal pressureforce.

2039 gibbous (adj.). See PHASES OF THE MOON.

96

2040 gimbals. A device consisting of two rings pivoted at right angles to each other, for supporting anything, such as aninstrument, in such a manner that it will remain essentially horizontal when the support tilts. Also called cardanicsuspension.

2041 glacier. A mass of SNOW and ICE continuously moving from higher to lower GROUND or, if afloat, continuouslyspreading. The principal forms of glaciers are: ICE SHEETS, ICE SHELVES, ICE CAPS, ICE PIEDMONTS, andvarious types of mountain glaciers.

2042 glacier ice. Any ICE floating on the SEA as an ICEBERG, which originates from a land GLACIER. Also called landice.

2043 glacier tongue. Projecting seaward extension of GLACIER, usually afloat.

2044 glaciology. The study of SNOW and ICE on the EARTH's surface, with specific concentration on the regime of activeGLACIERS.

2045 glen. A narrow, secluded VALLEY.

2046 glitter. The spots of light reflected from a point source by the surface of the SEA.

2047 global positioning system (GPS). A satellite navigation system intended to provide highly accurate position andvelocity information in three dimensions and precise time and time interval on a global basis continuously.

2048 Global Maritime Distress and Safety System (GMDSS). The GMDSS is a global communications service basedupon automated systems, both satellite based and terrestrial, to provide distress alerting and promulgation ofMARITIME SAFETY INFORMATION for mariners.

2049 globe. The EARTH; the world.A spherical structure on whose surface is depicted the geographical configuration of the EARTH (terrestrial globe).

2050 gnomon. Any object the shadow of which serves as an indicator, as the SHADOW PIN of a SUN COMPASS.

2051 gnomonic chart. See CHART.

2052 gnomonic projection. See PROJECTION.

2053 Gold slide. An adjustable attachment to a MERCURIAL BAROMETER for giving resultant of CORRECTIONS forINDEX ERROR, HEIGHT of instrument above SEA LEVEL, variation in GRAVITY due to LATITUDE, temperature.Invented by Col. E. Gold.

2054 gong buoy. See BUOY: SOUND.

2055 goniometer. An instrument for measuring angles.A pick-up coil which eliminates the necessity of having to rotate a radio direction finder antenna to determine

direction.

2056 gorge. A narrow opening, between HILLS, usually with precipitous sides.

2057 grab. A kind of closing scoop or bucket used in dredging or for bringing up BOTTOM SAMPLES.

2058 grab sampling. A random mode of collecting samples.

2059 grad (or grade). A unit of angle measurement, equal to the angle at the centre of a circle, subtended by one four-hundredth part of its circumference.

2060 grade. Rate of SLOPE or degree of INCLINATION, as a 2% grade. See also GRADIENT and GRAD.

2061 grade correction (tape). See TAPE: GRADE CORRECTION.

2062 gradient. The change of any quantity with distance in any given direction. See PRESSURE GRADIENT, TEMPERA-TURE LAPSE RATE.

The amount of SLOPE, INCLINATION to the HORIZONTAL, in road, railway, etc.

2063 gradient tints. See HYPSOMETRIC TINTS.

97

2064 graduation. The placing of marks on an instrument or device to represent standard values thereon. Also the marks soplaced.

The division and subdivision of LATITUDE and LONGITUDE shown on the borders of a CHART. See alsoSCALE.

2065 graduation error. See ERROR OF GRADUATION.

2066 gram(me). A CGS unit of mass equal to one one-thousandth of a KILOGRAMME.

2067 granite. Light-coloured, acidic IGNEOUS ROCK mineralogically composed primarily of QUARTZ and potassium-sodium rich feldspars in which the mineral grains are visible to the naked eye (phaneritic texture).

2068 graphic control survey. See SURVEY.

2069 graphic scale. See SCALE: BAR.

2070 graticule. The network of lines representing MERIDIANS and PARALLELS on a MAP, CHART, or PLOTTINGSHEET. See also GRID.

A SCALE at the FOCAL PLANE of an optical instrument to aid in the measurement of objects. See also RETICLE.

2071 graticule adjustment. The transportation or movement of the GRATICULE in order to change the ORIGIN ofreference.

2072 gravel. Loose detrital material ranging in size from 2 to 256 mm.

2073 gravimeter. A weighing device or instrument of sufficient sensitivity to register variations in the WEIGHT of aconstant mass when the mass is moved from place to place on the EARTH and thereby subjected to the influence ofGRAVITY at those places. Also called gravity meter.

2074 gravimetric survey. See SURVEY.

2075 graving dock. See DOCK: DRY.

2076 gravitation. In general, the mutual attraction between masses of matter (bodies).Gravitation is the component of GRAVITY which acts towards the EARTH.

2077 gravity. That force which tends to pull bodies towards the EARTH; that is to give bodies WEIGHT. Gravity is theresultant of two opposing forces: GRAVITATION and CENTRIFUGAL FORCE due to the ROTATION of theEARTH.

2078 gravity: centre of. A point at which the mass of the entire body may be regarded as being concentrated.

2079 gravity: direction of. The DIRECTION indicated by a PLUMB LINE. It is perpendicular to the surface of the GEOID.Also called direction of the force of gravity.

2080 gravity: intensity of. The magnitude with which GRAVITY acts, expressed in suitable units, usually as anACCELERATION, in GALS; as a force, in DYNES.

2081 gravity anomaly. The difference between the observed gravity value properly reduced to SEA LEVEL and thetheoretical GRAVITY obtained from gravity formula. See BOUGUER ANOMALY, FREE-AIR ANOMALY,ISOSTATIC ANOMALY.

2082 gravity anomaly chart. See CHART.

2083 gravity coring. A method to obtain cores by bottom penetration solely as a result of gravity.

2084 gravity meter. See GRAVIMETER.

2085 gravity wave. See WAVE.

2086 grease ice. A later stage of freezing than frazil ice when the crystals have coagulated to form a soupy layer on thesurface. Grease ice reflects little light, giving the sea a matt appearance.

2087 great circle. See CIRCLE.

98

2088 great circle: primary. See PRIMARY GREAT CIRCLE.

2089 great circle bearing. See BEARING.

2090 great circle chart. See CHART: GNOMONIC.

2091 great circle course. See COURSE.

2092 great circle sailing. See SAILING.

2093 great circle track. See TRACK.

2094 great diurnal range. See RANGE: DIURNAL.

2095 greatest elongation. See ELONGATION.

2096 great tropic range. See RANGE.

2097 Greenwich civil time. See TIME: GREENWICH MEAN.

2098 Greenwich hour angle. See HOUR ANGLE.

2099 Greenwich mean time (GMT). See TIME.

2100 Greenwich meridian. See MERIDIAN.

2101 Greenwich time. See TIME.

2102 grid. A series of lines, usually straight and parallel, superimposed on a CHART or PLOTTING SHEET to serve as adirectional reference for NAVIGATION. See also GRATICULE.

Two sets of mutually perpendicular lines dividing a MAP, CHART, or other representation of the EARTH's surface,into squares or rectangles to permit location of any point by a system of RECTANGULAR COORDINATES.

In electronics, an electrode with one or more openings to permit passage of electrons or ions.

2103 grid: parallactic. In PHOTOGRAMMETRY, a uniform pattern of rectangular lines drawn or engraved on some trans-parent material, usually glass, and placed either over the PHOTOGRAPHS of a STEREOSCOPIC PAIR or in theOPTICAL SYSTEM of a STEREOSCOPE, in order to provide a continuous floating-mark system.

2104 grid: perspective. In PHOTOGRAMMETRY, a network of lines, drawn or superimposed on a PHOTOGRAPH, torepresent the perspective of a systematic network of lines on the GROUND or DATUM PLANE. See also GRIDMETHOD.

2105 grid: Universal Transverse Mercator (UTM). A grid system in which a grid network is applied to TRANSVERSEMERCATOR PROJECTIONS of zones of the EARTH's surface extending to 80o N. and S. LATITUDES.

2106 grid azimuth. See AZIMUTH.

2107 grid bearing. See BEARING.

2108 grid coordinates. See COORDINATES.

2109 grid course. See COURSE.

2110 grid declination. The angular difference in DIRECTION between GRID NORTH and TRUE NORTH. It is measuredeast or west from TRUE NORTH.

2111 grid direction. See DIRECTION.

2112 gridiron. See CAREENING GRID.

2113 grid magnetic angle. See VARIATION: GRID.

2114 grid meridian. See MERIDIAN.

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2115 grid method. In PHOTOGRAMMETRY, a method of plotting detail from OBLIQUE PHOTOGRAPHS bysuperimposing a perspective of a map grid on a PHOTOGRAPH and transferring the detail by eye, that is, by using thecorresponding lines of the map grid and its perspective as placement guides. See also GRID: PERSPECTIVE.

2116 grid navigation. See NAVIGATION.

2117 grid north. See NORTH.

2118 grid ticks. Short lines indicating where selected grid lines intersect the NEAT LINE.

2119 grid variation. See VARIATION.

2120 grivation. See VARIATION: GRID.

2121 groin (or groyne). A low artificial wall-like structure of durable material extending from the LAND to seaward for aparticular purpose, such as to protect the COAST or to force a CURRENT to scour a CHANNEL. Sometimes called arip-rap in New England waters.

2122 gross error. See ERROR.

2123 ground. The BOTTOM of the SEA. The solid surface of the EARTH.

2124 ground (v.t. and i.). To touch BOTTOM or run aground. See STRAND.

2125 ground control. See CONTROL.

2126 ground control point. See CONTROL POINT.

2127 ground distance. The great-circle distance between two ground positions, as contrasted with SLANT DISTANCE.Also called ground range.

2128 ground effects. Influence exerted by the GROUND on the propagation of ELECTROMAGNETIC WAVES.

2129 ground ice. See ANCHOR ICE.

2130 ground layer. Thin layer of air adjacent to the EARTH's surface and of a depth which has been variously given as 10 mto 100 m. Also called surface boundary layer.

2131 ground log. See LOG.

2132 ground nadir. See NADIR.

2133 ground parallel. See PARALLEL.

2134 ground photograph. See PHOTOGRAPH: TERRESTRIAL.

2135 ground plane. In PHOTOGRAMMETRY, the horizontal plane passing through the GROUND NADIR of a CAMERASTATION.

2136 ground range. See GROUND DISTANCE.

2137 ground resolution. See RESOLUTION.

2138 ground speed. In AIR NAVIGATION, the rate of motion of an aircraft along its TRACK with relation to theGROUND. See also AIR SPEED.

2139 ground survey. See SURVEY.

2140 ground swell. A long high ocean swell; also, this SWELL as it rises to prominent HEIGHT in SHALLOW WATER,however, usually not so high or dangerous as BLIND ROLLERS.

2141 ground truth. The information acquired by a field study to calibrate or verify geographical data acquired by remotesensing.

100

2142 ground wave. See WAVE.

2143 group. A term distinguishing a LIGHT exhibiting two or more flashes or OCCULTATIONS from one exhibiting asingle flash or single OCCULTATION.

2144 group velocity. The VELOCITY of a wave disturbance as a whole, i.e., of an entire group of component simpleharmonic waves.

2145 growler. A piece of ICE almost awash, smaller than a BERGY BIT.

2146 guard lock. See TIDE LOCK.

2147 guidance. The exercise of directing influence over the movements of a craft or missile with particular reference to theselection of a flight PATH.

2148 gulf. A part of the SEA extending into the LAND, usually larger than a BAY.

2149 gully. Small VALLEY cut into soft SEDIMENTS on the CONTINENTAL SHELF or CONTINENTAL SLOPE.

2150 gust. A sudden brief increase in the SPEED of the WIND.

2151 gut. A narrow PASSAGE such as a STRAIT or INLET.A CHANNEL in otherwise less deep water, generally formed by water in motion.

2152 guyot. See TABLEMOUNT.

2153 gyre. A closed circulatory system, but larger than a WHIRLPOOL or EDDY.

2154 gyro. See GYROSCOPE.

2155 gyro: free. A two-degree-of-freedom GYRO or a GYRO the spin axis of which may be oriented in any specifiedattitude. Also called free gyroscope.

2156 gyro compass. See COMPASS: GYROSCOPIC.

2157 gyro pilot. An automatic pilot controlled by GYROSCOPES.An automatic device for steering a vessel by means of control signals from a GYROSCOPIC COMPASS.

2158 gyro repeater. An electrically operated DIAL repeating at a distance the indications of the MASTER GYROCOMPASS.

2159 gyroscope. A rapidly rotating mass free to move about one or both AXES perpendicular to the AXIS of ROTATIONand to each other. It is characterized by GYROSCOPIC INERTIA and PRECESSION. Sometimes shortened to gyro.

2160 gyroscope: directional. A gyroscopic device used to indicate a selected horizontal DIRECTION for a limited time. Itrequires periodic resetting dependent upon the gyro precession rate or change in the desired HEADING or horizontalDIRECTION.

2161 gyroscope: free. See GYRO: FREE.

2162 gyroscopic compass. See COMPASS.

2163 gyroscopic inertia. The property of a GYROSCOPE of resisting any force which tends to change its AXIS ofROTATION. A GYROSCOPE tends to maintain the DIRECTION of its AXIS of ROTATION in space.

2164 gyroscopic stabilization. EQUILIBRIUM in the attitude and/or COURSE of a ship or airborne vehicle maintained bythe use of GYROSCOPES. Also, the maintenance (by the use of GYROSCOPES) of a CAMERA in a desired attitudewithin an airborne vehicle.

2165 gyro sextant. See SEXTANT.

101

H

2166 hachures. Short lines drawn on a CHART or MAP for representing RELIEF. These lines are drawn in the direction ofthe SLOPE and intensified on the shaded side of the feature assuming the area is illuminated from the north-west.Unlike the CONTOUR LINES. they give no indication of the actual HEIGHT of LAND above SEA LEVEL. See alsoHILL SHADING.

2167 hack watch. See WATCH.

2168 hail. PRECIPITATION of small balls or pieces of ICE (HAILSTONES) with a diameter ranging from 5 to 50 mm (0.2to 2.0 in) or sometimes more, falling either separately or agglomerated into irregular lumps.

2169 hailstone. Globule or piece of ICE, with a diameter varying between 5 and 50 mm or even more, the fall of whichconstitutes HAIL. Hailstones consist almost entirely of transparent ICE, or of a series of LAYERS of transparent ICEwith a thickness of at least 1 mm, alternating with translucent LAYERS.

2170 halation. In PHOTOGRAPHY, a spreading of a photographic IMAGE beyond its proper boundaries, due especially toREFLECTION from the side of the film or plate support opposite to that on which the EMULSION is coated. Par-ticularly noticeable in PHOTOGRAPHS of bright objects against a darker background.

2171 half byte. 4 bits of a 8 BIT BYTE; usually used to encode the figures 0....9.

2172 half convergency. See ARC TO CHORD CORRECTION.

2173 half tide. See TIDE.

2174 half tide level. The LEVEL midway between MEAN HIGH WATER and MEAN LOW WATER. It may differ slightlyfrom MEAN SEA LEVEL. Also called mean tide level.

2175 halftone. In PHOTOGRAPHY or printing, a technique in which the solid IMAGE is broken up by the use of a screeninto evenly spaced dots of equal density but of varying size. This gives an illusion of continuous tone.

2176 halo (solar, lunar). Group of optical PHENOMENA, in the form of rings, arcs, pillars or bright spots, produced by theREFRACTION or REFLECTION of light by ice crystals suspended in the ATMOSPHERE (cirriform CLOUDS, ICEFOG, etc.).

2177 halo: large (halo of 46o). HALO much less frequent and always less luminous than the SMALL HALO. It comprises afaintly luminous ring centered on the SUN or MOON and has an apparent radius of 46o.

2178 halo: small (halo of 22o). White, or mostly white, luminous ring of 22o radius with the luminary at its centre. The smallhalo shows a faint red fringe on the inside and, in some rare cases, a violet fringe on the outside. The portion of the skyinside the ring is conspicuously darker than the rest of the sky.

2179 halocline. A vertical GRADIENT of salinity (which is usually positive) in some LAYER of the body of water, which isappreciably greater than the GRADIENTS above and below it.

2180 hand lead. See LEAD.

2181 hand lead sounding. See SOUNDING.

2182 hanging compass. See COMPASS.

2183 hanging traverse. See TRAVERSE: OPEN.

2184 harbour (or harbor). A natural or artificially improved body of water providing protection for vessels, and generallyANCHORAGE and docking facilities. See also PORT.

2185 harbour: inner. The part of a HARBOUR more remote from the SEA, as contrasted with the OUTER HARBOUR.These expressions are usually used only in a HARBOUR that is clearly divided into two parts, as by a narrow passa-geway or man-made structures. The inner harbour generally has additional protection and is often the principal berthingarea.

2186 harbour: island. A HARBOUR formed, or mainly protected, by ISLANDS.

102

2187 harbour: outer. The part of a HARBOUR towards the SEA, through which a vessel enters the INNER HARBOUR.

2188 harbour: tidal. A HARBOUR affected by the TIDES, in distinction from a HARBOUR in which the water level ismaintained by CAISSONS or gates.

2189 harbour chart. See CHART.

2190 harbour line. The line beyond which wharves and other structures cannot be extended.

2191 harbour master. A local official who has charge of mooring and berthing of vessels, collecting harbour fees, etc.

2192 harbour reach. The reach of a winding RIVER or ESTUARY which leads directly to the HARBOUR.

2193 harbour works. Permanent manmade structures built along the coast which form an integral part of the harbour systemsuch as jetties, moles, quays or other port facilities, coastal terminals, wharves, breakwaters, sea walls, etc.

2194 hard bottom. The sea floor not covered by unconsolidated SEDIMENT.

2195 hard iron. Iron or steel which is not readily magnetized by induction, but which retains a high percentage of theMAGNETISM acquired. The opposite is SOFT IRON.

2196 hardware. All physical material components of a computer system as e.g. central processing unit (CPU), peripheraldevices etc.. May also include individual chips.

2197 harmonic. A sinusoidal quantity having a FREQUENCY that is an integral multiple of the FREQUENCY of a periodicquantity to which it is related.

2198 harmonic analysis of tide. The mathematical process by which the observed TIDE at a place is analyzed by breaking itdown into a number of constituent tides of simple periodic forces, each having a fixed PERIOD. In this process the SUNand MOON are replaced by a number of hypothetical tide-producing bodies which move in circular ORBITS around theEARTH in the plane of the EQUATOR. See HARMONIC CONSTANTS, HARMONIC CONSTITUENT.

2199 harmonic constants. The AMPLITUDE and EPOCHS of the HARMONIC CONSTITUENT of the TIDE, or TIDALCURRENT at any place.

2200 harmonic constituent. One of the harmonic elements in a mathematical expression for the TIDE-PRODUCINGFORCE, and in corresponding formulae for the TIDE or TIDAL CURRENT. Each constituent represents a periodicchange or variation in the RELATIVE POSITIONS of the EARTH, SUN and MOON. Also called tidal constituent orcomponent.

2201 harmonic prediction. In tidal terminology, the method of predicting TIDES and TIDAL CURRENTS by combiningthe HARMONIC CONSTITUENTS into a single curve. The work was usually done mechanically by a machinedesigned for this purpose, called a TIDE PREDICTING MACHINE; nowadays the work is done by COMPUTERS.

2202 hatching. The drawing or engraving of fine, parallel or crossed lines to show SHADING.

2203 haven. An enclosed and protected HARBOUR.

2204 Hayford's spheroid. The SPHEROID based upon an investigation made in 1909 by Hayford, a geodesist of U.S. Coast& Geodetic Survey, using the large TRIANGULATION NET then existing in the U.S.A., and taking into account theinequal density of the EARTH's CRUST. The Geodetic and Geophysical Union (now the International Union ofGeodesy and Geophysics) adopted in 1924 the Hayford's spheroid (with some slight modifications) as theINTERNATIONAL ELLIPSOID OF REFERENCE.

2205 hazard beacon. See BEACON.

2206 haze. Suspension in the ATMOSPHERE of extremely small, dry particles which are invisible to the naked eye but arenumerous enough to give the sky an appearance of OPALESCENCE.

2207 head. A precipitous CAPE, or PROMONTORY. See also HEADLAND.That part of a RIP CURRENT that has widened out seaward of the BREAKERS.

2208 heading. The direction in which the longitudinal axis of a craft is pointed, usually expressed in DEGREES fromNORTH (TRUE, MAGNETIC or COMPASS).

103

2209 headland. A high steep PROMONTORY. Usually called HEAD when coupled with a specific name.

2210 head sea. A SEA in which the WAVES move in a direction approximately opposite to the HEADING.

2211 headwater(s). The STREAMS from the sources and upper parts of a RIVER.

2212 headway. Motion in a forward direction. Also called SEAWAY.

2213 head wind. WIND which blows in a direction opposite to that in which an object is moving, with respect to theEARTH's surface.

2214 heat equator. See EQUATOR: THERMAL.

2215 heat lightning. Lightning flash at a distance which may be observed as a short illumination of the sky or of a CLOUD,close to the HORIZON.

2216 heat wave. Marked warming of the air, or the invasion of very warm air, over a large area.

2217 heave. The oscillatory rise and fall of a ship due to the entire hull being lifted by the force of the sea.

2218 heave compensation. The removal of the effect of wave motion from the recorded seabed profile.

2219 heavenly body. See CELESTIAL BODY.

2220 heel. Transverse INCLINATION of a vessel due to the action of WAVES, WIND, a greater WEIGHT on one side, etc.See also LIST.

2221 heeling error. See ERROR.

2222 heeling magnet. A permanent MAGNET placed vertically in a tube under the centre of a marine MAGNETIC COM-PASS, to correct for HEELING ERROR.

2223 height. The vertical distance of a LEVEL, a point, or an object considered as a point, measured from a specifiedDATUM.

The vertical dimension of an object.

2224 height finder. A stereoscopic RANGE FINDER so constructed as to indicate vertical HEIGHTS rather than SLANTRANGE.

2225 height of cloud base. HEIGHT above the EARTH's surface, at a given point, of the base of the lowest CLOUD whoseCLOUD AMOUNT exceeds a given value. Called ceiling in U.S. terminology.

2226 height of eye correction. See DIP CORRECTION.

2227 height of tide. The vertical distance from the CHART DATUM to the level of the water at any TIME.

2228 heliocentric parallax. See PARALLAX: ANNUAL.

2229 heliograph. A device for reflecting the SUN's RAYS from one point to another for sending messages.In METEOROLOGY, instrument which records the time interval during which solar RADIATION reaches

sufficient intensity to cast distinct shadows. Also called sunshine recorder.

2230 heliostat. An instrument composed of one or more plane MIRRORS, so mounted and arranged that a BEAM of sunlightmay be reflected by it in any desired direction and kept there by continuous adjustment. It is used in geodeticSURVEYING for reflecting a BEAM of sunlight from a STATION towards another distant STATION, where it can beobserved with a THEODOLITE. Also called heliotrope, or Galton's sun signal.

2231 heliotrope. See HELIOSTAT.

2232 heli pad. A site on which helicopters may land and take off.

2233 helium magnetometer. See MAGNETOMETER.

2234 Helmert spheroid. See REFERENCE SPHEROID.

104

2235 hemispherical map. See MAP.

2236 henry. The unit of electrical inductance in the SI SYSTEM.

2237 hertz. A unit of FREQUENCY in the SI SYSTEM. See also CYCLE PER SECOND.

2238 hertzian waves. See WAVE(S): RADIO.

2239 high. See ANTICYCLONE.

2240 higher high water (H.H.W.). The higher of two HIGH WATERS occurring during a TIDAL DAY if the DIURNALINEQUALITY prevails.

2241 higher high water interval. See LUNITIDAL INTERVAL.

2242 higher low water (H.L.W.). The higher of two LOW WATERS occurring during a TIDAL DAY if DIURNALINEQUALITY prevails.

2243 higher low water interval. See LUNITIDAL INTERVAL.

2244 highest astronomical tide. The highest tidal level which can be predicted to occur under average meteorologicalconditions and under any combination of astronomical conditions.

2245 high frequency (HF). See FREQUENCY: RADIO.

2246 highland (or highlands). High or elevated LAND; a lofty HEADLAND or CLIFF. The mountainous or elevated part ofany country; occasionally also in the names of geographical districts.

2247 high noon. See NOON: LOCAL APPARENT.

2248 high oblique photograph. See PHOTOGRAPH.

2249 high seas. The open sea beyond the exclusive economic zone, the territorial sea or the archipelagic waters of anarchipelagic state.

2250 high tide. See HIGH WATER.

2251 high water (H.W.). The highest LEVEL reached at a place by the water surface in one OSCILLATION. Also calledhigh tide.

2252 high water: double. A HIGH WATER consisting of two maxima of nearly the same HEIGHT separated by a relativelysmall depression. See also TIDE: DOUBLE.

2253 high water: storm. The highest LEVEL to which the SEA rises during a STORM and usually marked by a line ofDEBRIS high up on the BEACH.

2254 high water full and change (H.W.F. and C.). See ESTABLISHMENT.

2255 high water inequality. See DIURNAL INEQUALITY.

2256 high water interval. See LUNITIDAL INTERVAL.

2257 high water line. See HIGH WATER MARK.

2258 high water lunitidal interval (H.W.I.). See LUNITIDAL INTERVAL.

2259 high water mark. The mark left by the TIDE at HIGH WATER. The line or LEVEL reached, especially the highestline ever reached. Also called high water line.

A permanent MARK which indicates the maximum observed stage of TIDE.

2260 high water quadrature (H.W.Q.). The average HIGH WATER INTERVAL when the MOON is at QUADRATURE.See LUNITIDAL INTERVAL.

2261 high water stand. See STAND OF TIDE.

105

2262 hill. A small isolated ELEVATION, smaller than a MOUNTAIN. See also KNOLL.

2263 hillock. A small HILL.

2264 hill shading. A method of indicating RELIEF on MAPS by shading normally the SLOPES facing south and east, i.e. itis assumed that the area mapped is illuminated from the north-west. The method is often used in association withCONTOURS.

2265 hinterland. The district behind that lying along the COAST; the back country.

2266 historic bay. Historic bays are those over which the coastal state has publicly claimed and exercised jurisdiction andthis jurisdiction has been accepted by other states. Historic bays need not match the definition of "bay" contained in theUnited Nations Convention on the Law of the Sea.

2267 hoarfrost. Deposit of ICE, having a crystalline appearance, generally assuming the form of scales, needles, feathers orfans. Hoarfrost is produced in a manner similar to DEW but at a temperature below O°C. Also written as two words.

2268 hodograph. Used in oceanography. The tidal current vector hodograph is the figure traced out by the tip of a vectorrepresenting the current over the tidal cycle.

2269 hodometer. See ODOMETER.

2270 holding ground. An expression usually used with a modifying adjective to indicate the quality of the holding power ofthe material constituting the BOTTOM of an ANCHORAGE; e.g., of good (or poor) holding ground.

2271 hole. An abrupt HOLLOW in the GROUND or ocean floor. See also PASSAGE.

2272 holiday. An unintentional unsurveyed area within a given hydrographic survey where the spacing between soundinglines or surveys exceeds the maximum allowable limits.

2273 hollow. TROUGH or depression between CRESTS of sea-waves.A cavity or HOLE.

2274 homing. NAVIGATION toward a point by maintaining constant some navigational COORDINATE(S), usually BEA-RING.

The procedure of using the direction-finding equipment of one RADIO STATION with the emission of anotherRADIO STATION, where at least one of the STATIONS is mobile, and whereby the MOBILE STATION proceedscontinuously towards the other STATION.

2275 hook. A sharp bend or angle in the course or length of anything; especially a bend in a RIVER.A projecting corner, POINT or SPIT of LAND.

2276 horizon. In general, the apparent or visible junction of the EARTH and sky, as seen from any specific position.

2277 horizon: apparent. The irregular line along which rays from the point of observation are tangent to the surface of theEARTH. Also called visible (or local) horizon.

2278 horizon: artificial. A device for indicating the HORIZONTAL, as a bubble, GYROSCOPE, PENDULUM, or the flatreflecting surface of a liquid.

2279 horizon: celestial. That circle of the CELESTIAL SPHERE, formed by the intersection of the CELESTIAL SPHEREand a plane through the centre of the EARTH, and perpendicular to the zenith-nadir line. Also called rational horizon.

2280 horizon: closing the. Measuring the last of a series of horizontal angles at a STATION required to make the seriescomplete around the HORIZON. See also ERROR OF CLOSURE OF HORIZON.

2281 horizon: dip of the. See DIP.

2282 horizon: false. A line resembling the APPARENT HORIZON but above or below it.

2283 horizon: radio. See RADIO.

2284 horizon: rational. See HORIZON: CELESTIAL.

106

2285 horizon: sensible. That circle of the CELESTIAL SPHERE formed by the intersection of the CELESTIAL SPHEREand a plane through any point, such as the eye of an observer, and perpendicular to the zenith-nadir line.

2286 horizon: true. In PHOTOGRAMMETRY, a horizontal plane passing through a point of vision or PERSPECTIVECENTRE. The APPARENT HORIZON approximates the true horizon only when the point of vision is very close toSEA LEVEL.

2287 horizon: visible (or local). See HORIZON: APPARENT.

2288 horizon glass. That glass of a MARINE SEXTANT, attached to the frame, through which the HORIZON is observed.That half of this glass nearer the frame is silvered to form the HORIZON MIRROR for reflecting the IMAGE of aCELESTIAL BODY; the other half is clear.

2289 horizon line. See HORIZON TRACE.

2290 horizon mirror. The mirror part of the HORIZON GLASS. The expression is sometimes used somewhat loosely torefer to the HORIZON GLASS.

2291 horizon photograph. In AERIAL PHOTOGRAPHY, a PHOTOGRAPH of the HORIZON, taken simultaneously witha VERTICAL PHOTOGRAPH and used to determine the RELATIVE TILT between adjacent VERTICALPHOTOGRAPHS.

2292 horizon shade. One of the pivoted coloured glasses which can be swung before the HORIZON GLASS of a SEXTANTto regulate the intensity of light.

2293 horizon system of coordinates. A set of CELESTIAL COORDINATES based on the CELESTIAL HORIZON as thePRIMARY GREAT CIRCLE; usually ALTITUDE and AZIMUTH or AZIMUTH ANGLE.

2294 horizontal (adj). Parallel to the plane of the HORIZON; perpendicular to the DIRECTION OF GRAVITY.

2295 horizontal axis. See AXIS.

2296 horizontal circle. See CIRCLE.

2297 horizontal control. See CONTROL.

2298 horizontal control datum. The position on the SPHEROID OF REFERENCE assigned to the HORIZONTALCONTROL (TRIANGULATION and TRAVERSE) of an area and defined by (1) the POSITION (LATITUDE andLONGITUDE) of one selected STATION in the area, and (2) the AZIMUTH from the selected STATION to anadjoining STATION. See also DATUM: GEODETIC.

2299 horizontal coplane. See COPLANE: BASAL.

2300 horizontal dilution of precision (HDOP). See DILUTION OF PRECISION.

2301 horizontal force instrument. See VIBRATING NEEDLE.

2302 horizontal intensity of the earth's magnetic field. The strength of the horizontal component of the EARTH'SMAGNETIC FIELD.

2303 horizontal parallax. See PARALLAX.

2304 horizontal photograph. See PHOTOGRAPH.

2305 horizontal plane. See PLANE.

2306 horizon trace. In PHOTOGRAMMETRY, an imaginary line, in the plane of a PHOTOGRAPH, which represents theIMAGE of the TRUE HORIZON; it corresponds to the intersection of the plane of a PHOTOGRAPH and thehorizontal plane containing the internal PERSPECTIVE CENTRE or rear NODAL POINT of the LENS. Also calledhorizon line.

2307 horn. FOG SIGNAL using compressed air or electricity to vibrate a diaphragm. Horns exist in a variety of types whichdiffer greatly in their SOUND and power. Further to the recent action taken by the HYDROGRAPHER of the U.K.such terms as NAUTOPHONE, TYPHON and others are gradually being embraced by the general term 'Horn'.

107

2308 horn buoy. See BUOY: SOUND.

2309 hour. A 24th part of a DAY.

2310 hour angle. ANGULAR DISTANCE west of a CELESTIAL MERIDIAN, the arc of the CELESTIAL EQUATOR, orthe angle at the CELESTIAL POLE, between the UPPER BRANCH of a CELESTIAL MERIDIAN and the HOURCIRCLE of a CELESTIAL BODY or the VERNAL EQUINOX, measured westward through 24 HOURS (360o). It isusually further designated as local, Greenwich, or sidereal, as the origin of measurement is the LOCAL orGREENWICH MERIDIAN, or the HOUR CIRCLE of the VERNAL EQUINOX.

2311 hour circle. A GREAT CIRCLE on the CELESTIAL SPHERE passing through the CELESTIAL POLES. Also calleddeclination circle, circle of declination, or circle of right ascension.

2312 hulk. Usually refers to an unrigged hull of a wrecked or condemned ship.

2313 humidity. Atmospheric water vapour content, expressed in any of several measures, such as relative humidity.

2314 humidity of the air. Water vapour content of the air.

2315 hummock. A natural ELEVATION of the EARTH's surface resembling a HILLOCK, but smaller and lower.In ice terminology, a mound of ICE raised by pressure. May be fresh or weathered.

2316 hummocked ice. SEA ICE piled haphazardly one piece over another, and which may be weathered.

2317 hummocking. Pressure process by which LEVEL ICE becomes broken up into HUMMOCKS.

2318 Hunter short base apparatus. A BASE APPARATUS designed for measuring short BASES by the subtendedmethod. It consists of a jointed STEEL TAPE made up of four sections suspended in CATENARY between regularlyspaced supports. The Hunter short base apparatus may be used over rough GROUND which is unsuitable for groundTAPING.

2319 hunting. Fluctuation about a midpoint due to INSTABILITY, as OSCILLATIONS of the needle of an instrument aboutthe zero point, or alternate LEAD and LAG of a synchronous motor with respect to the alternating current.

2320 hurricane. Term, derived from a Caribbean word, first applied to TROPICAL CYCLONES of the Caribbean Sea.WIND with a speed equal to or greater than 58 KNOTS. (Beaufort scale wind force 12.)

2321 hurricane warning. See WARNING.

2322 hydrograph. Used in oceanography. The tidal current vector hydrograph is the figure traced out by the tip of a vectorrepresenting the current over the tidal cycle.

2323 hydrographer. A person who studies and practices the science of HYDROGRAPHY; the term is often applied to theperson in charge of a hydrographic department or office of a country.

2324 hydrographic airborne laser sounder. A system for measuring the depth of water by sending a LASER pulse from anaircraft and recording the time difference between the sea surface and the bottom reflection.

2325 hydrographic chart. See CHART: NAUTICAL.

2326 hydrographic circle. See CIRCLE.

2327 hydrographic data acquisition. All on-site measurements to collect hydrographic data.

2328 hydrographic data processing system. A system for verification, selection, and calculation of hydrographic surveydata.

2329 hydrographic datum. See DATUM: CHART.

2330 hydrographic sextant. See SEXTANT: SOUNDING.

2331 hydrographic signal. See SIGNAL.

2332 hydrographic survey. See SURVEY.

108

2333 hydrographic survey examination. Review of a hydrographic survey that is directed toward evaluating compliancewith, and the adequacy of, standards, rather than a careful examination of the data and their quality. The survey iscarefully examined for adequacy with respect to data acquisition and conformance with applicable standards and projectinstructions. In addition, the overall condition of the records and the Descriptive Report are examined. The digital datarepresenting the survey is plotted and subjected to a cursory examination (spot check) to ensure that digital datastandards are being adhered to. The examination of the survey data is limited only to that necessary to evaluate theacquisition and processing procedures.

2334 hydrographic survey sheet. An inclusive term used to designate both BOAT SHEETS and SMOOTH SHEETS. Alsocalled survey sheet.

2335 hydrography. That branch of applied science which deals with the measurement and description of the physicalfeatures of the navigable portion of the EARTH's surface and adjoining coastal areas, with special reference to their usefor the purpose of NAVIGATION.

2336 hydrologic front. A relatively narrow transition zone separating two water masses in the ocean.

2337 hydrologic series. Simultaneous observations carried out to obtain oceanographic data at specified levels.

2338 hydrology. The scientific study of the waters of the EARTH, especially with relation to the effects ofPRECIPITATION and evaporation upon the occurrence and character of water in STREAMS, LAKES, and on orbelow the land surfaces.

2339 hydrometeor. METEOR consisting of an ensemble of liquid or solid water particles falling through or suspended in theATMOSPHERE, blown by the WIND from the EARTH'S surface or deposited on objects on the ground or in the freeair.

2340 hydrometer. An instrument for determining the SPECIFIC GRAVITY of liquids.

2341 hydrophone. An electroacoustic TRANSDUCER that responds to water-borne SOUND WAVES and delivers essenti-ally equivalent electric WAVES. See also PRESSURE HYDROPHONE.

2342 hydrophone streamer. A chain of separate receivers for seismic sound sources towed by a ship. The streamer containsmany individual hydrophone elements and is divided electronically into a number of discrete sections or channels.

2343 hydrophotometer. An instrument used to measure the EXTINCTION COEFFICIENT of transmission of light in water.It consists of a constant light source placed at a specific distance from a photocell. When placed in water, the electricalOUTPUT of the photocell is proportional to the amount of light striking the cell which, in turn, depends upon thetransparency of the water. The instrument is calibrated to read 100 percent light transmission in air.

2344 hydroplastic corer. See CORER.

2345 hydrosphere. The waters of the EARTH's surface collectively.The 'water sphere' in comparison and contrast with LITHOSPHERE and ATMOSPHERE.

2346 hydrostatic instability. See INSTABILITY: STATIC.

2347 hydrostatic levelling. The transfer of level between two points by the use of a pipe filled with water laid on the sea bedwith vertical sections at each end.

2348 hydrostatic pressure. See PRESSURE.

2349 hydrostatic stability. See STABILITY: STATIC.

2350 hygrograph. HYGROMETER which includes an arrangement for the time recording of atmosphere humidity.

2351 hygrometer. An instrument for measuring the HUMIDITY OF THE AIR. See also PSYCHROMETER.

2352 hygrothermograph. Instrument resulting from the combination of a THERMOGRAPH and a HYGROGRAPH andfurnishing, on the same DIAGRAM, simultaneous time recording of atmospheric temperature and humidity. Also calledthermohygrograph.

109

2353 hyperbola. An open curve with two parts, all points of which have a constant difference in distance from two fixedpoints called foci.

2354 hyperbolic line of position. A LINE OF POSITION in the shape of a hyperbola, determined by measuring thedifference in distance to two fixed points.

2355 hyperbolic navigation. See NAVIGATION.

2356 hyperbolic number(s). Numerical figures characterizing the hyperbolae of a pattern or LATTICE in a HYPERBOLICSYSTEM of RADIOLOCATION.

2357 hyperbolic system. Radiolocation system supplying HYPERBOLIC LINES OF POSITION.

2358 hypsograph. A circular instrument of the slide-rule type used to compute ELEVATIONS from vertical angles andhorizontal distances.

2359 hypsographic (or hypsometric) curve. A curve used to indicate the proportion of the area of the surface at variousELEVATIONS above or DEPTHS below a given DATUM.

2360 hypsometer. A device for measuring HEIGHTS of land surfaces by the boiling point of water.

2361 hypsometric curve. See HYPSOGRAPHIC CURVE.

2362 hypsometric tints. A succession of shades or colour graduations used to depict ranges of ELEVATIONS. Also calledgradient tints or elevation tints.

2363 hypsometry. The determination of ELEVATIONS above SEA LEVEL. Generally applied to the determination ofELEVATIONS through the measurement of air pressure by observing the boiling point of a liquid.

2364 hysteresis. Retardation or LAG in the effect of changing forces, as in ship's MAGNETISM induced by the EARTH'sMAGNETIC FIELD.

110

I

2365 ice. The solid form of water.

2366 iceberg. A large mass of floating or stranded ICE broken away from a GLACIER or from an ICE SHELF. Often ofconsiderable HEIGHT (in any case more than 5 m above the LEVEL of the SEA).

2367 ice blink. A typical whitish glare on low CLOUDS above an accumulation of distant ICE. It is especially glowing whenobserved on the HORIZON.

2368 icebound (adj.). Said of a HARBOUR, INLET, etc. when NAVIGATION by ships is prevented due to ICE, exceptpossibly with the assistance of an icebreaker.

Of a ship, surrounded so closely by ICE as to be incapable of proceeding.

2369 ice cake. A FLOE smaller than 10 m across. If less than 2 m, it is termed BRASH ICE or small ice cake.

2370 ice cap. See ICE SHEET.

2371 ice cluster. A concentration of SEA ICE covering hundreds of square kilometres, which is found in the same regionevery summer.

2372 ice concentration. In SEA ICE reporting, the ratio of the areal extent of ICE present to the total areal extent of ICE andwater. Concentration is usually reported in tenths. Also called ice cover.

2373 ice cover. See ICE CONCENTRATION.

2374 ice edge. The BOUNDARY at any given time between the open sea and SEA ICE of any kind, whether drifting or fast.

2375 ice field. Area of PACK ICE consisting of any size FLOES, which is greater than 10 km across.

2376 ice floe. See FLOE.

2377 ice fog. See FOG.

2378 icefoot. A narrow strip of ICE attached to the COAST, unmoved by TIDES and remaining after the FAST ICE hasbroken free.

2379 ice free. Water surface completely free of ICE.

2380 ice front. The vertical CLIFF forming the seaward face of an ICE SHELF. See ICE WALL.

2381 ice island. A form of tabular BERG found in the Arctic Ocean, with a thickness of 30 to 50 m and from a few thousandsquare metres to 500 sq. km in area. Ice islands are characterized by a regularly undulating surface, which gives them aribbed appearance from the air.

2382 ice limit. The average position of the ICE EDGE in any given month or period based on OBSERVATIONS over anumber of years.

2383 ice piedmont. ICE covering a coastal strip of low-lying LAND backed by MOUNTAINS. The surface of an icepiedmont slopes gently seawards and may be anything from 50 m to 50 km wide, fringing long stretches ofCOASTLINE with ice cliffs. Ice piedmonts frequently merge into ICE SHELVES.

2384 ice rind. A thin, elastic, shining crust of ICE, formed by the freezing of SLUDGE on a quiet sea surface. Thickness lessthan 5 cm.

2385 ice rise. A mass of ICE resting on ROCK and surrounded either by an ICE SHELF or partly by an ICE SHELF andpartly by SEA. No ROCK is exposed and there may be none above SEA LEVEL. Ice rises often have a dome-shapedsurface. The largest known is about 100 km across.

2386 ice sheet. A continuous mass of ICE and SNOW of considerable thickness and large area. Ice sheets may be resting onROCK (see INLAND ICE SHEET) or floating (see ICE SHELF). Ice sheets of less than 40,000 sq. km resting onROCK are called ice caps.

111

2387 ice shelf. A floating ICE SHEET of considerable thickness. Ice shelves are usually of great horizontal extent and have alevel or gently undulating surface. They are nourished by local snow accumulation and often also by the seawardextension of land glaciers. Limited areas may be aground. The initial stage is called BAY ICE until the surface is morethan about 2 m above SEA LEVEL. The seaward edge is termed an ICE FRONT.

2388 ice storm. Intense formation of ICE on objects by the freezing, on impact, of drops of RAIN or DRIZZLE.

2389 ice wall. An ice cliff forming the seaward margin of an INLAND ICE SHEET, ICE PIEDMONT or ICE RISE. Therock basement may be at or below SEA LEVEL. See ICE FRONT.

2390 identification beacon. See BEACON.

2391 igneous rock. ROCK formed by solidification of molten material or MAGMA.

2392 illuminated contours. An application of ILLUMINATED RELIEF in which CONTOURS appear lighter on illumina-ted SLOPES and darker on the shadow SLOPES.

2393 illuminated relief. The representation of RELIEF with the appearance of lighting from one or more directions, giving athree-dimensional impression. Also called shaded relief.

2394 illumination (at a point of a surface). Quotient of the LUMINOUS FLUX incident on an infinitesimal element of thesurface containing the point under consideration, by the area of that element.

2395 image. The optical counterpart of an object.

2396 image: erect. An IMAGE that appears upright or in the same relative position as the object.

2397 image: inverted. An IMAGE that appears upside down in relation to the object.

2398 image: real. An IMAGE actually produced and capable of being shown on a surface, as in a CAMERA.

2399 image: virtual. An IMAGE that cannot be shown on a surface but is visible, as in a MIRROR.

2400 image point. In PHOTOGRAMMETRY, the IMAGE on a PHOTOGRAPH corresponding to a definite object on theGROUND.

2401 imagery. Techniques to obtain, process or interpret images created by different types of radiation, e.g. light, infrared, x-rays.

2402 impedance. The total opposition offered to an alternating current. It may consist of any combination of resistance,inductive reactance, or capacitive reactance.

2403 impersonal micrometer. See MICROMETER: TRANSIT.

2404 impulse. See PULSE.

2405 inch. A unit of length equal to 1/12 of a FOOT or 2.54 cm.

2406 incidence: angle of. See ANGLE OF INCIDENCE.

2407 incident nodal point. See NODAL POINT.

2408 incineration areas (at sea). An offshore area officially designated as suitable for the burning of chemical waste byspecially equipped ships.

2409 inclination. The angle which a line or surface makes with the VERTICAL, horizontal or with another line or surface.

2410 inclination: magnetic. See DIP.

2411 inclination of the horizontal axis. The vertical angle between the HORIZONTAL AXIS of a surveying or astronomi-cal instrument and the plane of the HORIZON.

112

2412 inclinometer. An instrument for measuring the INCLINATION to the horizontal of the longitudinal axis of an aircraft.See DIP CIRCLE, WIRE ANGLE INDICATOR.

2413 indelta. Inland area where a RIVER subdivides (now in common use, especially in Australia).

2414 indentation. A recess in a COASTLINE.

2415 index arm. A slender bar carrying an index; particularly that bar which pivots at the centre of curvature of the arc of aMARINE SEXTANT and carries the index and the VERNIER or MICROMETER. Also called index bar.

2416 index bar. See INDEX ARM.

2417 index chart. See CHART.

2418 index contours. Certain CONTOUR LINES (usually every fifth) accentuated by use of a line heavier than theintervening ones.

2419 index correction. That CORRECTION due to INDEX ERROR.

2420 index error. See ERROR.

2421 index glass. See INDEX MIRROR.

2422 index mark. In PHOTOGRAMMETRY, a real mark (such as a cross or dot) lying in the plane or the object space of aPHOTOGRAPH and used singly as a reference mark in certain types of monocular instruments, or as one of a pair toform a FLOATING MARK (as in certain types of STEREOSCOPES).

2423 index map. See MAP.

2424 index mirror. The MIRROR attached to the INDEX ARM of a MARINE SEXTANT. Also called index glass.

2425 index of refraction. See REFRACTION: INDEX OF.

2426 index shade. On a SEXTANT, one of the pivoting coloured glasses which can be swung before the INDEX MIRRORfor reducing the glare of the SUN.

2427 Indian spring low water. An arbitrary TIDAL DATUM approximating the LEVEL of the mean of the LOWER LOWWATER at SPRING TIDES. It was first used in waters surrounding India. Also called Indian tide plane.

2428 Indian tide plane. See INDIAN SPRING LOW WATER.

2429 indicator. That part of electronic equipment in which the DATA obtained by the RECEIVER is presented for visualOBSERVATION. This is usually in the form of a SCOPE or DIAL.

2430 indirect levelling. See LEVELLING.

2431 indirect wave. See WAVE.

2432 induced magnetism. See MAGNETISM.

2433 inequality. A systematic departure from the mean value of a quantity. See ANNUAL INEQUALITY, DIURNAL INE-QUALITY, LUNAR INEQUALITY, PARALLAX INEQUALITY, PHASE INEQUALITY.

2434 inertia: gyroscopic. See GYROSCOPIC INERTIA.

2435 inertial navigation. See NAVIGATION.

2436 inferior conjunction. See CONJUNCTION.

2437 inferior mirage. See MIRAGE.

2438 inferior planet (s). See PLANET (S): INFERIOR.

2439 inferior transit. See MERIDIAN TRANSIT.

113

2440 infrared (adj.). Having a FREQUENCY immediately beyond the red end of the visible SPECTRUM said of RAYS oflonger WAVE LENGTH than visible light, but shorter than RADIO WAVES.

2441 infrasonic (adj.). Having a FREQUENCY below the audible RANGE.

2442 ingoing stream. See FLOOD STREAM.

2443 initial great circle course. See COURSE: GREAT CIRCLE.

2444 initial great circle direction. INITIAL GREAT CIRCLE COURSE.

2445 initial meridian. See MERIDIAN.

2446 initial point. The ORIGIN of a system in the rectangular system of SURVEYS of which a PRINCIPAL MERIDIANand a BASE LINE constitute the axes for a given area. Also called fundamental point.

2447 inland ice sheet. An ICE SHEET of considerable thickness and more than about 50,000 sq. km in area, resting onROCK. Inland ice sheets near SEA LEVEL may merge into ICE SHELVES.

2448 inlet. A narrow opening by which the water penetrates into the LAND.

2449 inner harbour. See under HARBOUR.

2450 inner neat line. See NEAT LINE.

2451 innocent passage. Navigation through certain sea areas of a coastal State without being prejudicial to the peace, goodorder or security of that State. See also TRANSIT PASSAGE.

2452 inogen light. Leading mark based on the interference effect of 3 separately placed rasterboards. An arrow pointing inthe direction of the line is displayed when not sailing along the leading line.

2453 input. What is put in, as electric current or other power put into a machine.DATA to be transferred from an external storage medium, such as punched cards, into the internal STORAGE of a

COMPUTER.

2454 input impedance. The complex opposition to alternating current between the input terminals of any device.

2455 inset. In CARTOGRAPHY: a) a small area outside the NEAT LINES of a MAP or CHART included within the NEATLINES or borders to avoid publishing a separate graphic of the small area alone; b) a representation of a small area on alarger SCALE (e.g., town-plan inset), or of a large area at a smaller SCALE (e.g., orientation inset); c) any information,not normally appearing within the geographic limits of a MAP, which has been enclosed by border lines and includedwithin the map neat lines. Insets are always placed in areas where important features will not be obscured.

2456 inshore (adj. and adv.). Near or towards the SHORE.

2457 inshore traffic zone. A routeing measure comprising a designated area between the landward boundary of a trafficseparation scheme and the adjacent coast, to be used in accordance with the provisions of the International Regulationsfor Preventing Collisions at Sea.

2458 insolation. Amount of direct solar RADIATION incident per unit horizontal area at a given level. Downward-directedsolar RADIATION (global RADIATION).

2459 instability. Property of the state of rest or continuous movement of a system such that any disturbance introduced intothis state grows. In METEOROLOGY, the term is often used as a synonym of STATIC INSTABILITY.

2460 instability: static. State of hydrostatic equilibrium of the ATMOSPHERE in which a particle of air moved from itsinitial level undergoes a hydrostatic force which tends to remove it further from this level. Also called hydrostaticinstability.

2461 installation: offshore. Manmade structure usually built for the exploration or exploitation of marine resources, marinescientific research, tide observations, etc.

2462 instrumental (or instrument) correction. That CORRECTION due to INSTRUMENTAL ERROR.

114

2463 instrumental (or instrument) error. See ERROR.

2464 instrumental (or instrument) parallax. See PARALLAX.

2465 insular. Of or pertaining to an island or islands.

2466 insular shelf. A zone around an island that extends from the low water line to a depth at which there is usually a markedincrease of slope towards oceanic depths.

2467 insulated (or insulating) water bottle. See WATER BOTTLE.

2468 intake. The place where water is taken into a channel or pipe from a RIVER or other body of water, to drive a mill, orsupply a CANAL, waterworks, etc.

2469 intended track. See TRACK.

2470 intensity: luminous (in a direction). Quotient of the LUMINOUS FLUX emitted by a source, or by an element of asource in an infinitesimal cone containing the given direction, by the solid angle of that cone.

2471 intensity: magnetic. See MAGNETIC FIELD INTENSITY.

2472 intensity: sound. At a point, the average rate of sound energy transmitted in a specified direction through a unit areanormal to this direction at the point considered. The square of the SOUND PRESSURE.

2473 intensity of gravity. See GRAVITY.

2474 intensity of magnetization. See MAGNETIZATION.

2475 intercardinal point. Any of the four directions midway between the CARDINAL POINTS: north-east, north-west,south-east, south-west.

2476 intercept. The name given in MARCQ ST. HILAIRE METHOD to the difference between the calculated ALTITUDEand the TRUE ALTITUDE of a CELESTIAL BODY. Also called altitude difference.

2477 intercept method. See MARCQ ST. HILAIRE METHOD.

2478 interface. A surface, usually plane, forming the BOUNDARY between adjacent solids, spaces or immiscible liquids.In data processing it describes all information (signals, data) and the specifications ("protocol") required for

exchanging data between two units of a system. Interfaces exist between hardware and software units but also betweencomputer and human user.

2479 interference. Extraneous SIGNALS, NOISES, etc., that hinder proper reception of the desired SIGNAL in electronicequipment. See WAVE INTERFERENCE.

2480 Intergovernmental Oceanographic Commission (IOC). Established within UNESCO to promote the scientificinvestigation of the oceans through the concerted action of Member States.

2481 interior orientation. See ORIENTATION.

2482 interior perspective centre. See PERSPECTIVE CENTRE.

2483 interlines (of sounding). Those SOUNDING LINES running between the normally spaced lines usually when theBOTTOM is known or suspected to be irregular or if more detailed information about the sea bottom relief is required.

2484 internal waters. Waters on the landward side of the baseline of the territorial sea and landlocked waters within thestate.

2485 internal wave. See WAVE.

2486 International Association for the Physical Sciences of the Ocean (IAPSO). Promotes the study of scientificproblems relating to the ocean and interactions at its boundaries.

2487 International Association of Geodesy (IAG). One association of IUGG.

115

2488 International Association of Geomagnetism and Aeronomy (IAGA). Promotes international coordination of effortsto further the sciences of geomagnetism and aeronomy.

2489 International Association for Hydraulic Research (IAHR). Stimulates and promotes hydraulic research, both basicand applied, in all its aspects.

2490 International Association of Lighthouse Authorities (IALA). Association of authorities responsible for the provisionor maintenance of lighthouses and other aids to marine navigation.

2491 International Association of Meteorology and Atmospheric Physics (IAMAP). Promotes meteorological researchand investigation including all aspects of atmospheric physics, particularly in those fields which require internationalcooperation.

2492 International Association of Scientific Hydrology (IASH). Promotes and develops the study of hydrology; initiates,facilitates and coordinates research on such hydrological problems requiring international cooperation.

2493 International Association of Seismology and Physics of the Earth's Interior (IASPEI). Develops the study ofseismology and physics of the earth's interior in the economic, social and scientific aspects.

2494 International Association of Theoretical and Applied Limnology (IAL). Furthers the progress of theoretical andapplied limnology by holding congresses, by publishing the proceedings of the congresses, and by providing a soundfoundation of methods and techniques of limnological investigations.

2495 International Association of Volcanology and Chemistry of the Earth's Interior (IAV). Promotes research in allmatters relating to volcanic activity; coordinates research in mineralogy, chemistry and physical chemistry for thebenefit of volcanologists.

2496 International Atomic Time. See ATOMIC TIME: INTERNATIONAL.

2497 International Bureau of Weights and Measures (IBWM). Establishment of international standards and ofmeasurement scales of physical sizes; determination of fundamental physical constants.

2498 International Cartographic Association (ICA). Advances the study of cartographic problems; instigates cartographicresearch involving cooperation between nations.

2499 International (INT) Chart. A nautical chart with limits and scale in conformity with an internationally agreed schemeof such charts; carries the INT number of that sheet; conforms to the Chart Specifications of the IHO.

2500 International Commission for the Scientific Exploration of the Mediterranean Sea (ICSEM). Serves as liaison forresearch in Mediterranean laboratories; promotes international activities on behalf of countries bordering theMediterranean.

2501 International Council for the Exploration of the Sea (ICES). Promotes research and investigations for the study ofthe sea, particularly programs related to living resources.

2502 International Council of Scientific Unions (ICSU). Coordinates and facilitates activities of International ScientificUnions in the field of exact and natural sciences; maintains relations with the specialized and related agencies of theUN.

2503 International Cooperative Investigations of the Tropical Atlantic (ICITA). An international expedition coordinatedby the IOC.

2504 international date line. See DATE LINE.

2505 International Ellipsoid of Reference. A reference ellipsoid having the following approximate dimensions: semimajoraxis 6378388.0 METRES; semiminor axis 6356911.9 METRES; and the flattening or ellipticity 1/297. See alsoHAYFORD'S SPHEROID and FLATTENING OF THE EARTH.

2506 International Federation of Surveyors (FIG). Brings together the leading national associations to exchange views ofgeneral interest to the profession and organizes the liaison between the various affiliated associations.

2507 International Geological Congress (IGC). Congresses held by IUGS.

2508 International Geographical Union (IGU). Founded in 1922 to promote the study of geographical problems, IGUinitiates and coordinates research and provides for scientific discussion and publication.

116

2509 International Geophysical Year (IGY). By international agreement, a period during which greatly increasedOBSERVATION of world-wide geophysical PHENOMENA is undertaken through the cooperation effort ofparticipating nations.

2510 International Hydrographic Bureau (IHB). Founded in 1921 in the Principality of Monaco, to establish a close andpermanent association among the hydrographic offices of its Member States; since 1970 the headquarters of theINTERNATIONAL HYDROGRAPHIC ORGANIZATION.

2511 International Hydrographic Organization (IHO). Coordinates the activities of national hydrographic offices;promotes standards and provides advice to developing countries in the fields of hydrographic surveying and productionof nautical charts and publications.

2512 International Ice Patrol (IIP). Activities of the International Ice Patrol include collecting and analyzing dataconcerning iceberg detection and movement. The Patrol's mission is to provide the latest information on icebergpositions and computer prediction of the extent of the iceberg danger zone to North Atlantic Mariners. Areas of studyinclude iceberg detection, drift and deterioration, surface object drift, and currents in the Grand Banks Region.

2513 International Indian Ocean Expedition (IIOE or IOE). An international expedition between 1959 and 1965 underthe general coordination of the IOC.

2514 International Map of the World (IMW). The Map of the World on the SCALE of 1/1 000 000 which was plannedunder the auspices of the International Geographical Union and carried out by very many of the countries of the world.

2515 International Maritime Organization (IMO). Formerly called IMCO, the IMO is the specialized agency of theUNITED NATIONS responsible for maritime safety and efficiency of navigation.

2516 International nautical mile. A unit of length equal to 1,852 METRES. This value was approved by the InternationalHydrographic Conference of 1929 and has been adopted by nearly all maritime states.

2517 International Scientific Radio Union (ISRU). Promotes and organizes international cooperative research in radioscience.

2518 International Telecommunications Union (ITU). Maintains and extends international cooperation for the improve-ment and rational use of telecommunications.

2519 International Union of Geodesy and Geophysics (IUGG). Promotes the study of all problems relating to theconfiguration and physics of the earth, its oceans and atmosphere; comprises a number of international associations.

2520 International Union of Geological Sciences (IUGS). Contributes to the advancement of investigations relating to thestudy of the Earth.

2521 International Union of Pure and Applied Physics (IUPAP). Creates and encourages international cooperation inphysics; secures international agreement on symbols, units, nomenclature, and standards.

2522 International waters. A nonlegal term that refers to those waters subject to the high seas freedom of navigation andoverflight, i.e., contiguous zone, EEZ, and high seas.

2523 interocular distance. See INTERPUPILLARY DISTANCE.

2524 interpolation. The process of determining intermediate values between given values in accordance with some known orassumed rate or system of variation.

2525 interpupillary distance. The distance between centres of the eye pupils of an individual. Also called eye base, and usedinterchangeable with interocular distance.

2526 interrogator. A radar transmitter which sends out a PULSE that triggers a TRANSPONDER. An interrogator is usuallycombined in a single unit with a RESPONSOR, which receives the reply from a TRANSPONDER and produces anOUTPUT suitable for feeding to a display system, the combined unit being called an interrogator-responsor.

Short for interrogator-responsor. Also called challenger.

2527 interrogator-responsor. See INTERROGATOR and RESPONSOR.

117

2528 intersected point. See STATION: INTERSECTION.

2529 intersection. In SURVEYING, the procedure of determining the horizontal POSITION of an unoccupied point(INTERSECTION STATION) by direction observations from two or more known POSITIONS.

In PHOTOGRAMMETRY, the procedure of determining the horizontal POSITION of a point by intersecting linesof direction obtained photogrammetrically. The lines of direction may be obtained directly from VERTICALPHOTOGRAPHS or by graphic or mathematical rectification of tilted PHOTOGRAPHS.

2530 intersection station. See STATION.

2531 intertidal zone. The zone generally considered to be between MEAN HIGH WATER and MEAN LOW WATERlevels.

2532 intertropical convergence zone. The zone which separates the trade winds.

2533 intervalometer. A timing device for automatically operating the SHUTTER of a CAMERA at selected intervals.

2534 invar. An alloy of nickel and steel which has a low coefficient of expansion.

2535 invar tape. See TAPE.

2536 invar wire. See WIRE.

2537 inverse Mercator projection. See PROJECTION.

2538 inversion layer. Atmospheric LAYER, horizontal or approximately so, in which the temperature increases withincreasing HEIGHT. See TEMPERATURE INVERSION.

2539 inverted compass. See COMPASS: HANGING.

2540 inverted image. See IMAGE.

2541 inverting eyepiece. See EYEPIECE: ERECTING.

2542 ionosphere. That part of the ATMOSPHERE, extending from about 70 to 500 km, in which ions and free electronsexist in sufficient quantities to reflect ELECTROMAGNETIC WAVES.

2543 ionospheric disturbance. A sudden outburst of ultraviolet light on the sun, known as a Solar Flare, which producesabnormally high ionization in the region of the D-layer. The result is a sudden increase in radio wave absorption, withparticular severity in the upper medium and lower high frequencies. It has negligible effects on the heights of thereflecting/refracting layers and, consequently, upon critical frequencies, but enormous transmission may occur.

2544 ionospheric storm. Violent changes in the state of the IONOSPHERE.

2545 ionospheric wave. See WAVE: SKY.

2546 ironbound (adj.). Said of a rugged, rocky COAST which affords no ANCHORAGE.

2547 irradiance (at a point of a surface). Quotient of the FLUX of RADIATION incident on an infinitesimal element ofsurface containing the point under consideration, by the area of that element.

2548 irradiation (at a point of a surface). Product of an IRRADIANCE and its duration.

2549 irregular error. See ERROR: ACCIDENTAL.

2550 island. A piece of LAND completely surrounded by water.

2551 island harbour. See under HARBOUR.

2552 island shelf. See CONTINENTAL SHELF.

2553 island slope. See CONTINENTAL SLOPE.

118

2554 isle. An ISLAND. Now more usually applied to an ISLAND of smaller size, except in established appellation, as the'British Isles'.

2555 islet. A small ISLAND.

2556 isobar. Line joining points of equal pressure on a given surface (level surface, vertical cross section, etc.).

2557 isobath. See DEPTH CURVE, FATHOM LINE.

2558 isobathytherm. A line or surface showing the DEPTHS in OCEANS or LAKES at which points have the sametemperature. Isobathytherms are usually drawn to show cross sections of the water mass.

2559 isobront. Line joining, on a CHART, places on the EARTH's surface where, on a given day, the first clap ofTHUNDER has been heard simultaneously.

In CLIMATOLOGY, line drawn on a CHART through places which have the same average number of days onwhich THUNDER is heard in a given period.

2560 isocentre. The unique point common to the plane of a PHOTOGRAPH, its PRINCIPAL PLANE, and the plane of anassumed truly VERTICAL PHOTOGRAPH taken from the same CAMERA STATION and having an equalPRINCIPAL DISTANCE.

The point of intersection on a PHOTOGRAPH of the true PRINCIPAL LINE and the ISOMETRIC PARALLEL.The point on a PHOTOGRAPH intersected by the bisector of the angle between the PLUMB LINE and the photo-

graph perpendicular. The isocentre is significant because it is the centre of radiation for DISPLACEMENTS ofIMAGES caused by TILT.

2561 isoclinal (or isoclinic) (adj.). Of or pertaining to equal MAGNETIC DIP.

2562 isoclinal (n.). See ISOCLINAL LINE.

2563 isoclinal (or isoclinic) line. A line connecting points of equal MAGNETIC DIP. Also called isoclinal.

2564 isodynamic (adj.). Of or pertaining to equality of force.

2565 isodynamic line. A line connecting points of equal MAGNETIC INTENSITY, either the total or any component.

2566 isogon. Line which is the locus of points at which the DIRECTION OF WIND is the same.

2567 isogonal (or isogonic) (adj.). Having equal angles.

2568 isogonal (or isogonic) (n.). See ISOGONIC LINE.

2569 isogonic line. A line connecting points of equal magnetic VARIATION. Also called isogonal (or isogonic).

2570 isogram. That line on a CHART or DIAGRAM connecting points having equal values of same PHENOMENON.

2571 isogriv. A line connecting points of equal GRID VARIATION.

2572 isohaline (or isohalsine). A line connecting points of equal SALINITY in the OCEAN.

2573 isohalsine. See ISOHALINE.

2574 isolated danger mark. A navigation mark used alone to indicate a danger, e.g. a reef or shoal.

2575 isoline. A line representing the intersection of the plane of a VERTICAL PHOTOGRAPH with the plane of anoverlapping OBLIQUE PHOTOGRAPH. If the VERTICAL PHOTOGRAPH were tilt-free, the isoline would be theISOMETRIC PARALLEL of the OBLIQUE PHOTOGRAPH.

2576 isomagnetic (adj.). Of or pertaining to lines connecting points of equality in some MAGNETIC ELEMENT.

2577 isomagnetic (n.). See ISOMAGNETIC LINE.

2578 isomagnetic line. A line connecting points of equality in some MAGNETIC ELEMENT.

2579 isometric (adj.). Of or pertaining to equal measure.

119

2580 isometric latitude. See LATITUDE.

2581 isometric parallel. See PARALLEL.

2582 isopleth. An ISOGRAM of equal or constant value of a given quantity, with respect to either time or space.

2583 isopor. See ISOPORIC LINE.

2584 isoporic line. A line connecting points of equal annual rate of change of a MAGNETIC ELEMENT. Also called isopor.

2585 isopycnic (adj.). Of or pertaining to equal density.

2586 isopycnic (n.). See ISOPYCNIC LINE.

2587 isopycnic line. A line connecting points of equal density. Also called isopycnic.

2588 isoradial. A RADIAL from the ISOCENTRE.

2589 isostasy. A condition of approximate EQUILIBRIUM in the outer part of the EARTH, such that the gravitational effectof masses extending above the surface of the GEOID in continental areas is approximately counterbalanced by adeficiency of density in the material beneath those masses while the effect of deficiency of density in the ocean waters iscounterbalanced by an excess of density in the material under the OCEANS.

2590 isostatic anomaly. The difference between an observed value of GRAVITY and a theoretical value at the point ofOBSERVATION which has been corrected for ELEVATION of the STATION above the GEOID and for the effect ofTOPOGRAPHY over the whole EARTH, and for its isostatic compensation.

2591 isotherm. In METEOROLOGY, the line which is the locus of points which have the same value of air temperature.

2592 isothermal (or isothermic) (adj.). Of or relating to equal temperature.

2593 isotope. Two or more nuclides having the same atomic number, hence constituting the same element, but differing inmass number.

2594 isotropic (or isotropus) (adj.). Having the same physical properties in all directions.

2595 isthmus. A narrow strip of LAND connecting two larger bodies of LAND.

120

J

2596 jack-up rig. A rig which provides a stable drilling structure in shallow waters by extending supporting legs onto theseabed. The legs are retracted when the rig is under tow.

2597 jellyfish. Any of various free-swimming COELENTERATES having a disc- or bell-shaped body of jellylike consisten-cy. Many have long tentacles with nematocysts (stinging cells). Some are capable of producing glowing ballLUMINESCENCE.

Any jellylike free-floating organism. The term 'jellyfish' often is applied to the ctenophores and may be applied tocertain tunicates. Also called medusa.

2598 jetsam. See JETTISON.

2599 jettison. The throwing overboard of objects, especially to lighten a craft in distress. Jettisoned objects that float aretermed flotsam; those that sink, jetsam; and heavy articles that are buoyed for future recovery, lagan. See DERELICT.

2600 jetty. In U.S. terminology, a structure, such as a WHARF or PIER, so located as to influence CURRENT or protect theENTRANCE to a HARBOUR or RIVER.

In British terminology, a PIER, usually of solid construction, intended as a berthing place for vessels. See DOCK,LANDING, WHARF.

2601 joule. The unit of work or energy in the SI SYSTEM.

2602 joystick. In computer systems, a pointing device operated by pressing a stick in a desired direction.

2603 Julian day. The number of each day, as reckoned consecutively since the beginning of the present Julian period onJanuary 1, 4713 BC. The Julian day begins at noon, 12 hours later than the corresponding civil day.

2604 junction. Place of meeting or joining, as that of a TRIBUTARY with a main RIVER, or of two CHANNELS in aWATERWAY.

In LEVELLING, the place where two or more LINES OF LEVELS are connected together.In HYDROGRAPHIC SURVEY, the joining of two adjacent survey sheets. See OVERLAP.

2605 junction buoy. A BUOY marking the inner end of a MIDDLE GROUND.Usually called a MIDDLE GROUND BUOY in British terminology, without regard to direction of travel.

121

K

2606 Kalman filter. In electronics, a mathematical-statistical method for predicting a time-variable signal in the presence ofdisturbances. The method exploits the fact that certain characteristic parameters of the process vary slowly with time, sothat a best estimate of the signal can be obtained as a function of time.

2607 katabatic wind. An offshore wind produced by cold air draining from high ground by convection. The cold air replacesthe lighter warm air which is rising over the sea.

2608 kay. See CAY.

2609 kedging. Moving a vessel by laying out an anchor and then hauling the vessel up to the anchor.

2610 keg buoy. A BUOY consisting of a keg to which is attached a small pole with a flag, used by fishermen to mark theposition of a trawl line. See DAN BUOY.

2611 kelp. One of an order of usually large blade-shaped or vinelike brown ALGAE.

2612 Kelvin temperature scale. Thermodynamic scale of temperature (ToK) defined by assigning to the triple point of purewater, considered as a fundamental fixed point, the temperature of 273.16o K. Also called absolute temperature scale.

2613 Kelvin wave. A long ocean wave whose amplitude decreases from right to left along the wave crest when viewed in thedirection of travel in the northern hemisphere and from left to right in the southern hemisphere. The component ofgravity acting down the slope is exactly balanced by the deflecting force of the earth's rotation, the CORIOLIS FORCE.

2614 key. See CAY.

2615 kilogram(me). The unit of mass in the SI system.

2616 kite. A mechanical contrivance towed beneath the surface to warn of arrival in water of a certain DEPTH. SeeSUBMARINE SENTRY.

A contrivance for holding the inner end of an OROPESA SWEEP to the required DEPTH.

2617 kite otter. A multi-plane device which can be used either as a KITE or as an OTTER depending on the way it is slung.See OROPESA SWEEP.

2618 klaxon. A small power FOG SIGNAL sometimes operated by hand.

2619 knoll. A relatively small isolated ELEVATION of a rounded shape.An ELEVATION rising less than 1,000 METRES from the SEA FLOOR and of limited extent across the SUMMIT.

Also called HILL.

2620 knot. A division of the LOG LINE, by which the ship's SPEED is measured.A nautical unit of SPEED. One knot is one NAUTICAL MILE per HOUR. The name is derived from the knots in

the LOG LINE.

2621 kymatology. The science of WAVES and wave motion.

122

L

2622 labour (or labor) (v.i.). To pitch and roll heavily under conditions which subject the ship to unusually heavy stressescaused by confused or turbulent seas or unstable stowage or cargo.

2623 lag. The delay between change of condition and the indication of the change on an instrument.Delay in human reaction.The amount one cyclic motion is behind another, expressed in DEGREES. The opposite is LEAD.

2624 lagging of the tide(s). The periodic retardation in the time of occurrence of high and low water due to changes in therelative positions of the moon and the sun.

2625 lagan. See JETTISON.

2626 lagging of the tide(s). The retardation in the TIME of occurrence of HIGH and LOW WATER due to changes in therelative positions of the MOON and the SUN. The opposite effect is called PRIMING OF THE TIDE(S). See alsoDAILY RETARDATION.

2627 lagoon. An enclosed area of salt or brackish water separated from the OPEN SEA by some more or less effective, butnot complete, obstacle such as low sand bank. The name most commonly used for the area of water enclosed by aBARRIER REEF or ATOLL.

2628 Lagrangian current observations. The method for observing ocean currents by tracking the movement of driftingfloats, buoys, tracers etc. The rate and direction of movement of a particular sample of water is measured. This is theopposite to EULARIAN CURRENT OBSERVATIONS where the movement of water past a fixed point is measured bya current meter.

2629 lake. A large body of water entirely surrounded by LAND.

2630 lake breeze. See SEA BREEZE.

2631 lake ice. ICE formed in LAKES.

2632 Lambert conformal conic projection. See PROJECTION.

2633 laminar flow. FLOW in which the fluid moves smoothly along STREAMLINES which are arranged in parallelLAYERS or sheets; opposite of TURBULENT FLOW. Also called streamline flow.

2634 lamp: signal. A compact, portable battery-operated electric lamp used as a TARGET for OBSERVATIONS orSURVEYS of high PRECISION. The parabolical reflector is mounted in a special case so as to facilitate pointing andadjusting.

2635 land. The solid portion of the EARTH's surface, as opposed to SEA, water.A part of the EARTH's surface marked off by natural or political BOUNDARIES.

2636 land breeze. WIND of coastal regions blowing at night from the LAND towards a large water surface as a result ofnocturnal cooling of the land surface.

2637 landfall. The first sighting of LAND when approached from seaward. By extension, the term is sometimes used to referto the first contact with LAND by any means, as by RADAR.

2638 landfall buoy. See FAREWELL BUOY.

2639 land ice. See GLACIER ICE.

2640 landing. A place where boats receive or discharge passengers, freight, etc.

2641 landing area. In aeronautical terminology, the part of the movement area intended for the landing or take-off run ofaircraft.

2642 landlocked (adj.). Almost or quite enclosed by LAND. Said of an area of water, as a BAY, HARBOUR, etc., entirelyprotected from the SEA.

123

2643 landmark. Any MONUMENT or fixed object used to mark a BOUNDARY on the GROUND.Any prominent object on LAND which can be used in determining a location or a DIRECTION. See also MARK.

2644 land mile. See STATUTE MILE.

2645 land navigation. See NAVIGATION.

2646 landslide (or landslip). The sliding down of a mass of land on a MOUNTAIN or cliff-side; land which has so fallen.

2647 land survey. See SURVEY: CADASTRAL.

2648 land territory. Continental or insular land masses that are above water at high tide.

2649 lane. An established route as an air lane or shipping lane.In an electronic radiolocation lattice, the zone between two lines on which measured values, expressed in terms of

the system's electronic unit (WAVE LENGTH or MICROSECOND), are whole numbers and are one unit apart. SeeLATTICE.

A LEAD.

2650 Laplace azimuth. See AZIMUTH.

2651 Laplace equation. See EQUATION.

2652 Laplace station. See STATION.

2653 lappened ice. See ANCHOR ICE.

2654 large correction (British terminology). A new printing of a CHART corrected from important information which is toocomprehensive to promulgate by Admiralty NOTICES TO MARINERS or to insert conveniently by hand on existingcopies of the CHART. When large corrections are made, the CHART is not revised throughout but all copies incirculation are cancelled.

2655 large halo. See HALO.

2656 Large Automatic Navigation Buoy (lanby). A large buoy designed to take the place of a lightship where constructionof an offshore light station is not feasible. See also BUOY: SUPER.

2657 large scale. See SCALE.

2658 large scale survey. See SURVEY.

2659 LASH. See LIGHTER ABOARD SHIP.

2660 laser. Light amplification by stimulated emission of RADIATION. A device that produces an intense BEAM ofmonochromatic, spatially coherent light.

2661 last quarter. See PHASES OF THE MOON.

2662 lateral system. A buoyage system generally used for well defined CHANNELS; buoyage marks indicate thePOSITION of dangers in relation to the route to be followed by mariners in their vicinity.

2663 latitude. ANGULAR DISTANCE from a PRIMARY GREAT CIRCLE or plane. One of the COORDINATES used todescribe a POSITION, the other being LONGITUDE.

2664 latitude: astronomical. ANGULAR DISTANCE between the DIRECTION OF GRAVITY and the plane of theEQUATOR.

Astronomical latitude is the LATITUDE which results directly from OBSERVATIONS on CELESTIAL BODIES,uncorrected for DEFLECTION OF THE VERTICAL.

2665 latitude: celestial. ANGULAR DISTANCE north or south of the ECLIPTIC; the arc of a CIRCLE OF LATITUDEbetween the ECLIPTIC and a point on the CELESTIAL SPHERE, measured northward or southward from theECLIPTIC through 90°, and labelled N or S to indicate the direction of measurement. Also called ecliptic latitude.

124

2666 latitude: geocentric. The angle at the centre of the EARTH between the plane of the EQUATOR and a line to a pointon the surface of the EARTH.

2667 latitude: geodetic. The angle which the NORMAL at a point on the SPHEROID makes with the plane of theGEODETIC EQUATOR.

2668 latitude: geographic(al). A general term, applying alike to ASTRONOMICAL and GEODETIC LATITUDES.

2669 latitude: geometric. See LATITUDE: PARAMETRIC.

2670 latitude: isometric. An auxiliary LATITUDE used in the conformal mapping of the SPHEROID on a sphere.

2671 latitude: magnetic. The angle whose tangent is one half the tangent of the magnetic inclination or DIP.

2672 latitude: parametric. The angle at the centre of a sphere which is tangent to the SPHEROID along the GEODETICEQUATOR, between the plane of the EQUATOR and the radius to the point intersected on the sphere by a straight lineperpendicular to the plane of the EQUATOR and passing through the point on the SPHEROID whose parametriclatitude is defined. Parametric latitude is an auxiliary LATITUDE used in problems of GEODESY andCARTOGRAPHY. Also called reduced latitude, geometric latitude or eccentric angle.

2673 latitude: reduced. See LATITUDE: PARAMETRIC.

2674 latitude: terrestrial. ANGULAR DISTANCE from the EQUATOR, measured northward or southward through 90°and labelled N or S to indicate the direction of measurement.

2675 latitude factor. The change in LATITUDE along a CELESTIAL LINE OF POSITION per 1' change in LONGITUDE.

2676 latitude line. A LINE OF POSITION extending in a generally east-west direction.

2677 latitude scale. See SCALE.

2678 lattice. A pattern formed by two or more families of intersecting lines, such as HYPERBOLIC LINES OF POSITIONor parts of circles, drawn on CHARTS, PLOTTING SHEETS or SOUNDING BOARDS.

2679 launch. Large open or half decked boat.

2680 lava. The fluid or semi-fluid matter flowing from a volcano.The substance that results from the cooling of the molten ROCK. Part of the ocean bed is composed of lava.

2681 layer. A single thickness, coat, fold or STRATUM.

2682 layer depth. In OCEANOGRAPHY, the thickness of the MIXED LAYER; or the DEPTH to the top of the THERMO-CLINE.

2683 layer depth prediction. Predetermining the DEPTH to the top of the THERMOCLINE over a given ocean area.

2684 layering. A method of emphasizing on a CHART differences in HEIGHT or DEPTH by use of varying tints. Alsoreferred to as layer tinting.

2685 layer of no motion. A LAYER, assumed to be at rest at some DEPTH in the OCEAN.

2686 layer tinting. See LAYERING.

2687 lead (lèd). A heavy, soft, malleable, bluish-grey metallic chemical element. Anything made of this metal, such as aWEIGHT for sounding DEPTHS of water.

2688 lead (led). A navigable PASSAGE through ICE.The amount one cyclic motion is ahead of another, expressed in DEGREES.

2689 lead: deep sea. A heavy SOUNDING LEAD the WEIGHT of which may be varied from about 15 to over 50 kilosdepending on the DEPTHS to be sounded. Also called dipsey lead.

2690 lead: drift. SOUNDING LEAD dropped to BOTTOM, to indicate movement of a vessel.

125

2691 lead: fish. A fish-shaped SOUNDING LEAD which reduces to a minimum the INCLINATION of the wire whentrailed, making unnecessary its removal from the water between SOUNDINGS.

2692 lead: hand. A light SOUNDING LEAD (3 to 5 kilos) having usually a line of not more than 40 to 50 METRES.

2693 lead: sounding. A LEAD attached to a line used for measuring the DEPTH of water. Also called plummet.

2694 leader cable. See CABLE.

2695 leading light(s). See LIGHT.

2696 leading line (British terminology). A line passing through two or more clearly defined charted objects, and along whicha vessel can approach safely.

2697 leading marks. (British terminology). AIDS TO NAVIGATION or other indicators so located as to indicate the path tobe followed. Leading marks identify a LEADING LINE when they are in TRANSIT. See also RANGE.

2698 lead line. A line, graduated with attached MARKS and fastened to a SOUNDING LEAD, used for determining theDEPTH of water when making SOUNDINGS by hand. Generally used in DEPTHS of less than 45 METRES (25 FAT-HOMS). Lead lines are braided or left-laid. Also called sounding line.

2699 lead line correction. A CORRECTION to be applied to the DEPTHS taken with a rope LEAD LINE to take into ac-count the shrinking and stretching. This difficulty has been overcome by inserting a wire heart in the rope.

2700 leadsman. A person using a SOUNDING LEAD to determine DEPTH of water.

2701 league. A varying measure of distance, usually about three MILES.

2702 leaper. See LIPPER.

2703 leap year. See YEAR: CIVIL.

2704 least count. The finest reading that can be made directly (without estimation) from a VERNIER or MICROMETER.

2705 least depth. The shoalest sounding value obtained on a feature.

2706 least squares. A method of adjusting OBSERVATIONS in which the sum of the squares of all the deviations orresiduals derived in fitting the OBSERVATIONS to a mathematical model, is made a minimum. Least squares have alsobeen designated as minimum squares.

2707 ledge. A shelf-like projection, on the side of a ROCK or MOUNTAIN.A rocky formation continuous with and fringing the SHORE.

2708 lee. That side towards which the WIND blows; the sheltered side.

2709 lee shore. SHORE that is to leeward of a vessel. See WEATHER SHORE.

2710 lee tide. See TIDE: LEEWARD.

2711 leeward (adj. and adv.). Toward the LEE, or in the general direction toward which the WIND is blowing. The oppositeis WINDWARD.

2712 leeward tidal current. See TIDE: LEEWARD.

2713 leeward tide. See TIDE.

2714 leeway. The leeward motion of a vessel due to WIND. It may be expressed as distance, SPEED, or angular differencebetween COURSE steered and COURSE through the water.

DRIFT ANGLE. Also called drift.

2715 leeway angle. The angle between true heading and track of a vessel caused by wind action.

2716 left bank. Of a RIVER, the BANK of the left-hand side as one proceeds downstream.

2717 left-right indicator. An instrument indicating the orthogonal distance of a position from a pre-defined track.

126

2718 leg. Each straight section of a TRAVERSE.One part of a craft's TRACK consisting of a single COURSE line.

2719 legend. A description, explanation, table of SYMBOLS, and other information, printed on a MAP or CHART toprovide a better understanding and interpretation of it. The TITLE of a MAP or CHART formerly was considered partof the legend, but this usage is obsolete.

2720 length correction (tape). See TAPE: LENGTH CORRECTION.

2721 lens. A plane, convex, or concave combination of glass or transparent material surfaces adapted for changing thedirection of LIGHT RAYS to enlarge or reduce the apparent size of objects.

See ACHROMATIC LENS, APOCHROMATIC LENS, ANALLACTIC LENS, OBJECTIVE LENS, EYEPIECE,LENS ELEMENT, LENS SYSTEM, MAGNIFYING GLASS.

2722 lens axis. See AXIS OF LENS.

2723 lens distortion. See DISTORTION.

2724 lens element. One LENS of a complex LENS SYSTEM.

2725 lens system. The combination of two or more LENSES placed in series on the same PRINCIPAL AXIS.

2726 levee. An EMBANKMENT to prevent the overflow of a RIVER.A landing place, PIER, QUAY.A depositional EMBANKMENT bordering a CANYON, VALLEY or deep-sea channel.

2727 level. A horizontal plane or line; especially, such a plane taken as a basis for the measurement of ELEVATION. SeeSEA LEVEL, MEAN SEA LEVEL, HALF TIDE LEVEL, STILL WATER LEVEL.

A level tract of land; a stretch of country approximately horizontal and unbroken by ELEVATIONS.In SURVEYING and LEVELLING, either a SPIRIT LEVEL or a LEVELLING INSTRUMENT.

2728 level (v.t.). To make perfectly horizontal by means of a LEVEL.

2729 level: bull's eye. See LEVEL: SPIRIT.

2730 level: circular. See LEVEL: SPIRIT.

2731 level: dumpy. See LEVELLING INSTRUMENT: DUMPY LEVEL.

2732 level: plate. A SPIRIT LEVEL attached to the plate of a surveying instrument for the purpose of levelling the graduatedCIRCLE or, indirectly, making the VERTICAL AXIS truly vertical.

2733 level: self aligning.See LEVELLING INSTRUMENT: SELF ALIGNING LEVEL.

2734 level: spirit. A small closed vessel of glass (vial), having the inside surface of its upper part curved in form; the vessel isnearly filled with a fluid of low viscosity (alcohol or ether), enough free space being left for the formation of a bubble ofair and gas, which will always assume a position at the top of the vessel. There are two types of spirit levels used inSURVEYING: one has the curved surface spherical in form, producing a bubble of circular outline; and is properlycalled a circular level (also universal level or bull's eye level). The other and much more generally used type has avessel in the form of circular tube, the longitudinal axis of which is also circular in form. It is the type usually referred towhen the term 'spirit level' is used. The spirit level is also called spirit bubble or sensitive bubble.

2735 level: striding. A SPIRIT LEVEL so mounted that it can be placed above and parallel with the HORIZONTAL AXISof a surveying or astronomical instrument, and so supported that it can be used to measure the INCLINATION OF THEHORIZONTAL AXIS to the plane of the HORIZON. Generally, its mounting has supports in the form of inverted eyeswhich rest directly upon the pivots on which the TELESCOPE of the instrument rotates.

2736 level: tilting. See LEVELLING INSTRUMENT: TILTING LEVEL.

2737 level: universal. See LEVEL: SPIRIT.

2738 level: Y. See LEVELLING INSTRUMENT: Y LEVEL.

127

2739 level axis. See AXIS OF SPIRIT LEVEL.

2740 level ice. SEA ICE with a flat surface which has never been hummocked.

2741 levelling (or leveling). The operation of determining differences of ELEVATION between points on the surface of theEARTH; the determination of the ELEVATION of points relative to some arbitrary or natural LEVEL SURFACEcalled a DATUM. Also called differential levelling.

2742 levelling: barometric. A method of determining differences of ELEVATION from differences of ATMOSPHERICPRESSURE observed with a BAROMETER.

2743 levelling: direct. The determination of differences of ELEVATION by a continuous series of short horizontal lines.Vertical distances from these lines to adjacent ground marks are determined by direct OBSERVATIONS on graduatedrods with a LEVELLING INSTRUMENT equipped with a SPIRIT LEVEL.

2744 levelling: geodetic. SPIRIT LEVELLING of a high order of ACCURACY, usually extended over large areas, to furnishaccurate VERTICAL CONTROL as a basis for the CONTROL in the vertical dimension for all surveying and mappingoperations.

2745 levelling: indirect. The determination of differences of ELEVATION from (a) vertical angles and horizontal distances,as in TRIGONOMETRIC LEVELLING; (b) comparative ELEVATIONS derived from values of ATMOSPHERICPRESSURE determined with a BAROMETER, as in BAROMETRIC LEVELLING; and (c) ELEVATIONS derivedfrom values of the boiling point of water determined with a HYPSOMETER, as in THERMOMETRIC LEVELLING.

2746 levelling: reciprocal. TRIGONOMETRIC LEVELLING wherein vertical angles have been observed at both ends ofthe line to eliminate ERRORS.

2747 levelling: spirit. The determination of ELEVATIONS of points with respect to each other or with respect to a commonDATUM, by use of a LEVELLING ROD and an instrument using a SPIRIT LEVEL to establish a horizontal LINE OFSIGHT.

2748 levelling: thermometric. The determination of ELEVATIONS above SEA LEVEL from observed values of the boilingpoint of water. See HYPSOMETER.

2749 levelling: trigonometric. The determination of differences of ELEVATIONS by means of observed vertical angles,combined with lengths of lines.

2750 levelling: water. A method of obtaining relative ELEVATIONS by observing HEIGHTS with respect to the surface ofa body of still water, such as a LAKE. The relative ELEVATIONS of objects along its SHORES are obtained by takingthe differences of their HEIGHTS with respect to the surface of the water.

2751 levelling instrument. An instrument designed for the accurate measurement of height differences by the process ofLEVELLING. Its essential parts are a TELESCOPE and a SPIRIT LEVEL. Also called a LEVEL.

2752 levelling instrument: dumpy level. A LEVELLING INSTRUMENT which has its TELESCOPE permanentlyattached to the levelling base, either rigidly or by a hinge that can be manipulated by a MICROMETER SCREW.

2753 levelling instrument: self-aligning level. A LEVEL in which a sensitive prism element inside the TELESCOPE actson the principle of the PENDULUM and ensures that the LINE OF SIGHT is horizontal, even if the axis of theTELESCOPE is several MINUTES out of the HORIZONTAL.

2754 levelling instrument: tilting level. A LEVELLING INSTRUMENT in which the final LEVELLING of the instrumentis obtained by small controlled amounts of rotation of the TELESCOPE about a HORIZONTAL AXIS.

2755 levelling instrument: Y level. A LEVELLING INSTRUMENT having the TELESCOPE, with attached SPIRITLEVEL, supported in Y's. The TELESCOPE can be rotated about its longitudinal axis in the Y's and it can be lifted andreversed, end for end.

2756 levelling plate. A plate designed to support the LEVELLING ROD as an aid to ACCURACY in LEVELLING.

2757 levelling rod. A straight rod or bar, designed for use in measuring a vertical distance between a point on the ground andthe LINE OF COLLIMATION of a LEVELLING INSTRUMENT which has been adjusted to horizontal position. Alsocalled levelling staff.

128

2758 levelling screw. One of the three screws of surveying and astronomic instruments used for levelling same. Also calledfootscrew.

2759 levelling staff. See LEVELLING ROD.

2760 level net. Lines of SPIRIT LEVELLING connected together to form a system of loops and circuits extending over anarea. Also called survey net.

2761 level surface. A surface which, at every point, is perpendicular to the DIRECTION OF GRAVITY. A level surface isan EQUIPOTENTIAL SURFACE. The GEOID or, in general, any surface parallel to it, is a level surface. If changes inELEVATION due to TIDES, WINDS, etc. are neglected, the surface of the SEA is a level surface. A level surface is nota plane surface, but it is sometimes so regarded in SURVEYS of limited areas.

2762 librations of moon. Motions of the MOON due to which certain margins round the MOON's LIMB occasionally passout of view while a corresponding margin on the other side comes into view. The librations enable 59 per cent of theMOON's surface to be seen at one time or another.

2763 lidar. An instrument that measures distance to a reflecting object by emitting timed pulses of light and measuring thetime between emission and reception of reflected pulses. The measured time interval is converted to distance.

2764 life buoy. See BUOY.

2765 life saving station. A place where equipment for saving life at SEA is maintained. Also called Coast Guard station.

2766 light. A luminous or lighted AID TO NAVIGATION.For internationally agreed abbreviations of light characteristics see the International Chart Specifications.

2767 light: aerial obstruction. LIGHT marking an obstacle which constitutes a danger to AIR NAVIGATION.

2768 light: aeromarine. A marine-type LIGHT in which a proportion of the BEAM is deflected to an angle of 10°-15°above the HORIZON for the use of aircraft.

2769 light: aeronautical ground. Any LIGHT specially provided as an AID TO NAVIGATION, other than a LIGHT dis-played on an aircraft.

2770 light: alternating. A signal LIGHT that shows, in any given direction, two or more colours in a regularly repeatedsequence with a regular periodicity.

2771 light(s): boundary. LIGHTS marking the BOUNDARY of a LANDING AREA.

2772 light: catoptric. A LIGHT concentrated into a narrow BEAM by means of one or more REFLECTORS (as opposed toDIOPTRIC).

2773 light: composite group-flashing. A flashing light in which a sequence of groups of one or more flashes, which arespecified in number, is regularly repeated, and the groups comprise different numbers of flashes.

2774 light: composite group-occulting. An occulting light in which a sequence of groups of one or more eclipses, which arespecified in number, is regularly repeated, and the groups comprise different numbers of eclipses.

2775 light: continuous ultra quick. A light showing 160 or more, usually 240 to 300, flashes per minute.

2776 light: continuous very quick. A light showing 80 to 159, usually either 100 or 120, flashes per minute.

2777 light: dioptric. A LIGHT concentrated into a parallel BEAM by means of refracting LENSES and prisms (as opposedto CATOPTRIC).

2778 light: direction (or directional). A LIGHT illuminating a sector of very narrow angle and intended to mark aDIRECTION to be followed.

2779 light: equal-interval. A LIGHT with all durations of light and darkness equal. The term 'equal-interval light' has beenreplaced by isophase light in many countries.

2780 light: fixed. A signal light that shows continuously, in any given direction, with constant luminous intensity and colour.

129

2781 light: fixed and flashing. A rhythmic light in which a fixed light is combined with a flashing light of higher luminousintensity.

2782 light: fixed and group-flashing. A FIXED LIGHT varied at regular intervals by a GROUP of two or more flashes ofgreater brilliance. This CHARACTER is being gradually suppressed.

2783 light: flashing. A rhythmic LIGHT in which the total duration of light in a PERIOD is clearly shorter than the totalduration of darkness and all the appearances of light are of equal duration.

2784 light: front. See LIGHT(S): RANGE.

2785 light: group-flashing. A flashing LIGHT in which a group of two or more flashes, which are specified in number, isregularly repeated.

2786 light: group-occulting. An occulting LIGHT in which a group of two or more eclipses, which are specified in number,is regularly repeated.

2787 light: group quick. A light in which flashes are combined in groups including the same number of quick flashes(repetition rate : 50-79 per minute) and in which groups are repeated at regular intervals.

2788 light: group very quick. A light in which very quick flashes are combined in groups including the same number offlashes (repetition rate : 80-159 per minute) and in which groups are repeated at regular intervals.

2789 light: intermittent. See LIGHT: RHYTHMIC.

2790 light: interrupted quick-flashing. A quick LIGHT in which the sequence of flashes is interrupted by regularlyrepeated ECLIPSES of constant and long duration.

2791 light: interrupted ultra quick. A light in which the ultra quick flashes (160 or more per minute) are interrupted atregular intervals by eclipses of long duration.

2792 light: interrupted very quick. A light in which the very rapid alterations of light and darkness are interrupted atregular intervals by eclipses of long duration.

2793 light: isophase. See LIGHT: EQUAL-INTERVAL.

2794 light(s): leading. Two or more LIGHTS associated so as to form a LEADING LINE to be followed.

2795 light: linear. A luminous SIGNAL having perceptible length, as contrasted with a POINT LIGHT.

2796 light: long-flashing. A FLASHING LIGHT in which a single flash of not less than two SECONDS duration is regu-larly repeated.

2797 light: marine. A LIGHT intended primarily for MARINE NAVIGATION.

2798 light: Morse code. A rhythmic LIGHT in which appearances of light of two clearly different durations are grouped torepresent a character or characters in the Morse code.

2799 light: obstruction. LIGHT indicating the presence of an OBSTRUCTION.

2800 light: occasional. A LIGHT not regularly exhibited.

2801 light: occulting. A rhythmic LIGHT in which the total duration of light in a PERIOD is clearly longer than the totalduration of darkness and all the eclipses are of equal duration.

2802 light: point. A luminous SIGNAL appearing without perceptible length, as contrasted with a LINEAR LIGHT.

2803 light: quick-flashing. A LIGHT exhibiting without interruption very rapid regular alternations of light and darkness.

2804 light(s): range. Two or more LIGHTS in the same horizontal DIRECTION, particularly those LIGHTS so placed asNAVIGATIONAL AIDS to mark any line of importance to vessels, as a CHANNEL. The one nearest the observer isthe front light and the one farthest from the observer is the rear light. See LIGHT(S): LEADING.

2805 light: rear. See LIGHT(S): RANGE.

130

2806 light: revolving. See LIGHT: ROTATING.

2807 light: rhythmic. A signal LIGHT that shows intermittently, in any given direction, with a regular periodicity. Alsoreferred to as intermittent light.

2808 light: rotating (or revolving). A LIGHT which has one or more BEAMS which rotate.

2809 light(s): runway. LIGHTS marking a RUNWAY.

2810 light: sector (or sectored). A LIGHT presenting different appearances (in particular, different colours) over variousparts of the HORIZON of interest to maritime NAVIGATION.

2811 light: short-flashing. A FLASHING LIGHT having individual flashes of less than two seconds duration.

2812 light(s): tidal. LIGHTS shown at the ENTRANCE of a HARBOUR to indicate TIDE and TIDAL CURRENTconditions within the HARBOUR.

2813 light: undulating. A continuously luminous LIGHT which alternatively increases and decreases in brightness in cyclicsequence.

2814 light: unwatched. A navigational LIGHT, usually automatic in operation, without any permanently-stationed personnelto superintend it.

2815 light(s): vertical. Two or more LIGHTS disposed vertically to give a different CHARACTER or appearance.

2816 light (or lighted) beacon. See BEACON.

2817 light beam. A group of PENCILS of light, as those originating at the many points of an illuminated surface. A BEAMof parallel LIGHT RAYS is a special case in which each PENCIL is of such small cross-section that it may be regardedas a RAY.

2818 light (or lighted) buoy. See BUOY.

2819 light characteristics. See CHARACTERISTICS OF LIGHT.

2820 lighted sound buoy. See BUOY.

2821 light float. A boat-like structure used instead of a light buoy in waters where strong streams or currents are experienced,or when a greater elevation than that of a light buoy is necessary.

2822 lighthouse. A distinctive structure on or off a COAST exhibiting a major LIGHT designed to serve as an AID TONAVIGATION.

2823 light list. See LIST OF LIGHTS.

2824 lightning. Luminous manifestations accompanying a sudden electrical discharge which takes place from or inside aCLOUD or, less often, from high structures on the GROUND or from MOUNTAINS.

2825 light pencil. A bundle of LIGHT RAYS originating at, or directed to, a single point.

2826 light ray. The geometrical concept of a single element of light propagated in a straight line and of infinitesimal cross-section; used in analytically tracing the path of light through an OPTICAL SYSTEM.

2827 light sector. A sector in which a navigational LIGHT is visible. See ARC OF VISIBILITY.

2828 lightship. A distinctively marked vessel anchored or moored at a charted point, to serve as an AID TO NAVIGATION.By night it displays a characteristic LIGHT(S), and is usually equipped with other devices, such as a FOG SIGNAL,SUBMARINE SOUND SIGNAL, and RADIOBEACON, to assist NAVIGATION. Also called light vessel.

2829 light vessel. See LIGHTSHIP.

2830 light-year. A unit of length equal to the distance light travels in one YEAR. This unit is used as a measure of stellardistance.

131

2831 Lighter Aboard Ship (LASH). A cargo carrying system using specially built ships and lighters. Cargoes are loadedinto LASH lighters which are towed to a LASH ship where they are embarked. At their destination the LASH lightersare disembarked and towed away to their unloading berths. Special berths or anchorages are sometimes designated forLASH ships.

2832 limb. The graduated curved part of an instrument for measuring angles, as that part of a MARINE SEXTANT carryingthe altitude scale, or arc.

The circular outer edge of a CELESTIAL BODY. The half with the greatest ALTITUDE is called the upper limband the half with the least ALTITUDE, the lower limb.

2833 limestone. A ROCK which consists chiefly of calcium carbonate.

2834 limnology. The scientific study of the biological, chemical, geographical, and physical features of fresh waters,especially of LAKES and PONDS.

2835 linear light. See LIGHT.

2836 linear parallax. See PARALLAX: ABSOLUTE STEREOSCOPIC.

2837 linear scale. See SCALE: BAR.

2838 linear speed. See SPEED.

2839 line of apsides. The line connecting the two orbital points (called apsides) that are nearest and farthest from the centreof attraction, as the PERIGEE and the APOGEE in the case of an ORBIT about the EARTH, and the PERIHELION andthe APHELION in the case of an ORBIT about the SUN.

2840 line of collimation. See COLLIMATION.

2841 line of constant scale. Any line on a PHOTOGRAPH which is parallel to the TRUE HORIZON or to the ISOMETRICPARALLEL.

2842 line of delimitation. A line drawn on a map or chart depicting the separation of any type of maritime jurisdiction.

2843 line of equidistance. See MEDIAN LINE.

2844 line of flow. The vector line showing the DIRECTION OF CURRENT at a given TIME. Also called STREAMLINE.

2845 line of force. A line indicating the direction in which a force acts, as in a MAGNETIC FIELD. See MAGNETICLINES OF FORCE.

2846 line of levels. A continuous series of measured differences of ELEVATION.

2847 line of nodes. The straight line connecting the two points of intersection of the ORBIT of a PLANET, PLANETOID, orCOMET and the ECLIPTIC, or the line of intersection of the planes of the ORBITS of a SATELLITE and its primary.See NODE.

2848 line of position. A line indicating a series of possible POSITIONS of a craft, determined by OBSERVATION ormeasurement. Also called position line. See CELESTIAL LINE OF POSITION, CIRCLE OF POSITION, COURSELINE, ELECTRONIC LINE OF POSITION, FIX, HYPERBOLIC LINE OF POSITION, LATITUDE LINE,LEADING LINE, LONGITUDE LINE, RETIRED LINE OF POSITION, SONIC LINE OF POSITION, SPEED LINE,SUMNER LINE, SURFACE OF POSITION, VISUAL LINE OF POSITION.

2849 line of sight. The straight line between two points. This line is in the direction of a GREAT CIRCLE, but does notfollow the curvature of the EARTH. See also COLLIMATION: LINE OF.

2850 line of sounding. A series of SOUNDINGS along a predetermined line, and at stated intervals, when SURVEYING.Also referred to as sounding line. See CHECK LINES, CROSS LINES, CROSS-SECTION LINES, INTER LINE,SPLIT LINE, STARRED LINES, TRACK LINE OF SOUNDING, SYSTEMS OF SOUNDING LINES.

2851 line spacing. Distance between sounding lines. Spacing usually depends on the type of survey, the instruments used,and the demanded accuracy.

2852 line squall. SQUALL which occurs along a SQUALL LINE.

132

2853 lipper. Slight ruffling or roughness on a water surface.Light spray from small WAVES. Also called leaper.

2854 liquid compass. See COMPASS.

2855 list. INCLINATION to one side, as of a vessel until she has found EQUILIBRIUM in an inclined position. Listing is astatic condition, as distinguished from heeling which is dynamic. See HEEL.

2856 list of lights. A publication tabulating navigational LIGHTS, with their locations, CANDLE POWERS, CHARACTE-RISTICS, etc. to assist in their identification, and details of any accompanying FOG SIGNAL. A list of lights maycontain other information useful to a navigator. Also called light list.

2857 list of radio signals. A publication tabulating and combining particulars of: coast radio stations, port radio stations,RADIO DIRECTION FINDING STATIONS, RADIOBEACONS, etc., as well as other information on radio servicesuseful to a navigator.

2858 lithification. The process of induration, cementation, petrification, consolidation, and crystallization which convertMAGMA and newly deposited SEDIMENTS into ROCK.

2859 lithographic image. An ink-receptive image on the lithographic press plate, either photographic or direct hand ortransfer image. The design or drawing on stone or metal plate.

2860 lithographic transfer. A PROOF of design or type matter pulled with special transfer ink on transfer paper from aLITHOGRAPHIC IMAGE or a relief image. Such PROOFS are used to transfer an IMAGE to the PLATE or stone.

2861 lithography. The art or process of printing from a flat stone or metal PLATE by a method based on the repulsionbetween grease and water. The original lithographic material was Bavarian LIMESTONE; but zinc and aluminium arenow used.

2862 lithology. The scientific study of ROCKS. Literally, the science of STONES.

2863 lithosphere. The solid, rocky part of the EARTH; EARTH's CRUST distinguished from ATMOSPHERE,HYDROSPHERE.

2864 litre (or liter). The basic unit of capacity in the METRIC SYSTEM. It is equal to one cubic decimetre.

2865 littoral (adj.). Of or pertaining to a SHORE, especially of the SEA.

2866 littoral (n.). A coastal region.INTERTIDAL ZONE.

2867 local apparent noon. See NOON.

2868 local apparent time. See TIME.

2869 local attraction. LOCAL MAGNETIC DISTURBANCE.Also deflection of the PLUMB LINE due to a MOUNTAIN or other irregularity in EARTH's CRUST.

2870 local frequency oscillator. An oscillator used to derive an intermediate frequency by beating with the signal carryingfrequency in Superheterodyne reception.

2871 local horizon. See HORIZON: APPARENT.

2872 local hour angle. See HOUR ANGLE.

2873 localizer. A radio facility which provides SIGNALS for lateral GUIDANCE of aircraft with respect to a RUNWAYcentre line.

2874 local magnetic disturbance. An ANOMALY of the MAGNETIC FIELD of the EARTH, extending over a relativelysmall area, due to local magnetic influences. Also called local attraction or magnetic anomaly.

2875 local mean time. See TIME.

2876 local meridian. See MERIDIAN.

133

2877 local sidereal time. See TIME.

2878 local time. See TIME.

2879 local transit. See TRANSIT.

2880 loch. The Scottish term for (1) a LAKE, and (2) a FIORD.

2881 lock. A WET DOCK in a WATERWAY, permitting a ship to pass from one level to another. See TIDE LOCK.

2882 lock gate(s). Pair of massive hinged doors at each end of a LOCK.

2883 locking. In ELECTRONIC NAVIGATION, a term describing the condition of two TRANSMITTERS when theirSIGNALS maintain a stable phase relationship. One STATION is then said to be locked to the other.

2884 locus (pl. loci). All possible positions of a point or curve satisfying stated conditions.

2885 log. An instrument for measuring the SPEED or distance or both travelled by a vessel.

2886 log (v.t.). Said of a ship moving at a specified SPEED according to the indications given by the LOG.

2887 log: chip. A LOG consisting essentially of a weighted wooden quadrant attached to a bridle in such a manner that it willfloat in a vertical position, and a LOG LINE, SPEED being measured by casting the quadrant overboard and countingthe KNOTS in the line paid out in unit time.

2888 log: Dutchman's. A buoyant object thrown overboard to determine the SPEED of a vessel by timing its passagebetween two marks, of known distance apart, on the vessel.

2889 log: electric. A graphic recording of the various electrical properties of a SEDIMENT or ROCK through which a holehas been drilled.

2890 log: electric submerged. An electrically-operated LOG installed inside the hull of the vessel with a tube extendingthrough the hull into the water.

2891 log: electromagnetic. A LOG consisting of a pulsating MAGNET mounted on a probe pointing downwards with twocontacts on the sides of the probe. The movement of the water through the MAGNETIC FIELD is reflected by thecontacts and the ripple of voltage thus induced can be converted to measure the SPEED of the craft.

2892 log: ground. A device for determining the COURSE and SPEED made good over the GROUND in SHALLOWWATER, consisting of a LEAD attached to a line. The LEAD is thrown overboard and allowed to rest on theBOTTOM. The COURSE being made good is indicated by the direction the line tends and the SPEED by the amount ofline paid out in unit time.

2893 log: patent. Former name for a towed LOG.Any mechanical LOG, particularly a TAFFRAIL LOG.

2894 log: taffrail. A LOG consisting essentially of a rotator towed through the water by a braided LOG LINE attached to adistance registering device usually secured at the TAFFRAIL, the railing at the stern. Also called PATENT LOG.

2895 logarithmic scale. See SCALE.

2896 log book. Book in which events connected with the ship are entered. Also written as one word.

2897 log boom. Heavy logs chained or lashed together and moored or anchored so as to enclose and contain rafted logs. Seealso BOOM.

2898 log chip. The wooden quadrant forming part of a CHIP LOG. Also called log ship.

2899 log factor. The ratio represented by the true distance divided by the LOG distance.

2900 log factor scale. See SCALE.

2901 logging. 1. Gauging ship's speed with a log. 2. Record events.

134

2902 log line. A graduated line used to measure the SPEED of a vessel through the water or to measure the SPEED of aCURRENT from a vessel at anchor. The line secured to a LOG. See CURRENT POLE.

2903 log ship. See LOG CHIP. Also written as one word.

2904 long baseline acoustic system. An underwater position fixing system which consists of an array of seabed transponderswith baselines of several kilometres.

2905 longitude. ANGULAR DISTANCE, along a PRIMARY GREAT CIRCLE, from the adopted REFERENCE POINT.One of COORDINATES used to describe a POSITION the other being LATITUDE.

2906 longitude: astronomical. The angle between the plane of the CELESTIAL MERIDIAN and the plane of an INITIALMERIDIAN, arbitrarily chosen. Astronomical longitude is the LONGITUDE which results directly from OBSERVA-TIONS on CELESTIAL BODIES, uncorrected for DEFLECTION OF THE VERTICAL.

2907 longitude: celestial. ANGULAR DISTANCE east of the VERNAL EQUINOX, along the ECLIPTIC; the arc of theECLIPTIC or the angle at the ecliptic pole between the CIRCLE OF LATITUDE of the VERNAL EQUINOX and theCIRCLE OF LATITUDE of a point on the CELESTIAL SPHERE, measured eastward from the CIRCLE OFLATITUDE of the VERNAL EQUINOX, through 360°.

2908 longitude: geodetic. The angle between the plane of the GEODETIC MERIDIAN and the plane of an INITIALMERIDIAN, arbitrarily chosen.

2909 longitude: geographic(al). A general term, applying alike to ASTRONOMICAL and GEODETIC LONGITUDE.

2910 longitude: terrestrial. The intercepted arc of EQUATOR, or the angle at the POLE, between the MERIDIAN of apoint on the EARTH, and the PRIME MERIDIAN, measured eastward or westward from the PRIME MERIDIANthrough 180° and labelled E or W to indicate the direction of measurement.

2911 longitude factor. The change in LONGITUDE along a CELESTIAL LINE OF POSITION per 1' change inLATITUDE.

2912 longitude line. A LINE OF POSITION extending in a generally north-south direction.

2913 longitude signal. See SIGNAL.

2914 longitudinal wave. See WAVE.

2915 long range navigation. See NAVIGATION.

2916 longshore current. A current paralleling the shore largely within the surf zone. It is caused by the excess water broughtto the zone by the small net mass transport of wind waves. Longshore currents feed into rip currents.

2917 lookout station. A distinctive structure or place on SHORE from which personnel keep watch upon events at SEA oralong the COAST.

2918 lookout tower. Any tower surmounted by a small house in which a watch is habitually kept, as distinguished from anOBSERVATION TOWER in which no watch is kept.

2919 loom. The glow of a light which is below the HORIZON, caused by REFLECTION by solid particles in the air.Vague first appearance of LAND at SEA.

2920 looming. An apparent ELEVATION of distant terrestrial objects by abnormal ATMOSPHERIC REFRACTION.Because of looming, objects below the HORIZON are sometimes visible. The opposite is SINKING.

The appearance indistinctly of an object during a period of low VISIBILITY.

2921 loop. In OCEANOGRAPHY, that part of a STANDING WAVE where the vertical motion is greatest and the horizontalVELOCITIES are least. Loops (sometimes called antinodes) are associated with CLAPOTIS and with SEICHE actionresultant from resonant wave reflecting in a HARBOUR or BAY.

2922 loop antenna. A DIRECTIONAL ANTENNA consisting essentially of one or more complete turns of wire.

2923 'lost' station. See STATION: UNRECOVERABLE.

2924 lough. Irish equivalent of the Scottish LOCH.

135

2925 louver shutter. See SHUTTER.

2926 love wave (or Q-wave). See WAVE.

2927 low. See DEPRESSION.

2928 lower branch. That half of a MERIDIAN or CELESTIAL MERIDIAN from POLE to POLE which passes through theANTIPODE or NADIR of a place.

2929 lower culmination. See MERIDIAN TRANSIT.

2930 lower high water (L.H.W.). The lower of two HIGH WATERS occurring during a TIDAL DAY if the DIURNALINEQUALITY prevails.

2931 lower high water interval. See LUNITIDAL INTERVAL.

2932 lower limb. See LIMB.

2933 lower low water (L.L.W.). The lower of two LOW WATERS occurring during a TIDAL DAY if the DIURNALINEQUALITY prevails.

2934 lower low water interval. See LUNITIDAL INTERVAL.

2935 lower transit. See MERIDIAN TRANSIT.

2936 lowest astronomical tide. The lowest tide level which can be predicted to occur under average meteorologicalconditions and under any combination of astronomical conditions.

2937 lowest low water. An arbitrary LEVEL conforming to the lowest TIDE observed at a place, or somewhat lower.

2938 low frequency (LF). See FREQUENCY: RADIO.

2939 lowland. Low and relatively level LAND at a lower ELEVATION than adjoining districts.

2940 low oblique photograph. See PHOTOGRAPH.

2941 low tide. See LOW WATER.

2942 low-tide elevation. A naturally formed area of land which is surrounded by and above water at low tide, but submergedat high tide.

2943 low velocity layer. Any LAYER in which the VELOCITY of compressional wave propagation is lower than in theadjacent LAYERS. Such a LAYER can act as an efficient channel for the propagation of elastic WAVES for greatdistances.

2944 low water (L.W.). The lowest LEVEL reached at a place by the water surface in one OSCILLATION. Also called lowtide.

2945 low water: double. A LOW WATER consisting of two minima separated by a relatively small elevation. See TIDE:DOUBLE.

2946 low water full and change (L.W.F. and C.). The average interval of time between the TRANSIT (upper or lower) ofthe full or new MOON and the next LOW WATER.

2947 low water inequality. See DIURNAL INEQUALITY.

2948 low water interval. See LUNITIDAL INTERVAL.

2949 low water line. See LOW WATER MARK.

2950 low water lunitidal interval (L.W.I.). Low water interval. See LUNITIDAL INTERVAL.

2951 low water mark. The intersection of the plane of LOW WATER with the SHORE. The line along a COAST, orBEACH, to which the SEA recedes at LOW WATER. Also called low water line.

136

2952 low water quadrature (L.W.Q.). The average low water interval when the MOON is at QUADRATURE.

2953 low water stand. See STAND OF TIDE.

2954 loxodrome. A curve, on the surface of a sphere, intersecting all GREAT CIRCLES of the sphere at a constant obliqueangle, theoretically never reaching the POLE while closely approaching it. See RHUMB LINE.

2955 loxodromic curve. A LOXODROME. A RHUMB LINE.

2956 lubber (or lubber's) line. A REFERENCE LINE on any direction-indicating instrument, marking the reading whichcoincides with the HEADING.

2957 lull. A momentary decrease in the SPEED of the WIND.

2958 lumen. The unit of LUMINOUS FLUX in the SI SYSTEM.

2959 luminance (at a point of surface, in a direction). Quotient of the LUMINOUS INTENSITY, in a given direction, of aninfinitesimal element of the surface containing the point under consideration, by the orthogonally projected area of theelement on a plane perpendicular to the given direction.

2960 luminescence. Any emission of light at temperatures below that required for incandescence. SeeBIOLUMINESCENCE, FLUORESCENCE, PHOSPHORESCENCE.

2961 luminous flux. See FLUX.

2962 luminous intensity. See INTENSITY.

2963 luminous range. See RANGE.

2964 luminous signal. See SIGNAL.

2965 lunar day. See DAY.

2966 lunar distance. The angle, at an observer on the EARTH, between the MOON and another CELESTIAL BODY. Thiswas the basis of a method formerly used to determine LONGITUDE at SEA.

2967 lunar eclipse. See ECLIPSE.

2968 lunar inequality. Variation in the MOON's motion in its ORBIT, due to attraction by other bodies of the SOLARSYSTEM. See EVECTION, PERTURBATION.

A minute fluctuation of a MAGNETIC NEEDLE from its mean position, caused by the MOON.

2969 lunar month. See MONTH: SYNODICAL.

2970 lunar rainbow. See MOONBOW.

2971 lunar tide. See under TIDE.

2972 lunation. See MONTH: SYNODICAL.

2973 lunicurrent interval. The interval between the moon's transit (upper or lower) over the local or Greenwich meridianand a specified phase of the tidal current following the transit. Examples are strength of flood interval and strength ofebb interval, which may be abbreviated to flood interval and ebb interval, respectively. The interval is described as localor Greenwich according to whether the reference is to the moon's transit over the local or Greenwich meridian. Whennot otherwise specified, the reference is assumed to be local.

2974 lunitidal interval. The interval of time between the TRANSIT (upper or lower) of the MOON over the LOCAL orGREENWICH MERIDIAN and the next HIGH WATER or LOW WATER at a place. The interval is assumed to belocal unless otherwise specified. The average of all high water intervals is called mean high water lunitidal interval,high water interval, or corrected establishment. The average of all low water intervals is called mean low waterlunitidal interval, or low water interval. The expressions higher high water interval, lower high water interval, higherlow water interval, and lower low water interval are used when there is considerable DIURNAL INEQUALITY. SeeESTABLISHMENT.

2975 lurch. A sudden ROLL to one side. See LIST.

137

2976 lux. Unit of ILLUMINATION in the SI SYSTEM.

2977 L-Z graph. A graph used to determine 'in situ' DEPTHS of oceanographic OBSERVATIONS by the wire depths minusTHERMOMETRIC DEPTH method.

138

M

2978 madrepore. A branching or stag-horn CORAL, also any perforated stone coral.

2979 magenta (adj.). A mixture of red and blue in equal parts.

2980 magma. The molten material in a state of fusion under the CRUST of the EARTH from which IGNEOUS ROCKS areformed.

2981 magnet. A body which produces a MAGNETIC FIELD around itself. It has the property of attracting certain materialscapable of being magnetized. See HEELING MAGNET.

2982 magnetic annual change. The amount of SECULAR CHANGE in the EARTH'S MAGNETIC FIELD which occurs inone YEAR. Also referred to as annual change.

2983 magnetic annual variation. The small systematic TEMPORAL VARIATION in the EARTH'S MAGNETIC FIELDwhich occurs after the trend for SECULAR CHANGE has been removed from the average monthly values. Alsoreferred to as annual variation.

2984 magnetic anomaly. See LOCAL MAGNETIC DISTURBANCE.

2985 magnetic azimuth. See AZIMUTH.

2986 magnetic bearing. See BEARING.

2987 magnetic chart. See CHART.

2988 magnetic compass. See COMPASS.

2989 magnetic compensation. See COMPENSATION OF MAGNETIC COMPASS.

2990 magnetic compensator. See COMPENSATOR.

2991 magnetic course. See COURSE.

2992 magnetic daily (or diurnal) variation. OSCILLATIONS of the EARTH'S MAGNETIC FIELD which have aperiodicity of about a day and that depend to a close approximation only on LOCAL TIME and GEOGRAPHICLATITUDE.

2993 magnetic declination. See VARIATION.

2994 magnetic deviation. See DEVIATION.

2995 magnetic dip. See DIP.

2996 magnetic element. VARIATION, DIP, or MAGNETIC INTENSITY.

2997 magnetic equator. See EQUATOR.

2998 magnetic field. The space in which a magnetic influence exists.

2999 magnetic field intensity. The magnetic force exerted on a UNIT MAGNETIC POLE placed at any specified point ofspace. It is a vector quantity. Its direction is taken as the direction toward which a north MAGNETIC POLE would tendto move under the influence of the field. Also called magnetic intensity, magnetic field strength.

3000 magnetic field strength. See MAGNETIC FIELD INTENSITY.

3001 magnetic inclination. See DIP.

3002 magnetic induction. The act or process by which material becomes magnetized when placed in a MAGNETIC FIELD.

3003 magnetic intensity. See MAGNETIC FIELD INTENSITY.

139

3004 magnetic latitude. See LATITUDE.

3005 magnetic lines of force. Closed lines indicating by their direction the direction of magnetic influence.

3006 magnetic meridian. See MERIDIAN.

3007 magnetic moment. The quantity obtained by multiplying the distance between two MAGNETIC POLES by theaverage strength of the poles.

3008 magnetic needle. A small, slender magnetized bar which tends to align itself with MAGNETIC LINES OF FORCE.

3009 magnetic north. See NORTH.

3010 magnetic observation(s). See OBSERVATION.

3011 magnetic observatory. A place where buildings are equipped for observing temporal changes in the EARTH'SMAGNETIC FIELD.

3012 magnetic permeability. The property of a magnetizable substance that determines the degree in which it modifies themagnetic flux in the region occupied by it.

3013 magnetic pole. Either of the two places on the surface of the EARTH where the magnetic DIP is 90°, that in thenorthern hemisphere being designated north magnetic pole, and that in the southern hemisphere being designated southmagnetic pole. The magnetic poles are not fixed and do not coincide with the GEOGRAPHICAL POLES.

Either of those two points of a MAGNET where the magnetic force is greatest.

3014 magnetic range. See RANGE.

3015 magnetic retentivity. The ability to retain MAGNETISM after removal of the magnetizing force. Also calledremanence.

3016 magnetic secular change. The temporal variation in the value of a MAGNETIC ELEMENT which occurs over aperiod of years.

3017 magnetic station. See STATION.

3018 magnetic storm. A disturbance in the EARTH'S MAGNETIC FIELD, associated with abnormal solar activity, andcapable of seriously affecting both radio and wire transmission.

3019 magnetic survey. See SURVEY.

3020 magnetic temporal variation. Any change in the EARTH'S MAGNETIC FIELD which is a function of time. Alsoreferred to as temporal variation.

3021 magnetic variation. See VARIATION.

3022 magnetism. The ability to attract magnetic material, notably iron and steel.

3023 magnetism: induced. MAGNETISM acquired by a piece of magnetic material while it is in a MAGNETIC FIELD.

3024 magnetism: permanent. MAGNETISM which is retained for long periods without appreciable reduction, unless theMAGNET is subjected to a demagnetizing force. Because of the slow dissipation of such MAGNETISM, it is some-times called subpermanent magnetism.

3025 magnetism: residual. MAGNETISM which remains after removal of the magnetizing force.

3026 magnetism: subpermanent. See MAGNETISM: PERMANENT.

3027 magnetism: temporary. MAGNETISM of craft which changes in amount with every change of HEADING. Alsocalled transient magnetism.

3028 magnetism: terrestrial. See GEOMAGNETISM.

3029 magnetism: transient. See MAGNETISM: TEMPORARY.

140

3030 magnetization. The degree to which a body is magnetized. The MAGNETIC MOMENT per unit volume. Also calledintensity of magnetization.

3031 magnetize (v.t.). To produce magnetic properties. The opposite is DEMAGNETIZE.

3032 magnetogram. An analogue time record of the temporal variations in a MAGNETIC ELEMENT measured at aMAGNETIC OBSERVATORY.

3033 magnetograph. A MAGNETIC OBSERVATORY instrument consisting of one or more (usually 3) VARIOMETERS,one for each MAGNETIC ELEMENT recorded, and a recorder.

3034 magnetometer. An instrument for measuring the intensity and/or the direction of the EARTH'S MAGNETIC FIELD.

3035 magnetometer: cesium. An OPTICAL PUMPING MAGNETOMETER which employs a cesium absorption cell.

3036 magnetometer: flux-gate. An instrument which uses a wire wound saturable core inductor to measure the EARTH'SMAGNETIC FIELD.

3037 magnetometer: helium. An OPTICAL PUMPING MAGNETOMETER which employs a helium absorption cell.

3038 magnetometer: nuclear precession. A MAGNETOMETER that utilizes the precessional characteristics of hydrogennuclei when in an ambient MAGNETIC FIELD. Also known as proton precession magnetometer.

3039 magnetometer: optical pumping. A highly sensitive MAGNETOMETER which employs a metastable helium oralkali metal absorption cell whose ATOMS absorb maximum energy from an infrared BEAM when a frequencymodulation oscillator is tuned to its resonant FREQUENCY. The resonant FREQUENCY is proportional to the strengthof the EARTH'S MAGNETIC FIELD.

3040 magnetometer: proton precession. See MAGNETOMETER: NUCLEAR PRECESSION.

3041 magnetometer: quartz horizontal. A MAGNETOMETER in which the torque exerted on a suspended MAGNET bythe EARTH's horizontal MAGNETIC FIELD is balanced against the torque exerted by the quartz suspension fibre whentwisted through 360o.

3042 magnetometer: rubidium vapour. An OPTICAL PUMPING MAGNETOMETER which employs a rubidium vapourabsorption cell.

3043 magnetometer: transit. An instrument used primarily for observing the declination of the EARTH'S MAGNETICFIELD.

3044 magnetometer: unifilar. A MAGNETOMETER, designed for use on SHORE, employed to measure the horizontalintensity of the EARTH's MAGNETISM; it can also be used to determine VARIATION.

3045 magnetometer: vector airbone. A FLUX-GATE MAGNETOMETER which measures the intensity and direction ofthe EARTH'S MAGNETIC FIELD from an aircraft.

3046 magnetometer survey. See SURVEY.

3047 magnetometric zero balance. A MAGNETIC OBSERVATORY instrument used for accurate measurement of thevertical component of the EARTH'S MAGNETIC FIELD.

3048 magnetostriction. The PHENOMENON wherein ferromagnetic material experience an elastic strain when subjected toan external MAGNETIC FIELD. Also, the converse PHENOMENON in which mechanical stresses cause a change inthe MAGNETIC INDUCTION of a ferromagnetic material.

3049 magnetostriction transducer. See TRANSDUCER.

3050 magnetron. An electron tube for converting direct-current energy into radio-frequency energy by means of aMAGNETIC FIELD.

3051 magnification. The apparent enlargement of anything. In optics, the term relates to the properties of LENSES andLENS SYSTEMS to produce an IMAGE which differs in dimension from the related object.

141

3052 magnifying glass. A LENS or combination of LENSES that increases the apparent size of an object seen through it.

3053 magnifying power. The ratio of the apparent length of a linear dimension as seen through an optical instrument to thatseen by the unaided eye. Sometimes shortened to power.

3054 magnitude. The importance, quality or size of something.In ASTRONOMY, relative brightness of a CELESTIAL BODY. According to the accepted classification, a STAR

of the 1st magnitude is a hundred times as bright as a STAR of the 6th magnitude (the faintest STAR that can be seenwith the unaided eye) and the distribution of magnitudes is logarithmic in terms of brightness. Thus each reduction inmagnitude means an increase of brightness of about 2 1/2.

3055 mainland. The principal LAND or largest part of a CONTINENT, as distinguished from a relatively small ISLAND orPENINSULA.

3056 main scheme station. See STATION.

3057 maintained depth. The depth at which a channel is kept by human influence, usually by dredging.

3058 maintenance. All procedures and activities required to keep equipment operational.

3059 major planet. See PLANET.

3060 make the land. To sight and approach or reach LAND from seaward.

3061 make way. To progress through the water.

3062 making way. Progressing through the water.

3063 manoeuvre. Planned and controlled movement of a vehicle.

3064 mangrove. One of several genera of tropical trees or shrubs which produce many prop roots and grow along low-lyingCOASTS into SHALLOW WATER.

3065 manometer. An instrument for measuring pressure of gases and vapours.

3066 mantle. The relatively plastic region between the CRUST and CORE of the EARTH. Also called asthenosphere.

3067 manuscript map. See MAP.

3068 map. A representation (usually on a flat medium) of all or a portion of the EARTH or other CELESTIAL BODY,showing the relative size and position of features to some given SCALE or PROJECTION; also, a representation of allor part of the CELESTIAL SPHERE. A map may emphasize, generalize, or omit the representation of certain featuresto satisfy specific requirements. Maps are frequently categorized and referred to according to the type of informationwhich they are primarily designed to convey, to distinguish them from maps of other types.

3069 map (v.t.). To prepare a MAP or engage in a mapping operation.

3070 map: anaglyphic. A MAP specially printed in two complementary colours in such a way that, when viewed through atwin EYEPIECE of the same colours, a three-dimensional impression of RELIEF is seen.

3071 map: base. A MAP showing certain fundamental information, used as a base upon which additional data of specializednature are compiled. Also, a MAP containing all the information from which MAPS showing specialized informationcan be prepared; a source map.

3072 map: cadastral. A MAP showing the BOUNDARIES of subdivision of LAND, usually with the BEARINGS andlengths thereof and the area of individual tracts, for purposes of describing and recording ownership. A cadastral mapmay also show culture, drainage, and other features relating to the value and use of LAND. See also PLAT.

3073 map: contour. A TOPOGRAPHIC MAP which portrays RELIEF by means of CONTOURS.

3074 map: hemispherical. A MAP of one half of the EARTH's surface, bounded by the EQUATOR, or by MERIDIANS.

3075 map: index. In AERIAL PHOTOGRAPHY, a MAP showing the location and numbers of FLIGHT STRIPS andPHOTOGRAPHS.

142

3076 map: manuscript. The original drawing of a MAP as compiled or constructed from various DATA, such as GROUNDSURVEYS, PHOTOGRAPHS, etc.

3077 map: planimetric. A MAP which presents only the horizontal positions for the features represented; distinguished froma TOPOGRAPHIC MAP by the omission of RELIEF in measurable form.

3078 map: reconnaissance. The plotted result of a RECONNAISSANCE SURVEY.

3079 map: relief. A MAP emphasizing RELIEF or relative ELEVATIONS. See RELIEF MODEL.

3080 map: sketch. A MAP drawn freehand and greatly simplified which, although preserving the essential spacerelationships, does not truly preserve SCALE or ORIENTATION.

3081 map: special purpose. Any MAP designed primarily to meet specific requirements.

3082 map: substitute. A hasty reproduction of wide-coverage AERIAL PHOTOGRAPHS, PHOTOMAPS, or MOSAICS,or of provisional MAPS, or any document used in place of a MAP, when the precise requirements of a MAP cannot bemet.

3083 map: thematic. A MAP designed to demonstrate particular features or concepts. In conventional use, this term excludesTOPOGRAPHIC MAPS.

3084 map: topographic. A MAP which represents the horizontal and vertical positions of the features represented;distinguished from a PLANIMETRIC MAP by the addition of RELIEF in measurable form. A topographic map usesCONTOURS or comparable SYMBOLS to show MOUNTAINS, VALLEYS and PLAINS.

3085 map nadir. See NADIR.

3086 map parallel. See AXIS OF HOMOLOGY.

3087 map projection. See PROJECTION.

3088 map scale. See SCALE.

3089 Marcq St. Hilaire method. The establishing of a LINE OF POSITION from the OBSERVATION of the ALTITUDEof a CELESTIAL BODY, and the calculation of its ALTITUDE and AZIMUTH by the use of an ASSUMEDPOSITION. First introduced by Commandant Marcq St. Hilaire of the French Navy. See INTERCEPT, SUB-POINT.Also referred to as Saint Hilaire method or intercept method.

3090 maregraph plongeur. An instrument for measuring changes in SEA LEVEL by automatically recording changes inpressure at sea bottom. Two BOURDON TUBES are open to pressure and a mechanism records the resultant move-ments of their free ends.

3091 margin. In CARTOGRAPHY, that area of a MAP or CHART lying outside the border. 'Margin' is preferred to the term'surround'.

3092 marginal information (or data). The notations printed in the MARGINS or borders of MOSAICS, PLANS, or MAPS.

3093 marigram. See TIDE CURVE.

3094 marigraph. A recording TIDE GAUGE.

3095 marina. A harbour facility for small boats, yachts, etc., where supplies, repairs, and various services are available.

3096 marine. An adjective meaning relating to navigation or shipping; relating to or connected with the sea; used, or adoptedfor use at sea. Sometimes called maritime, but maritime is more frequently applied to that which borders on the sea.

3097 marine barometer. See BAROMETER.

3098 marine biology. See BIOLOGY.

3099 marine borer. Any marine invertebrate that excavates tunnels, holes, or depressions in one or more of a variety ofmaterials by abrasive, chewing, or chemical action.

143

3100 marine chart. See CHART: NAUTICAL.

3101 marine farm. See FISH FARM.

3102 marine light. See LIGHT.

3103 marine navigation. See NAVIGATION.

3104 marine nature reserve (U.S. marine sanctuary). An officially designated area in which there may be restrictions onentry, fishing, anchoring, and other activities in order to protect the marine environment.

3105 marine railway. A track, cradle, and winding mechanism for hauling vessels out of the water so that the hull can beexposed as in a dry dock. This is also called a patent slip in British terminology.

3106 marine sextant. See SEXTANT.

3107 marine stabilizer. See STABILIZER.

3108 maritime. Bordering on, concerned with, or related to the sea.

3109 maritime boundary delimitation. The process of establishing LINE(S) OF DELIMITATION at sea.

3110 maritime meteorology. Branch of the METEOROLOGY which is concerned with the interaction between the SEAand ATMOSPHERE.

3111 Maritime Safety Information (MSI). Navigational and meteorological warnings, meteorological forecasts, distress al-erts and other urgent safety related messages broadcast to ships.

3112 mark. One of the bits of leather, cloth, etc. indicating a specified length of a LEAD LINE.A charted conspicuous object, structure, or LIGHT serving as an indicator for guidance or warning of a craft; a

BEACON.A definite object ,such as an imprinted metal disk, used to designate a survey point and sometimes refers to the entire

survey monument. Mark is used with a qualifying term such as station, reference, azimuth, or bench. See also BENCHMARK and REFERENCE MARK.

3113 mark: geodetic. See MARK.

3114 mark boat. A temporary SOUNDING MARK consisting of a whaler moored head and stern, with a SIGNAL on board.

3115 marker. That which marks something.A MARKER BEACON. See also RADIOBEACON.

3116 marker beacon. A RADIOBEACON which radiates a SIGNAL to define an area, as an AID TO NAVIGATION.Usually called marker radiobeacon or marker.

3117 marker buoy. A temporary BUOY used in SURVEYING to mark a location of particular interest such as a SHOAL orREEF. See also STATION BUOY.

3118 marker radiobeacon. See MARKER BEACON.

3119 marks and deeps. The divisions used in marking a hand LEAD LINE.

3120 marl. A crumbling, earthy DEPOSIT, particularly one of CLAY mixed with SAND, lime, decomposed SHELLS, etc.Sometimes a LAYER of marl becomes quite compact. Part of the ocean bed is composed of marl.

3121 Marsden chart. A system introduced by Marsden early in the nineteenth century for showing the distribution ofmeteorological DATA on a CHART especially over the OCEANS. A Mercator map projection is used; the worldbetween 90o N and 80o S being divided into Marsden 'squares' each of 10 degrees latitude by 10 degrees longitude.These squares are systematically numbered to indicate position. Each square may be divided into quarter squares, or into100 one-degree subsquares numbered from 00 to 99 to give the position to the nearest degree.

3122 marsh. A tract of low-lying LAND flooded at times and generally swampy.

144

3123 massif. A large mountain-mass; the central mass of a MOUNTAIN; a compact and more or less independent portion ofa RANGE.

3124 mass transport. In OCEANOGRAPHY, the voluminous transfer of water from one region to another.

3125 master. An instrument which controls another similar instrument called a REPEATER.A MASTER STATION.

3126 master compass. See COMPASS.

3127 master gyro compass. See COMPASS.

3128 master station. The governing station of two or more synchronized transmitting stations. Often shortened to MASTER.See SLAVE STATION.

3129 masthead. The top of a mast.

3130 masthead angle. The vertical angle subtended by a ship's mast.

3131 mathematical model. A mathematical representation, usually of a process, device, or concept, which permitsmathematical manipulation of variables as a means to determine how the process, device, or concept would behave invarious situations as e.g. the application of a specific simulation.

3132 Matthews table. An echo sounding correction table.

3133 matrix. In OCEANOGRAPHY, ROCK or SEDIMENT in which larger grains are imbedded in a mass of smaller grai-ns.

In printing, a metal plate, usually of copper, for moulding the face of a type.In graphic arts, a mold in which type characters are cast in line-casting machines and foundry type.

3134 matte. Conglomerate of SAND, MUD and SEAWEED forming a SHOAL or REEF.

3135 maximum thermometer. See THERMOMETER.

3136 meander (Often in plural). A winding, convolution or winding COURSE, as of a STREAM.A deviation of the flow pattern of a CURRENT.

3137 mean diurnal high water inequality. See DIURNAL INEQUALITY.

3138 mean diurnal low water inequality. See DIURNAL INEQUALITY.

3139 mean draft. See DRAFT.

3140 mean higher high water (M.H.H.W.). The average HEIGHT or HIGHER HIGH WATERS at a place over a 19-yearperiod.

3141 mean high water (M.H.W.). The average HEIGHT of all HIGH WATERS at a place over a 19-year period.

3142 mean high water lunitidal interval (M.H.W.I.). See LUNITIDAL INTERVAL.

3143 mean high water neaps (M.H.W.N.). The average HEIGHT of the HIGH WATERS of NEAP TIDES. Also calledneap high water.

3144 mean high water springs (M.H.W.S.). The average HEIGHT of the HIGH WATERS of SPRING TIDES. Also calledspring high water.

3145 mean lower low water (M.L.L.W.). The average HEIGHT of the LOWER LOW WATERS at a place over a 19-yearperiod.

3146 mean lower low water springs (M.L.L.W.S.). The average HEIGHT of lower low water springs at a place.

3147 mean low water (M.L.W.). The average HEIGHT of all LOW WATERS at a place over a 19-year period.

3148 mean low water lunitidal interval (M.L.W.I.). See LUNITIDAL INTERVAL.

145

3149 mean low water neaps (M.L.W.N.). The average HEIGHT of the LOW WATERS of NEAP TIDES. Also called neaplow water.

3150 mean low water springs (M.L.W.S.). The average HEIGHT of the LOW WATERS of SPRING TIDES. This LEVELis used as a TIDAL DATUM in some areas. Also called spring low water.

3151 mean neap range. See NEAP RANGE.

3152 mean neap rise. The HEIGHT of MEAN HIGH WATERS NEAPS above the CHART DATUM. Also called neaprise.

3153 mean noon. See NOON.

3154 mean range (of tide). The difference in the HEIGHT between MEAN HIGH WATER and MEAN LOW WATER.

3155 mean rise (of tide). The HEIGHT of MEAN HIGH WATER above the CHART DATUM.

3156 mean sea level. The average HEIGHT of the surface of the SEA at a TIDE STATION for all stages of the TIDE over a19-year period, usually determined from hourly height readings measured from a fixed predetermined reference level(CHART DATUM).

3157 mean solar day. See DAY: SOLAR.

3158 mean solar time. See TIME.

3159 mean sounding velocity. Mean values for VELOCITY OF SOUND through the vertical water column of specificDEPTHS based on different VELOCITIES OF SOUND in different sections of the column. These values yield morenearly correct DEPTHS when sonic DEPTH FINDERS are used. The VELOCITY OF SOUND at any specific DEPTHmay differ considerably from the mean.

3160 mean sphere depth. The uniform DEPTH to which the water would cover the EARTH if the solid surface were smoo-thed off and were parallel to the surface of the GEOID. The accepted figure for this DEPTH is 2,440 METRES. Themean DEPTH of the SEA is 3,800 METRES.

3161 mean spring range. See SPRING RANGE.

3162 mean spring rise. The HEIGHT of MEAN HIGH WATER SPRINGS above the CHART DATUM. Also called springrise.

3163 mean square error. See ERROR.

3164 mean sun. See SUN.

3165 mean tide level. See HALF TIDE LEVEL.

3166 mean time. See TIME: MEAN SOLAR.

3167 mean water level. The average surface level of a body of water.

3168 measured distance. A COURSE at SEA, whose ends are indicated by RANGES ashore, and whose length has beenaccurately measured for determining the SPEED of vessels.

3169 measured mile. A MEASURED DISTANCE of one NAUTICAL MILE.

3170 measurement. The (detailed) dimensions of a physical quantity.

3171 median line. A line every point of which is equidistant from the nearest points on the baselines of two or more statesbetween which it lies.

3172 median valley. The axial DEPRESSION of the mid-oceanic RIDGE system. See RIFT.

3173 medimarimeter. An apparatus for recording the MEAN SEA LEVEL.

3174 medium frequency (MF). See FREQUENCY: RADIO.

146

3175 medusa. See COELENTERATE and JELLYFISH.

3176 megacycle. One million CYCLES; one thousand KILOCYCLES. The term is often used incorrectly as the equivalent ofone million CYCLES PER SECOND.

3177 megahertz. One million HERTZ; one thousand KILOHERTZ. The term is used as the equivalent of one millionCYCLES PER SECOND.

3178 melting point. Temperature of change from the solid to the liquid phase of a substance. It is a function of pressure.

3179 menu. A list of commands and/or options. The user selects the desired command by moving the CURSOR to therespective position on the menu or by entering an appropriate code at the command entry line of the DISPLAY.

3180 Mercator: transverse. See PROJECTION: TRANSVERSE MERCATOR.

3181 Mercator chart. See CHART.

3182 Mercator projection. See PROJECTION.

3183 Mercator sailing. See SAILING.

3184 mercurial (or mercury) barometer. See BAROMETER.

3185 meridian. A north-south reference line, particularly a GREAT CIRCLE through the GEOGRAPHICAL POLES of theEARTH, from which LONGITUDES (or DEPARTURES) and AZIMUTHS are reckoned; or a plane normal to theGEOID or SPHEROID defining such a line. The term usually refers to the UPPER BRANCH.

3186 meridian: astronomical. A line on the surface of the EARTH connecting points having the same ASTRONOMICALLONGITUDE. Because the DEFLECTION OF THE VERTICAL is not the same at all points, an astronomicalmeridian is an irregular line, not lying in a single plane. Also called terrestrial meridian.

3187 meridian: celestial. A GREAT CIRCLE of the CELESTIAL SPHERE through the CELESTIAL POLES and theZENITH. The expression usually refers to the UPPER BRANCH.

3188 meridian: central. See MERIDIAN: INITIAL.

3189 meridian: compass. A line through the north-south points of a MAGNETIC COMPASS. The compass card axis lies inthe compass meridian.

3190 meridian: geodetic. A line on a SPHEROID which has the same GEODETIC LONGITUDE at every point.

3191 meridian: geographic(al). A general term, applying alike to an ASTRONOMICAL or a GEODETIC MERIDIAN.

3192 meridian: Greenwich. The ASTRONOMICAL MERIDIAN through the centre of the TRANSIT instrument of theobservatory of Greenwich, England, serving as the reference for GREENWICH TIME, in contrast with LOCALMERIDIAN.

3193 meridian: grid. One of the GRID lines extending in a grid north-south direction.

3194 meridian: initial. A MERIDIAN through the ORIGIN of a system of GRID COORDINATES and used as y-AXIS ofthe system. Also called central meridian.

3195 meridian: local. The MERIDIAN through any particular place or observer, serving as the reference for LOCAL TIME,in contrast with GREENWICH MERIDIAN.

3196 meridian: magnetic. A line of horizontal magnetic force of the EARTH. The vertical plane in which a freelysuspended, symmetrically magnetized NEEDLE, influenced by no transient, artificial, magnetic disturbance, will cometo rest.

3197 meridian: photograph. In PHOTOGRAMMETRY, the IMAGE on a PHOTOGRAPH of any horizontal line in theobject space which is parallel to the PRINCIPAL PLANE. Since all such lines meet at infinity, the IMAGE of themeeting point is at the intersection of the PRINCIPAL LINE and the HORIZON TRACE, and all photograph meridianspass through that point.

147

3198 meridian: prime. The MERIDIAN of LONGITUDE 0o, used as the origin for measurement of LONGITUDE. TheMERIDIAN of Greenwich, England, is almost universally used for this purpose.

3199 meridian: principal (U.S. terminology). A true north-south line (a MERIDIAN) extending both north and south of theINITIAL POINT in the rectangular system of SURVEYS. Together with the BASE LINE they constitute the axes of asystem and the INITIAL POINT the ORIGIN of that system. See PRINCIPAL LINE.

3200 meridian: standard. The MERIDIAN used for reckoning STANDARD TIME.A MERIDIAN of a map projection, along which the SCALE is as stated.

3201 meridian: terrestrial. See MERIDIAN: ASTRONOMICAL.

3202 meridian: true. A MERIDIAN. The expression is used to distinguish the GREAT CIRCLE through theGEOGRAPHICAL POLES from MAGNETIC MERIDIAN, COMPASS MERIDIAN, or GRID MERIDIAN.

3203 meridian: zone. The MERIDIAN used for reckoning ZONE TIME.

3204 meridian altitude. See ALTITUDE.

3205 meridian angle. ANGULAR DISTANCE east or west of the local CELESTIAL MERIDIAN; the arc of theCELESTIAL EQUATOR, or the angle at the CELESTIAL POLE, between the UPPER BRANCH of the localCELESTIAL MERIDIAN and the HOUR CIRCLE of a CELESTIAL BODY, measured eastward or westward from thelocal CELESTIAL MERIDIAN through 180o, and labelled E or W to indicate the direction of measurement. See HOURANGLE.

3206 meridian-arc method. A method of determining the dimensions of the ELLIPSOID by measuring (by TRIANGULA-TION) the linear distances between three points on a MERIDIAN and the ANGULAR DISTANCES between themfrom observed differences in their ASTRONOMICAL LATITUDES.

3207 meridian distance. In ASTRONOMY, the HOUR ANGLE of a CELESTIAL BODY when close to but not exactly onthe ASTRONOMICAL MERIDIAN.

In PLANE SURVEYING, the perpendicular distance in a horizontal plane of a point from a MERIDIAN ofreference.

3208 meridian distance: double. The algebraic sum of the perpendicular distances from the two ends of any line of aTRAVERSE to the initial, or reference, meridian.

3209 meridian passage. See MERIDIAN TRANSIT.

3210 meridian sailing. See SAILING.

3211 meridian telescope. See TELESCOPE.

3212 meridian transit. The apparent passage of a CELESTIAL BODY across a CELESTIAL MERIDIAN. Upper orsuperior transit is the passage across the UPPER BRANCH of the CELESTIAL MERIDIAN. Lower or inferior transitis the passage across the LOWER BRANCH. The lower transit may take place either above the HORIZON as is thecase of CIRCUMPOLAR CELESTIAL BODIES, or across that branch of the MERIDIAN lying below the observer'sHORIZON. Also called TRANSIT, meridian passage, CULMINATION.

3213 meridional difference. The difference between the MERIDIONAL PARTS of any two given PARALLELS.

3214 meridional distance. The difference of LATITUDE between two points on a map PROJECTION's GRATICULE con-verted to suitable linear units.

3215 meridional orthographic projection. See PROJECTION.

3216 meridional parts. The length of the arc of a MERIDIAN between the EQUATOR and a given PARALLEL on aMERCATOR CHART, expressed in units of 1 MINUTE of LONGITUDE at the EQUATOR.

3217 meridional plane. Any plane containing the polar AXIS of the EARTH.

3218 meroplankton. Chiefly the floating development stages (eggs and larvae) of the BENTHOS and NEKTON.

3219 mesa. A high TABLELAND (S.W. and W. United States).

148

3220 messenger. In oceanographic terminology, a cylindrical metal WEIGHT, usually hinged and with a latch so that it canbe fastened around a wire, and sent down to actuate WATER BOTTLES and CURRENT METERS after they havebeen lowered to the desired DEPTH.

3221 metacentre (or metacenter). That point in a floating body at which a vertical line drawn through its CENTRE OFBUOYANCY when it is upright meets the vertical line drawn through its CENTRE OF BUOYANCY when it is tipped.CENTRE OF GRAVITY of the unsubmerged part of a floating body; for stable EQUILIBRIUM the metacentre must beabove the CENTRE OF GRAVITY.

3222 meteor. PHENOMENON other than a CLOUD, observed in the ATMOSPHERE or on the surface of the EARTH,which consists of a precipitation, a suspension or a deposit of aqueous or non-aqueous liquid or solid particles, or aPHENOMENON of the nature of an optical or electrical manifestation.

3223 meteoric dust. Atmospheric dust originating from METEORS.

3224 meteorite. A METEOR that reaches the surface of the EARTH as a solid particle.

3225 meteorological element. Atmospheric variable or PHENOMENON which characterize the state of the WEATHER at aspecific place at a given time: air temperature, ATMOSPHERIC PRESSURE, WIND, HUMIDITY,THUNDERSTORM, FOG, PRECIPITATION, etc.

3226 meteorological observations. See OBSERVATION.

3227 meteorological optical range table. A table from the International Visibility Code which gives the code number ofmeteorological visibility and the meteorological visibility for several weather conditions such as thin fog (Code No. 4-meteorological visibility 1/2-1 nautical mile); clear (Code No. 7-meteorological visibility 5 1/2-11 nautical miles).

3228 meteorological tide. See TIDE.

3229 meteorological visibility. See VISIBILITY.

3230 meteorology. Science of the ATMOSPHERE.

3231 meter. (1) A person who measures.(2) An instrument or apparatus for measuring.(3) A device for measuring a specified thing as in THERMOMETER or BAROMETER.See also METRE.

3232 meter wheel. A special block used to measure payed out sampling line at OCEANOGRAPHIC STATIONS.

3233 method of squares. See SQUARES: METHOD OF.

3234 metonic cycle. A PERIOD of 19 YEARS, after which the various PHASES OF THE MOON fall on approximately thesame DAYS of the YEAR as in the previous CYCLE.

3235 metre (or meter). The basic unit of length in the SI SYSTEM.

3236 metric system. A system of weights and measures, based upon the metre.

3237 metrology. The science of weights and measures.

3238 micrometer. An auxiliary device to provide measurement of very small angles or dimensions, by an instrument such asa TELESCOPE. See MICROMETER: FILAR, MICROMETER: TRANSIT.

3239 micrometer: filar. A device attached to a TELESCOPE or microscope, consisting of a wire thread (filament)connected with a screw in such manner that as the screw is turned, the wire moves through a continuous succession ofparallel positions, all in the FOCAL PLANE of the instrument. Any small angle may be measured by noting the numberof turns and fractions of a turn of the screw which are made in moving the wire from one bisection to another, andmultiplying this by the value of one turn. See MICROMETER: OCULAR, MICROMETER MICROSCOPE, andMICROMETER COMB.

3240 micrometer: impersonal. See MICROMETER: TRANSIT.

149

3241 micrometer: ocular. A FILAR MICROMETER so placed that its wire moves in the principal FOCAL PLANE of aTELESCOPE. Also called an eyepiece micrometer.

3242 micrometer: run of. See ERROR OF RUN.

3243 micrometer: transit. A form of registering MICROMETER with its movable wire placed in the FOCAL PLANE of anastronomic TRANSIT and at right angles to the direction of motion of the IMAGE of a STAR which is observed at ornear CULMINATION. The transit micrometer is also termed an impersonal micrometer, because it almost completelyeliminates the effect of the PERSONAL EQUATION on time observations made with it.

3244 micrometer comb. A notched SCALE placed at right angles to the movable wire of a MICROMETER and so designedthat one turn of the MICROMETER SCREW will move the micrometer wire across one notch of the comb; the centralnotch of the comb in conjunction with the zero of the micrometer head furnishes a fiducial point from which allmicrometer readings are reckoned. The comb is used for keeping count of whole turns of the MICROMETER SCREW,parts of turns being read on the graduated micrometer head.

3245 micrometer drum. A graduated drum by which the motion of a traveling wire in a microscope can be measured.

3246 micrometer microscope. A FILAR MICROMETER so placed that its wire moves in the FOCAL PLANE of a micro-scope.

3247 micrometer screw. A finely-threaded screw of definite PITCH, with a head graduated to show how much the screw hasbeen moved in or out; used in MICROMETERS.

3248 micron. One millionth of a METRE; one thousandth of a millimetre.

3249 microphone. An electroacoustic device containing a TRANSDUCER which is actuated by SOUND WAVES and deli-vers essentially equivalent electric WAVES.

3250 microprocessor. Component of a data processing device made up of microcircuits integrated on a single chip.

3251 microsecond. One millionth of a SECOND.

3252 microseisms. More or less persistent feeble earth tremours caused by natural sources such as atmospheric pressuresystems or waves.

3253 microwave. A very short RADIO WAVE, usually 30 cm to 0.3 mm.

3254 microwave distance measurement. Any method of ELECTROMAGNETIC DISTANCE MEASUREMENT based ona comparison of radio microwave signals.

3255 mid-channel buoy. A BUOY for marking the middle of a CHANNEL.

3256 middle ground. A shallow area in otherwise deeper water with CHANNELS on both sides of it.

3257 middle ground buoy. A BUOY marking a SHOAL with CHANNELS on both sides. See BIFURCATION BUOY,JUNCTION BUOY.

3258 middle latitude. Half the arithmetical sum of the LATITUDES of two places on the same side of the EQUATOR.

3259 middle latitude sailing. See SAILING.

3260 mid-extreme tide. See TIDE.

3261 midnight. Twelve HOURS from NOON, or the instant the time reference crosses the LOWER BRANCH of the refe-rence CELESTIAL MERIDIAN.

3262 mil. 1. 1/6400 of the circumference of a circle. Approximately 1/1000 radian.2. 1/1000 of an inch.

3263 mile. A unit of distance. See INTERNATIONAL NAUTICAL MILE, SEA MILE, STATUTE MILE.

3264 military practice area. See EXERCISE AREA.

150

3265 Milky Way. A broad, faintly luminous band seen across the sky at night, consisting of innumerable STARS andnebulae, so distant as to be indistinguishable without a TELESCOPE.

3266 millibar. A unit of pressure equal to 1/1,000 of a BAR. The millibar is much used in METEOROLOGY.

3267 milligal. A unit of ACCELERATION equal to 1/1,000 of a GAL, or 1/1,000 centimetre per SECOND, per SECOND.This unit is used in gravity measurement, being approximately one-millionth of the average GRAVITY at the EARTH'ssurface.

3268 mineral development platform. See PLATFORM: MINERAL DEVELOPMENT.

3269 minimum squares. See LEAST SQUARES.

3270 minimum thermometer. See THERMOMETER.

3271 minor control. See CONTROL: PHOTOGRAMMETRIC.

3272 minor control plot. See RADIAL TRIANGULATION.

3273 minor planet. An ASTEROID.

3274 minus soundings. See SOUNDING.

3275 minute. The sixtieth part of an HOUR; sixty SECONDS.The sixtieth part of a DEGREE of arc; sixty SECONDS.

3276 mirage. Optical PHENOMENON consisting essentially of steady or wavering, single or multiple, upright or inverted,vertically enlarged or reduced, IMAGES of distant objects.

3277 mirage: inferior. Particular case of MIRAGE, occurring over stretches of water, land, SHORES, roads, etc. stronglyheated by INSOLATION, and in which the virtual IMAGE is found below the actual object. The case in which objectssituated on or slightly above the normal HORIZON become invisible is popularly termed SINKING.

3278 mirage: superior. Special case of MIRAGE, observable over snow-fields, cold SEAS, etc., in which the virtualIMAGE is above the actual object. The case in which objects below and beyond the normal HORIZON become visibleis popularly termed LOOMING.

3279 mire. A piece of wet swampy GROUND.

3280 mirror. Any surface which produces IMAGES by REFLECTION of LIGHT RAYS.

3281 mist. Suspension in the air of microscopic water droplets or wet hygroscopic particles, reducing the VISIBILITY at theEARTH's surface.

3282 mixed current. See CURRENT.

3283 mixed layer. In OCEANOGRAPHY, the surface layer of virtually isothermal water, which frequently exists above theTHERMOCLINE. The thickness of the layer is dependent on the temperature gradient.

3284 mixed seas. The state of the sea resulting from the interaction of wind, waves, and swell.

3285 mixed tide. See TIDE.

3286 moat (or sea moat). An annular depression that may not be continuous, located at the base of many SEAMOUNTS orISLANDS.

3287 mobile rig. See RIG.

3288 mobile station. A radiolocation station intended to be used at unspecified points. As opposed to FIXED STATION.

3289 mock moons. Name given sometimes to PARASELENAE, PARANTISELENAE and ANTISELENAE when they arebright.

3290 mock suns. Name often given to PARHELIA, PARANTHELIA and ANTHELIA when they are particularly bright.

151

3291 mock-up. A model of something to be used for testing or study

3292 modulated wave. See WAVE.

3293 modulating wave. See WAVE.

3294 modulation. VARIATION of some characteristic of a WAVE, called the CARRIER WAVE, in accordance withinstantaneous values of another WAVE, called the MODULATING WAVE. Variation of AMPLITUDE is amplitudemodulation, variation of FREQUENCY is frequency modulation, and variation of PHASE is phase modulation. SeeMODULATION: PULSE and MODULATION: SPACE.

3295 modulation: amplitude. The process of changing the amplitude of a carrier wave in accordance with the variations of amodulating wave.

3296 modulation: frequency. Angle modulation of a sine-wave carrier in which the instantaneous frequency of themodulated wave differs from the carrier frequency by an amount proportional to the instantaneous value of themodulating wave.

3297 modulation: pulse. The process of forming very short bursts of a CARRIER WAVE, separated by relatively longperiods during which no CARRIER WAVE is transmitted.

3298 modulation: sideband. A particular kind of amplitude modulation in which one of the two frequency bands on bothsides of the carrier (and possibly the carrier itself) is suppressed, allowing a considerable reduction of the spectral widthof the signal.

3299 modulation: space. The combining of SIGNALS in space to form a SIGNAL of desired characteristics.

3300 moire. Undesirable patterns occurring when REPRODUCTIONS are made from halftone PROOFS or steelENGRAVINGS, caused by conflict between the RULING of the HALFTONE screen and the dots or lines of theoriginal; a similar pattern occurring in multicolour halftone reproductions and usually due either to incorrect screenangles or misregister of the colour impressions during printing.

3301 moiré effect. The effect created by transmitting light through two separate, overlapping families of parallel lines. Seealso INOGEN LIGHT.

3302 mole. A massive structure of masonry or large stones serving as a PIER or BREAKWATER, or both.Unit of quantity of matter in the SI system.

3303 moment. The tendency or degree of tendency to produce motion about an AXIS. Numerically it is the quantity obtainedby multiplying the force, SPEED, or mass, by the distance from the point of application or CENTRE OF GRAVITY tothe AXIS. See MAGNETIC MOMENT.

3304 moment of inertia. The quantity obtained by multiplying the mass of each small part of a body by the square of itsdistance from an AXIS, and adding all the results.

3305 momentum. Quantity of motion. That property of a particle which is given by the product of its mass with itsVELOCITY.

3306 monitoring. Checking of the operation and performance of an electronic system through reception of its SIGNALS.

3307 monochromatic (adj.). Of, or having one colour. Of, or producing light of one WAVE LENGTH.

3308 monsoon. WIND of the general circulation of the ATMOSPHERE, typified by the seasonal persistence of a given winddirection and by a pronounced change in this direction from one season to another. The term is generally confined tothose cases in which the primary cause is the differential heating (changing in nature from summer to winter) of aCONTINENT relative to a neighbouring OCEAN.

3309 montage. A series of related pieces of copy appearing as one to tell a complete story.

3310 month. A measure of TIME based on the motion of the MOON in its ORBIT.

3311 month: anomalistic. The interval of time between two successive passages of the MOON in its ORBIT throughPERIGEE. The length of the anomalistic month is 27.55455 mean SOLAR DAYS.

152

3312 month: dracontic. See MONTH: NODICAL.

3313 month: lunar. See MONTH: SYNODICAL.

3314 month: nodical. The interval of time between two successive passages of the MOON through the same NODE of itsORBIT. The length of the nodical month averages 27.21222 mean SOLAR DAYS. Also called dracontic month.

3315 month: sidereal. The interval of time between two successive passages of the MOON past as FIXED STAR. Thelength of the sidereal month averages 27.321661 mean SOLAR DAYS.

3316 month: synodical. The interval of time between two successive CONJUNCTIONS or OPPOSITIONS of the MOON.The length of the synodical month is 29.5305888 mean SOLAR DAYS. Also called lunation or lunar month.

3317 month: tropical. The average period of REVOLUTION of the MOON about the EARTH with respect to the VERNALEQUINOX. The length of the tropical month averages 27.321582 mean SOLAR DAYS.

3318 monument (U.S. terminology). In SURVEYING, a structure used or erected to mark the POSITION of a STATION;permanence is implied.

3319 moon. The SATELLITE of the EARTH. See LIBRATIONS OF MOON and PHASES OF THE MOON.

3320 moonbow. RAINBOW whose formation is the same as that of the ordinary RAINBOW, but whose light comes fromthe MOON and not from the SUN. Also called lunar rainbow.

3321 moor (v.t.). To secure a boat or other floating thing to a fixed object or the seabottom.

3322 mooring. A place where a vessel may be secured.(Usually in pl.) The equipment used to secure a vessel.The process of securing a vessel or oceanographic instruments other than anchoring with a single anchor.

3323 mooring buoy. See BUOY.

3324 morse code light. See LIGHT.

3325 mosaic. In PHOTOGRAMMETRY, as assembly of AERIAL PHOTOGRAPHS whose edges usually have been torn, orcut, and matched to form a continuous photographic representation of a portion of the EARTH's surface.

3326 mosaic: controlled. A MOSAIC oriented and scaled to horizontal GROUND CONTROL; usually assembled fromrectified PHOTOGRAPHS. See also ORTHOPHOTOMOSAIC, UNCONTROLLED MOSAIC.

3327 most probable value. That value of a quantity which is mathematically determined from a series of observations and ismore probably free of the effects of blunders and errors than any other value that might stem from the same series ofobservations.

3328 mother vessel. The lead vessel when surveying with several vessels.

3329 mount. A large HILL or MOUNTAIN, usually a detached, characteristically conical mass of earth. The term 'mount' isalways used instead of MOUNTAIN when it precedes a proper name.

3330 mountain. A natural ELEVATION of the EARTH's surface rising more or less abruptly from the surrounding LEVEL,and attaining an ALTITUDE which, relatively to adjacent ELEVATIONS, is impressive or notable.

3331 mountains. On the sea floor, a well-delineated subdivision of a large and complex positive feature. See CORDILLERA.

3332 mountain chain. A complexity of several MOUNTAIN RANGES.

3333 mountain range. A line of MOUNTAINS. Also referred to as range of mountains.

3334 mouse. In computer systems, a pointing device operated by moving on a flat surface. See also CURSOR, JOYSTICK.

3335 mouth. The place of discharge of a STREAM into the OCEAN or ENTRANCE to a BAY from the OCEAN.

153

3336 mud. Pelagic or terrigenous detrital material consisting mostly of silt and clay-sized particles (less than 0.06 millimetre)but often containing varying amount of SAND and/or organic materials. It is a general term applied to any sticky fine-grained SEDIMENT whose exact size classification has not been determined.

3337 mud: red. A reddish-brown terrigenous deep-sea MUD which accumulates on the SEA FLOOR in the neighbourhoodof deserts and off the MOUTHS of great RIVERS; contains calcium carbonate up to 25 per cent.

3338 mud snapper. See SNAPPER.

3339 multi-beam echo sounder. Wide swath echo sounder for use in seabed mapping and surveying using the multi-beamprinciple. Depths are measured directly below and transverse to the ship's track. The width of the swath is a function ofthe number of beams and their aperture.

3340 multiple echo(es). See ECHO.

3341 multiple lens camera. See CAMERA.

3342 multiple lens photograph. See PHOTOGRAPH.

3343 multiple sea sampler. An instrument which comprises six WATER BOTTLES arranged so that they trip and takeWATER SAMPLES at different selected DEPTHS from a vessel under way.

3344 multiple tide staff. See TIDE STAFF.

3345 multiplex. In PHOTOGRAMMETRY, an instrument for making a TOPOGRAPHIC MAP from AERIALPHOTOGRAPHS. A three-dimensional optical model of the TERRAIN to be mapped is produced, in miniature, bydirect optical PROJECTION from a pair of overlapping PHOTOGRAPHS. This instrument uses the ANAGLYPHprinciple.

3346 multispectral imagery. Images obtained simultaneously in a number of discrete bands in the electromagnetic spectrum.

3347 multispectral scanner. A remote-sensing device which is capable of recording data in the ultraviolet and visibleportions of the electromagnetic spectrum, as well as the infrared.

154

N

3348 nadir. That point on the CELESTIAL SPHERE directly beneath the observer, and directly opposite to the ZENITH.

3349 nadir: ground. The point on the GROUND vertically beneath the PERSPECTIVE CENTRE of the camera lens.

3350 nadir: map. The map position for the GROUND NADIR.

3351 nadir: photograph. In PHOTOGRAMMETRY, the point at which a vertical line through the PERSPECTIVECENTRE of the camera lens pierces the plane of the PHOTOGRAPH. Also called nadir point, or plumb point.

3352 nadir point. See NADIR: PHOTOGRAPH.

3353 nadir radial. See RADIAL.

3354 name. The label of a numerical value used particularly to refer to the N (north) or S (south) label of LATITUDE andDECLINATION. When LATITUDE and DECLINATION are both N or both S, they are said to be of the same name,but if one is N and the other S, they are said to be of contrary name.

3355 nanosecond. A thousand millionth of a SECOND.

3356 Nansen bottle. A device used by oceanographers to obtain subsurface samples of SEA WATER. The bottle is loweredby wire; its valves are open at both ends. It is then closed 'in situ' by allowing a weight (called a MESSENGER) to slidedown the wire and strike the reversing mechanism. This causes the bottle to turn upside down, closing the valves andreversing the REVERSING THERMOMETERS which are mounted in a special thermometer case on it. If, as is usuallydone, a series of bottles is lowered, then the reversal of each bottle releases another MESSENGER to actuate the bottlebeneath it.

3357 Napier's (or Napier) diagram. A DIAGRAM on which compass deviation is plotted for various HEADINGS, and thepoints connected by a smooth curve, permitting a ready conversion of deviations from compass to magneticHEADINGS, or vice versa.

3358 narrow-beam echo sounder. A sounder with a transducer of small aperture (typically 2° to 5°) required for highresolution mapping.

3359 narrows. A NAVIGABLE narrow part of a BAY, STRAIT, RIVER, etc.

3360 natural frequency. See FREQUENCY.

3361 natural scale. See SCALE.

3362 natural year. See YEAR: TROPICAL.

3363 nature of the bottom. See BOTTOM: NATURE OF.

3364 nautical (adj.). Of or pertaining to ships, NAVIGATION (chiefly marine) or seamen. In contrast, navigational refers toNAVIGATION only.

3365 nautical almanac. See ALMANAC.

3366 nautical astronomy. See ASTRONOMY.

3367 nautical chart. See CHART.

3368 nautical mile. A unit of length used principally in NAVIGATION. See INTERNATIONAL NAUTICAL MILE.

3369 nautical plan. A large scale nautical chart or a large scale inset of a nautical chart.

3370 nautical twilight. See TWILIGHT.

3371 nautophone. A HORN having a diaphragm oscillated by electricity.

3372 NAVAREA. The short title of an area in the WORLD-WIDE NAVIGATIONAL WARNING SERVICE.

155

3373 navigability. Of a vessel: capability of being controlled and steered.Of a water area: capability of being navigated.

3374 navigable (adj.). Affording passage to a craft; capable of being navigated.

3375 navigable area surveys (NAS). Navigable Area Surveys are basic hydrographic surveys with restricted area coverage.The coverage is reduced by omitting requirements for:

1. Development of the 0-foot depth curve and foul, nearshore areas not considered navigable;2. Complete field edit of the survey area. Navigable Area Surveys may also be restricted to the main navigable

channel or corridor.

3376 navigable semicircle. See DANGEROUS SEMICIRCLE.

3377 navigable waters. Waters usable, with or without improvements, as routes for commerce in the customary means oftravel on water.

3378 navigation. The process of directing the movement of a craft from one point to another.

3379 navigation: acoustic. See NAVIGATION: SONIC.

3380 navigation: aerial. See NAVIGATION: AIR.

3381 navigation: air. The NAVIGATION of aircraft. Occasionally called aerial navigation or navigation. SeeNAVIGATION: SURFACE.

3382 navigation: astro. See NAVIGATION: CELESTIAL.

3383 navigation: celestial. NAVIGATION with the aid of CELESTIAL BODIES. Also called astro-navigation.

3384 navigation: coastwise. NAVIGATION in the vicinity of a COAST, in contrast with OFFSHORE NAVIGATION.

3385 navigation: Doppler. NAVIGATION using equipment employing Doppler shift measurement techniques with respectto transmitted and reflected SIGNALS to automatically compute DRIFT ANGLE and GROUND SPEED. DRIFTANGLE and GROUND SPEED are used in DEAD RECKONING performed either manually or, as in manyinstallations, by dead reckoning COMPUTERS. In either case, craft HEADING is a necessary INPUT.

3386 navigation: electronic. NAVIGATION by means of electronic equipment. The expression 'electronic navigation' ismore inclusive than radio navigation, since it includes navigation involving any electronic device or instrument.

3387 navigation: grid. NAVIGATION by the use of GRID DIRECTIONS.

3388 navigation: hyperbolic. NAVIGATION by the use of HYPERBOLIC LINES OF POSITION.

3389 navigation: inertial. DEAD RECKONING performed automatically by a device which gives a continuous indicationof POSITION by double integration of ACCELERATION since leaving a starting point.

3390 navigation: land. NAVIGATION of vehicles across LAND or ICE. The expression is generally used in connectionwith the crossing of a region devoid of roads or LANDMARKS, so that methods similar to those employed in AIR orMARINE NAVIGATION must be employed.

3391 navigation: long range. NAVIGATION requiring only aids usable at long RANGE, relatively few of which couldprovide world coverage. The opposite is SHORT RANGE NAVIGATION.

3392 navigation: marine. NAVIGATION of water craft.

3393 navigation: offshore. NAVIGATION at distance from a COAST, in contrast with COASTWISE NAVIGATION.

3394 navigation: radio. See NAVIGATION: ELECTRONIC.

3395 navigation: short range. NAVIGATION employing only aids usable at short RANGES. The opposite is LONGRANGE NAVIGATION.

3396 navigation: sonic. NAVIGATION by means of SOUND WAVES whether or not they are within the audible RANGE.Also called acoustic navigation.

156

3397 navigation: submarine. NAVIGATION of a submarine, whether submerged or surfaced.

3398 navigation: surface. NAVIGATION of a craft on the surface of the EARTH. The expression is used particularly withreference to NAVIGATION of vessels on the surface of water. See NAVIGATION: LAND and NAVIGATION:UNDERWATER.

3399 navigation: underwater. The NAVIGATION of a submerged vessel. Occasionally called SUBMARINENAVIGATION.

3400 navigational(adj.). See NAUTICAL.

3401 navigational aid (U.S. terminology). An instrument, device, CHART, method, etc., intended to assist in theNAVIGATION of a craft. An AID TO NAVIGATION is a navigational aid but the latter expression should not beconfused with the former which refers only to devices external to a craft.

3402 navigational astronomy. See ASTRONOMY: NAUTICAL.

3403 navigational drafts. See DRAFT.

3404 navigational observation. Observations necessary to fix the position of a vessel.

3405 navigational parameters. All data characterising the position and motion of a vessel in a system of coordinates.

3406 navigational planet(s). See PLANET(S): NAVIGATIONAL.

3407 navigational (or navigation) satellite. An artificial earth-orbiting SATELLITE designed for navigational purposes.

3408 navigational stars. Selected bright STARS whose POSITIONS are precisely tabulated in NAUTICAL ALMANACSfor navigational purposes.

3409 navigational triangle. See ASTRONOMICAL TRIANGLE.

3410 navigational warning. A broadcast message containing urgent information relevant to safe navigation.

3411 navstar global positioning system. A satellite navigation system.

3412 NAVTEX. Narrow-band direct-printing telegraphy system for transmission of MARITIME SAFETYINFORMATION.

3413 neaped. See BENEAPED.

3414 neap high water. See MEAN HIGH WATER NEAPS.

3415 neap low water. See MEAN LOW WATER NEAPS.

3416 neap range. The mean semidiurnal RANGE OF TIDE when NEAP TIDES are occurring; the mean difference inHEIGHT between neap high water and neap low water. Sometimes called mean neap range.

3417 neap rise. See MEAN NEAP RISE.

3418 neap tide(s). See TIDE.

3419 nearshore. Close to the SHORE.

3420 neat line. Line, usually grid or graticule, bounding the detail of a MAP. Also referred to as inner neat line todifferentiate from border drawn outside of neat line. Also written as one word.

3421 nebula (pl. nebulae). An aggregation of matter outside the SOLAR SYSTEM, large enough to occupy a perceptiblearea but which has not been resolved into individual STARS.

3422 neck. In OCEANOGRAPHY, the narrow band of water flowing swiftly seaward through the SURF. See CURRENT:RIP.

In geography, a narrow piece of LAND with water on each side; an ISTHMUS.

3423 needle. Short for MAGNETIC NEEDLE.

157

3424 negative. A photographic IMAGE on film, PLATE, or paper, in which the subject tones to which the EMULSION issensitive are reversed or complementary.

3425 negative altitude. See ANGLE OF DEPRESSION.

3426 negative engraving. A chart reproduction process in which the compilation manuscript, or other data, is photographedonto a plastic or glass negative. The art of creating FACSIMILE images on a reproduction media by utilizing handengraving techniques.

3427 negative gradient. In OCEANOGRAPHY, a LAYER of water where temperature decreases with DEPTH.

3428 nekton. Those animals of the PELAGIC DIVISION that are active swimmers, such as most of the adult squids, fishes,and marine mammals.

3429 neritic province. See PELAGIC DIVISION.

3430 network. In SURVEYING, a pattern or configuration of STATIONS called geodetic network.In geography, a complex system of RIVERS, CANALS, etc.In RADIO, a CHAIN of transmitting RADIO STATIONS owned and operated as a unit.

3431 new chart. See CHART.

3432 new edition (of chart). In British terminology, a new printing of an existing CHART embodying the latest informationreceived in the Hydrographic Department. The CHART is entirely revised throughout and modernized in style, allcopies of the CHART in circulation being cancelled. When a new edition is published the LARGE and SMALLCORRECTIONS dates are removed (these corrections being embodied in the new edition).

In U.S. terminology, a new printing of an existing CHART embodying corrections that have become so extensive orof such importance to NAVIGATION as to render all previous printings obsolete.

3433 new ice. A general term which includes FRAZIL ICE, SLUSH, SLUDGE, PANCAKE ICE and ICE RIND.

3434 new moon. See PHASES OF THE MOON.

3435 new print (of chart). In U.S. terminology, a new issue of a CHART embodying only minor corrections. Suchcorrections are applied to the NEGATIVES and a new printing plate made. New prints do not render previous printingsof the current edition obsolete.

3436 new print date (of chart). In U.S. terminology, the date when a NEW PRINT of a CHART is issued which embodieschanges or corrections of a relatively minor character. The new print date is added to the right of the edition date in thelower left-hand corner of the CHART.

3437 new survey. See RESURVEY.

3438 newton. The unit of force in the SI SYSTEM.

3439 niggerhead. Large blocks of CORAL torn loose from the outer face of a REEF and tossed on to the REEF FLAT bySTORM WAVES or TSUNAMIS. The blocks are blackened by a crust of lichens after detachment from the REEF.

3440 night effect (or error). See ERROR: POLARIZATION.

3441 nimbostratus. See CLOUD GENERA.

3442 nipped (adj. and adv.). Said of a ship which is BESET when ICE forcibly presses against it. See BESET, ICEBOUND.

3443 nipping. The closing of ICE around a ship so that the ship is BESET and subjected to pressure from the ICE.

3444 no-bottom sounding(s). See SOUNDING.

3445 no-check position. See STATION: INTERSECTION.

3446 noctiluca. A genus of usually pale pink luminescent dinoflagellates large enough to be seen by the unaided eye. Thisparticular organism is responsible for much of the sheet-type LUMINESCENCE noted in coastal waters of various partsof the world OCEAN.

158

3447 nocturnal arc. See ARC: ASTRONOMICAL.

3448 nodal point. In optics, one of the two points on the OPTICAL AXIS of a LENS (or a system of LENSES) such that aRAY emergent from the second point is parallel to the RAY incident at the first. The first nodal point is referred to asthe front nodal point or incident nodal point, and the second nodal point as the rear nodal point or emergent nodalpoint.

An AMPHIDROMIC POINT.In ASTRONOMY, a NODE.

3449 nodal point: emergent. See NODAL POINT.

3450 nodal point: front. See NODAL POINT.

3451 nodal point: incident. See NODAL POINT.

3452 nodal point: rear. See NODAL POINT.

3453 node. A point that is the start or end point of a line segment.In ASTRONOMY, one of the two points of intersection of the ORBIT of a PLANET, PLANETOID, or COMET

with the ECLIPTIC, or of the ORBIT of a SATELLITE with the plane of the orbit of its primary. That point at whichthe body crosses to the north side of the reference plane is called the ascending node; the other, the descending node.The line connecting the nodes is called LINE OF NODES. Also called NODAL POINT. See REGRESSION OF THENODES.

In OCEANOGRAPHY, that part of a STANDING WAVE where the vertical motion is least and the horizontalVELOCITIES are greatest. See PARTIAL NODE.

In optics, a NODAL POINT.

3454 nodical month. See MONTH.

3455 nodules. In OCEANOGRAPHY, concretionary lumps of manganese, cobalt, iron, and nickel found widely scattered onthe ocean floor. ROCKS of various sizes and shapes often are encrusted with these metals.

3456 noise. Any undesired SOUND. By extension, noise is any unwanted disturbance within a useful FREQUENCY BAND,such as undesired electric WAVES in a transmission channel or device.

3457 nominal range. See RANGE.

3458 nomogram (or nomograph). A DIAGRAM showing, to SCALE, the relationship between several variables in suchmanner that the value of one which corresponds to known values of the others can be determined graphically. SeeCONVERSION SCALE, CONVERSION TABLE.

3459 nonharmonic constants. Tidal or tidal current constants such as LUNITIDAL INTERVALS, ranges, and inequalities,which can be derived directly from OBSERVATION without regard to the HARMONIC CONSTITUENTS.

3460 nonharmonic method. In TIDE PREDICTION, an approximate method based on the principle that 'the TIDE followsthe MOON'. It makes use of the close relationship that exists between the TIME of TIDE at most places and theMOON's MERIDIAN TRANSIT.

3461 nontidal current. See CURRENT.

3462 noon. The instant at which a time reference is over the UPPER BRANCH of the reference MERIDIAN.

3463 noon: apparent. Twelve o'clock APPARENT TIME.

3464 noon: local apparent. Twelve o'clock LOCAL APPARENT TIME. Sometimes called high noon.

3465 noon: mean. Twelve o'clock MEAN TIME.

3466 normal. In general, a straight line perpendicular to a surface or to another line.In GEODESY, a straight line perpendicular to the surface of the SPHEROID.

3467 normal equation. See EQUATION.

159

3468 normal water. A standard SEA WATER preparation, the CHLORINITY of which lies between 19.30 and 19.50 permille (0/00) and has been determined to within ± 0.001 per mille. Normal water is used as a convenient comparisonstandard for chlorinity measurements of sea water samples by TITRATION. It is prepared by the HydrographicalLaboratories, Copenhagen, Denmark. Also called standard sea water.

3469 north: compass. The direction north as indicated by a MAGNETIC COMPASS.

3470 north: grid. An arbitrary REFERENCE DIRECTION used with GRID NAVIGATION.The northerly or zero direction indicated by the grid datum of directional reference.

3471 north: magnetic. The direction indicated by the north seeking POLE of a freely suspended MAGNETIC NEEDLEinfluenced only by the EARTH's MAGNETIC FIELD.

3472 north: true. The DIRECTION of the north GEOGRAPHIC POLE.

3473 north celestial pole. See POLE: CELESTIAL.

3474 north geographical pole. See POLE: GEOGRAPHICAL.

3475 northing. The distance a craft makes good to the north. The opposite is SOUTHING.In a cartesian reference system the value of one coordinate, usually expressed in metres, measured from the origin in

north-south direction; used e.g. in the UTM system.

3476 north magnetic pole. See MAGNETIC POLE.

3477 notch (or gap). A narrow passage between two ELEVATIONS, as MOUNTAINS; a deep close PASS; a DEFILE.

3478 Notice to Mariners. A periodical or casual notice issued by hydrographic offices, or other competent authorities,regarding changes in AIDS TO NAVIGATION, dangers to NAVIGATION, important new SOUNDINGS, and, ingeneral, all such information as affects NAUTICAL CHARTS, SAILING DIRECTIONS, LIGHT LISTS and othernautical publications.

3479 nuclear precession magnetometer. See MAGNETOMETER.

3480 numerical scale. See SCALE: NATURAL.

3481 nunatak. An isolated hill or mountain peak of rock projecting above an INLAND ICE SHEET.

3482 nun buoy. A BUOY the above water part of which is in the shape of a cone or a truncated cone.

3483 nutation. Irregularities in the precessional motion of the EQUINOXES because of varying positions of the MOON,and, to a lesser extent, of other CELESTIAL BODIES, with respect to the ECLIPTIC. Because of nutation theEARTH's AXIS nods like a top, describing an irregular circle about the mean pole in a period of about 19 years.

3484 Nyquist frequency. The highest frequency that may be uniquely resolved in a time series from a given samplinginterval. The Nyquist frequency is equal to twice the sampling interval. Analyzing frequencies for which the Nyquistcriterion is not met leads to ALIASING.

160

O

3485 object-glass. See OBJECTIVE LENS.

3486 objective. See OBJECTIVE LENS.

3487 objective lens. In TELESCOPES and microscopes,the optical component which receives light from the object andforms the first or primary IMAGE. In a CAMERA, the IMAGE formed by the objective lens is the final IMAGE. In aTELESCOPE or microscope used visually, the IMAGE formed by the objective lens is magnified by the EYEPIECE.Also called objective or object-glass.

3488 oblate spheroid. See SPHEROID.

3489 oblique photograph. See PHOTOGRAPH.

3490 oblique plotting instrument. An instrument (usually monocular) for plotting from OBLIQUE PHOTOGRAPHS.

3491 obliquity of the ecliptic. The acute angle between the plane of the ECLIPTIC and the plane of the CELESTIALEQUATOR, about 23° 27'.

3492 obscured(adj.). Said of the arc of a LIGHT SECTOR designated by its limiting BEARINGS in which the LIGHT is notvisible from seaward.

3493 observation. The act or practice of noting and recording facts and events as for some scientific study. The measure of aquantity whose value is desired. The DATA so noted and recorded.

A single measure, at a single setting of an apparatus.

3494 observation(s): astronomical. OBSERVATIONS of CELESTIAL BODIES, with appropriate instruments.In NAVIGATION, the measurement of ALTITUDE and AZIMUTH of CELESTIAL BODIES.

3495 observation(s): magnetic. Measurement of any of the MAGNETIC ELEMENTS.

3496 observation(s): meteorological. Evaluation or measurement of one or more meteorological elements.

3497 observation(s): tidal. A series of measurements taken in order to provide the information required for theREDUCTION OF SOUNDINGS, and the DATA for TIDAL PREDICTION and investigation.

3498 observation equation. See EQUATION.

3499 observation spot (or pillar). A position ashore determined from celestial observations.

3500 observation tower. A tower from which one's surroundings can be observed, but at which a watch is not habituallymaintained, as at a LOOKOUT TOWER. Such towers are temporarily erected for observations in geodetic networks.

3501 observed altitude. See ALTITUDE.

3502 observed tide. Those data from tide observing equipment such as tide gauges, staffs, etc. "Actual tides" is an expressionoften used in referring to "observed tides".

3503 obstruction. In MARINE NAVIGATION, anything that hinders or prevents movement, particularly anything thatendangers or prevents passage of a vessel. The term is usually used to refer to an isolated danger to NAVIGATION,such as a SUNKEN ROCK or PINNACLE.

3504 obstruction buoy. A BUOY marking an OBSTRUCTION.

3505 obstruction light. See LIGHT.

3506 occasional light. See LIGHT.

3507 occluded front. See OCCLUSION.

161

3508 occlusion. Process of progressive decrease of area of warm sector at the EARTH's surface, and its ultimatedisappearance, by the junction of the cold AIR MASSES which initially precede the WARM FRONT and follow theCOLD FRONT.

FRONT between these two cold AIR MASSES, after their junction (also termed occluded front).

3509 occultation. The concealment or extinguishment of the light of an AID TO NAVIGATION during the dark periods ofits CYCLE.

In ASTRONOMY, the concealment of a CELESTIAL BODY by another which crosses the line of view. Thus, theMOON occults a STAR when it passes between the observer and the STAR.

3510 occupy (v.t.). In SURVEYING, to observe with a surveying instrument at a STATION; also to set a surveyinginstrument over a point for the purpose of making OBSERVATIONS.

In OCEANOGRAPHY, to stop a ship at a selected location, an OCEANOGRAPHIC STATION, for the purpose ofgathering oceanographic observations.

3511 ocean. The vast body of water on the surface of the GLOBE, which surrounds the LAND; the main or great SEA.One of the main areas into which this body of water is divided geographically.

3512 ocean acoustic tomography. A method to determine the ocean structure based on precise measurements of transit timeof acoustic waves between a net of transmitters and receivers.

3513 ocean current. See CURRENT.

3514 ocean eddy. A moving or stationary local rotating formation where water predominantly moves along circular paths.

3515 ocean remote sensing. A kind of remote sensing especially for conducting investigations and surveys of the ocean andthe sea.

3516 oceanic circulation. A unique interrelated system of major ocean currents within a large area; usually a closed circularpattern such as in the North Atlantic, Mediterranean, etc.

3517 oceanic plateau. A comparatively flat topped elevation of the sea-bed which rises steeply from the ocean floor on allsides, and is of considerable extent across the summit.

3518 oceanic province. See PELAGIC DIVISION.

3519 oceanic ridge. A long elevation of the ocean floor with either irregular or smooth topography and steep sides, oftenseparating ocean basins.

3520 oceanographic cast. See CAST.

3521 oceanographic features. Physical qualities characterising the state of environment in water bodies of the World'sOceans. These features include temperature and salinity, electric conductivity, density, hydrostatic pressure,transparency, the range and character of tidal and other water level oscillations, the speed and direction of currents, thecharacteristics of waves and ice, etc.

3522 oceanographic slide rule. A specially designed SLIDE RULE used for calculating the CORRECTION of thermalexpansion of a deep sea REVERSING THERMOMETER.

3523 oceanographic section. A successive series of oceanographic stations carried out in a specified direction.

3524 oceanographic station. See STATION.

3525 oceanographic survey. See SURVEY.

3526 oceanography. The study of the SEA, embracing and integrating all knowledge pertaining to the SEA's physicalboundaries, the chemistry and physics of SEA WATER, MARINE BIOLOGY, and SUBMARINE GEOLOGY.

In strict usage oceanography is the description of the marine environment, whereas oceanology is the study of theOCEANS and related sciences.

3527 oceanology. See OCEANOGRAPHY.

162

3528 ocean station. As defined by the International Civil Aviation Organization, a specifically located area of ocean surface,roughly square and 200 NAUTICAL MILES on a side. An ocean station vessel on patrol is said to be 'on station' whenit is within the perimeter of the area.

3529 ocean station vessel (OSV). Ship assigned to operate at the location of an OCEAN WEATHER STATION. Also calledweather ship.

3530 ocean water. Water having the physical-chemical characteristics of the OPEN SEA, where continental influences are ata minimum.

3531 ocean weather station. STATION aboard a suitably equipped and staffed ship which tries to maintain a fixed maritimelocation and which observes and reports for international exchange the OBSERVATIONS of specified elements.

3532 octant. A double-reflecting instrument for measuring angles. It is similar to a SEXTANT but has an arc of only 45°.

3533 ocular micrometer. See MICROMETER.

3534 odograph. A mechanical instrument containing a distance-measuring element which is moved or turned by an amountproportional to the actual distance travelled; a compass element which provides a fixed-reference direction, and anintegrator which provides for the resolution of the direction of motion into components and for the summation orintegration of the distance components.

Also an instrument for recording the distance travelled by a vehicle or pedestrian.

3535 odometer (or hodometer). An instrument for measuring the distance travelled by a craft. See also SPEEDOMETER.

3536 oersted. The CGS unit of MAGNETIC FIELD INTENSITY in the SI system.

3537 offing. That part of the visible SEA a considerable distance from the SHORE, or that part just beyond the limits of thearea in which a pilot is needed.

3538 offset. In SURVEYING, a short line perpendicular to a surveyed line, measured to a line or point for which DATA aredesired, thus locating the second line or point with reference to the first or surveyed line. An offset is also a jog in aSURVEY or other line, the line having approximately the same direction both before and after passing the jog. Offsetsare measured from a surveyed line or lines to the edges of an irregular-shaped body of water, or to any irregular linewhich it is desired to locate.

3539 offset lithography. A method of printing in which an image is transferred from a printing PLATE on to a rubber-surfaced blanket and thence on the paper. The image on the plate is a RIGHT READING IMAGE.

3540 offshore(adj. and adv.). Away from the SHORE.

3541 offshore(n.). The comparatively flat zone of variable width which extends from the outer margin of the rather steeplysloping shoreface to the edge of the CONTINENTAL SHELF.

3542 offshore navigation. See NAVIGATION.

3543 offshore wind. WIND blowing from the LAND toward the SEA. See LAND BREEZE.

3544 off soundings. Said of a vessel navigating beyond the 100-FATHOM LINE. In earlier times, said of a vessel in waterdeeper than could be sounded with the HAND LEAD.

3545 ohm. The unit of electrical resistance in the SI system.

3546 oil field. An area in which oil occurs in quantities worthy of exploitation.

3547 old ice. Any SEA ICE more than 1 year old.

3548 omnibearing. The MAGNETIC BEARING of an OMNIRANGE.

3549 omnidirectional antenna. See ANTENNA.

3550 omnidirectional beacon. See OMNIRANGE.

163

3551 omnidirectional range. See OMNIRANGE.

3552 omnirange. A radio AID TO NAVIGATION providing direct indication of the MAGNETIC BEARING(OMNIBEARING) of that STATION from any direction. Also called omnidirectional range or omnidirectionalbeacon.

3553 one-way traffic lane. See TRAFFIC LANE.

3554 onshore wind. WIND blowing from SEA toward the LAND. See SEA BREEZE.

3555 on soundings. Said of a vessel navigating within the 100-FATHOM LINE. In earlier times, said of a vessel in watersufficiently shallow for sounding by HAND LEAD.

3556 on the beam. Bearing approximately 090° relative (on the starboard beam) or 270° relative (on the port beam). Theexpression is often used loosely for broad on the beam, or bearing exactly 090° or 270° relative. Also called abeam. Seealso BROAD ON THE BEAM.

3557 on the bow. Bearing approximately 045° relative (on the starboard bow) or 315° relative (on the port bow). Theexpression is often used loosely for broad on the bow, or bearing exactly 045° or 315° relative. See also BROAD ONTHE BOW.

3558 on the quarter. Bearing approximately 135° relative (on the starboard quarter) or 225° relative (on the port quarter).The expression is often used loosely for broad on the quarter, or bearing exactly 135° or 225° relative. See also BROADON THE QUARTER.

3559 ooze. A soft MUD or SLIME.A fine-grained pelagic SEDIMENT containing undissolved SAND- or silt-sized, calcareous or siliceous skeletal

remains of small marine organisms in proportion of 30 percent or more, the remainder being amorphous clay-sizedmaterial.

3560 opacity (optical). The ratio of the incident to the transmitted (or reflected) light. It varies with the use of scattered orspecular light.

3561 opalescence. Whitish colour of the ATMOSPHERE and a slight change of the apparent colour of objects with respectto normal (for example, bluish colour of an object normally black or dark) produced by the SCATTERING of light bysmall particles held in suspension in the ATMOSPHERE.

3562 open coast. A COAST that is not sheltered from the SEA.

3563 open end traverse. See TRAVERSE: OPEN.

3564 opening. A break in a COASTLINE or a PASSAGE through a REEF, between SHOALS, etc.Any break in SEA ICE which reveals the water.

3565 open pack ice. FLOES seldom in contact with many LEADS and POOLS. ICE CONCENTRATION 4/10th to 9/10th.

3566 open port. A PORT which is not ICEBOUND during winter.

3567 open roadstead. A ROADSTEAD with relatively little protection from the SEA.

3568 open sea. That part of the OCEAN not enclosed by HEADLANDS, within narrow STRAITS, etc. See HIGH SEA.

3569 open sound. A BAY similar to a LAGOON but with large OPENINGS between the protecting ISLANDS.

3570 open traverse. See TRAVERSE.

3571 open water. A relatively large area of free navigable water in an ice-encumbered SEA.In U.S. terminology, an area of SEA with less than one-tenth ICE CONCENTRATION.

3572 operating system. System which controls and monitors the execution of programs in the COMPUTER.

3573 operative sheet. See FIELD BOARD.

164

3574 opisometer. A recording device designed to measure by revolutions of a small wheel, continuous linear distances on aMAP. Used in measuring the length of SHORELINE by closely following all the INDENTATIONS and sinuosities ofthe SHORE.

3575 opposite coasts. The geographical relationship of the coasts of two States facing each other.

3576 opposition. The situation of two periodic quantities differing by half a CYCLE.In ASTRONOMY, the situation of two CELESTIAL BODIES having either CELESTIAL LONGITUDES or

sidereal HOUR ANGLES differing by 180°. The term is usually used only in relation to the position of a SUPERIORPLANET or the MOON with reference to the SUN.

3577 optical axis. See AXIS OF LENS.

3578 optical centre. The point of intersection of lines which represent within the LENS those RAYS whose emergentdirections are parallel to their respective incident directions. This point lies on the optical axis.

3579 optical flat. A surface, usually of glass, ground and polished plane within a fraction of a WAVE LENGTH of light. Anoptical element or glass blank with an optical flat is used to test the flatness of other surfaces.

3580 optical parallax. See PARALLAX: INSTRUMENTAL.

3581 optical path. The path followed by a RAY of light through an OPTICAL SYSTEM.

3582 optical pumping magnetometer. See MAGNETOMETER.

3583 optical square. A small hand instrument used to set off a right angle.

3584 optical system. A series of LENSES, APERTURES, prisms, MIRRORS, etc., so arranged as to perform a definiteoptical function.

3585 orange peel sampler. A bottom sediment sampling device capable of obtaining as much as 1/2 cubic foot of samplefrom the ocean floor. Named for its likeness to an orange that has its peel quartered.

3586 orbit. The path of a body or particle under the influence of a gravitational or other force. For instance, the orbit of aCELESTIAL BODY is its path relative to another body around which it revolves.

In water waves, the path of a water particle affected by the wave motion.

3587 ordinate. See COORDINATES: PLANE RECTANGULAR.

3588 Organization for Economic Cooperation and Development (OECD). Member States promote economic growth,employment, and improved standards of living for a sound and harmonious development of world economy.

3589 orientation. The act of establishing the correct relationship in direction with reference to the POINTS of theCOMPASS. The state of being in correct relationship in direction with reference to the POINTS of the COMPASS. AMAP is in orientation when the map SYMBOLS are parallel with their corresponding ground features. A PHOTO-GRAPH is in orientation when it correctly presents the perspective view of the ground or when IMAGES on thePHOTOGRAPH appear in the same direction from the point of observation as do the corresponding map symbols. SeeORIENTATION OF PLANE TABLE, ORIENTATION OF SURVEYING INSTRUMENT, ORIENTATION:PHOTOGRAMMETRIC.

3590 orientation: absolute. In PHOTOGRAMMETRY, the scaling, levelling, and ORIENTATION to GROUNDCONTROL (in a photogrammetric instrument) of a relatively oriented stereoscopic model or group of models.

3591 orientation: basal. In PHOTOGRAMMETRY, the establishment of the POSITION of both ends of an AIR BASE withrespect to a ground system of COORDINATES. In all, six elements are required. These are essentially the three-dimensional COORDINATES of each end of the BASE.

3592 orientation: exterior. In PHOTOGRAMMETRY, the determining (analytically or in a photogrammetric instrument) ofthe POSITION of the CAMERA STATION and the attitude of the taking CAMERA at the instant of EXPOSURE. Instereoscopic instrument practice, exterior orientation is divided into two parts, RELATIVE and ABSOLUTE ORIEN-TATION. Also called outer orientation.

3593 orientation: inner. See ORIENTATION: INTERIOR.

165

3594 orientation: interior. In PHOTOGRAMMETRY, the determining (analytically or in a photogrammetric instrument) ofthe interior perspective of the PHOTOGRAPH as it was at the instant of EXPOSURE. Elements of interior orientationare the CALIBRATED FOCAL LENGTH, location of the calibrated PRINCIPAL POINT, and the calibrated LENSDISTORTION. Also called inner orientation.

3595 orientation: outer. See ORIENTATION: EXTERIOR.

3596 orientation: photogrammetric. The recreation of natural terrain features at a miniature SCALE by the opticalprojection of overlapping PHOTOGRAPHS. The model is formed when all corresponding LIGHT RAYS from the twoPROJECTORS intersect in space. See also ORIENTATION: ABSOLUTE, ORIENTATION: BASAL,ORIENTATION: EXTERIOR, ORIENTATION: INTERIOR.

3597 orientation: relative. In PHOTOGRAMMETRY, the determining (analytically or in a photogrammetric instrument) ofthe POSITION and attitude of one of a pair of overlapping PHOTOGRAPHS with respect to the other PHOTOGRAPH.

3598 orientation of plane table. Placing the PLANE TABLE in such a position that every line drawn on the SURVEYSHEET from the point which represents the POSITION of the table on the ground to any other point on the sheet, isparallel to the corresponding line in nature.

3599 orientation of surveying instrument. Placing the instrument in such position that the HORIZONTAL CIRCLE reads0° when the LINE OF COLLIMATION is parallel to the direction it had at an earlier (initial) position of the instrument,or to a standard line of reference. If the line of reference is a MERIDIAN, the CIRCLE will show AZIMUTHS referredto that MERIDIAN.

3600 origin. In SURVEYING, the reference position from which angles or distances are reckoned. See alsoCOORDINATES: ORIGIN OF.

3601 origin: false. See COORDINATES: FALSE.

3602 original. See REPRODUCTION.

3603 original chart. See CHART.

3604 origin of coordinates. See COORDINATES: ORIGIN OF.

3605 orography. The branch of physical geography which deals with the formation and features of MOUNTAINS; thedescription of MOUNTAINS.

3606 oropesa sweep. Towed wire used for sweeping sea bottom when surveying SHOALS or WRECKS. It consists of a wirerunning from the stern of the vessel and held out on the water by an OTTER which also pulls down against a towedFLOAT. The amount of wire between FLOAT and OTTER determines the DEPTH of the outer end of the sweep. Theinner end of the sweep is held to the required DEPTH and in line with the stern of the vessel by a KITE.

3607 orthochromatic(adj.). Said of photographic surfaces sensitive to ultra-violet, blue, yellow, green and orange RAYS.Insensitive to red RAYS.

3608 orthodrome. Any line on a CHART representing a GREAT CIRCLE TRACK between two points.

3609 orthogonal(adj.). At right angles; rectangularly meeting, crossing, or lying at right angles.

3610 orthographic projection. See PROJECTION.

3611 orthometric correction. The CORRECTION to ELEVATIONS which takes into account the spheroidal shape of theEARTH and brings the ELEVATIONS to their true HEIGHT above MEAN SEA LEVEL.

3612 orthomorphic(adj.). Preserving the correct shape; of a PROJECTION which gives no local DISTORTION. SeePROJECTION: CONFORMAL.

3613 orthomorphic projection. See PROJECTION: CONFORMAL.

3614 orthophotograph. A photographic copy, prepared from a perspective PHOTOGRAPH, in which theDISPLACEMENTS of IMAGES due to TILT and RELIEF have been removed.

166

3615 orthophotomap. A PHOTOMAP made from an assembly of ORTHOPHOTOGRAPHS. It may incorporate specialcartographic treatment, photographic edge enhancement, colour separation, or a combination of these.

3616 orthophotomosaic. An assembly of ORTHOPHOTOGRAPHS forming a uniform-scale mosaic.

3617 orthophotoscope. A photomechanical device, used in conjunction with a double-projection anaglyphic instrument, forproducing ORTHOPHOTOGRAPHS.

3618 oscillation. The variation, usually with time, of the magnitude of a quantity with respect to a specified reference whenthe magnitude is alternately greater and smaller than the reference.

Half an oscillatory CYCLE, consisting of a fluctuation or VIBRATION in one direction; half a VIBRATION.

3619 oscillator. A device for producing OSCILLATIONS, especially one of the non-rotating type, as the sound generator of asonic DEPTH FINDER or a radio frequency generator. A submarine oscillator is a large, electrically-operateddiaphragm horn which produces a SOUND for transmission through water.

3620 oscillatory wave. See WAVE.

3621 oscillograph. A device for recording or indicating OSCILLATIONS or changes in an electric current.

3622 oscilloscope. A type of OSCILLOGRAPH that visually records an electrical WAVE on a fluorescent SCREEN, as of aCATHODE-RAY TUBE.

3623 otter. Plane surface towed forward of its middle length so that it will incline and dive. Used in fishing, minesweeping orSURVEYING. See KITE, KITE OTTER, and OROPESA SWEEP.

3624 outcrop. Naturally protruding, or erosionally exposed or uncovered part of a ROCK, most of which is covered byoverlying material.

3625 outer harbour. See HARBOUR.

3626 outer limit. The extent to which a coastal State claims or may claim a specific jurisdiction in accordance with theprovisions of International Law.

3627 outfall diffuser. A structure on an outfall through which liquids are discharged. The structure will usually project abovethe level of the outfall and can be an obstruction to navigation.

3628 outflow. The FLOW of water from the RIVER or its ESTUARY to the SEA.Total volume for any given period of time.

3629 outgoing stream. See EBB STREAM.

3630 output. The amount of power or energy produced by a machine.

3631 overfalls. Short, breaking WAVES occurring when a strong CURRENT passes over a SHOAL or other submarineOBSTRUCTION or meets a contrary CURRENT or WIND. See RIPS.

3632 overlap. In CARTOGRAPHY and SURVEYING, the amount by which one CHART or SURVEY SHEET extends alsoover an adjoining CHART or sheet.

The amount by which different sets of survey data, obtained with the same system, cover the same area; customarilyexpressed as a percentage.

3633 overlapping pair. In PHOTOGRAMMETRY, two PHOTOGRAPHS taken at different exposure stations in such amanner that a portion of one PHOTOGRAPH shows the same terrain as shown on a portion of the otherPHOTOGRAPH. This term covers the general case and does not imply that the PHOTOGRAPHS were taken forstereoscopic examination. See also STEREOSCOPIC PAIR.

3634 overlay. In mapping, a record on a transparent medium to be superimposed on another record.

3635 overlay tracing. A plotting on TRACING CLOTH at the SCALE of the smooth sheet used in conjunction with thesmooth sheet. See FAIR CHART.

167

3636 overprint. In CARTOGRAPHY, an additional PLATE, generally in a distinctive colour, printed down on a MAP orCHART which is already complete in itself. Overprints may be used to reproduce specialized information (e.g., to addaeronautical information to a TOPOGRAPHIC MAP) or to incorporate revision data without the amendment of existingPLATES.

3637 overtide. A shallow water harmonic tide constituent with a SPEED that is a multiple of the SPEED of one of the basicconstituents of the TIDE-PRODUCING FORCE. See HARMONIC CONSTITUENT, and SHALLOW WATERCONSTITUENT.

3638 ozalid print. See PRINT: OZALID.

168

P

3639 pack ice. Term used in a wide sense to include any area of SEA ICE, other than FAST ICE, no matter what form ittakes or how it is disposed. Pack ice may be described as VERY OPEN PACK ICE (1/10ths-3/10ths), OPEN PACKICE (4/10ths-6/10ths), CLOSE PACK ICE (7/10ths-9/10ths), VERY CLOSE PACK ICE (almost 10/10th with little ifany water visible).

3640 pair of compasses. See COMPASS.

3641 paleomagnetism. Remanent MAGNETISM produced by the EARTH's field when a material was initially formed.

3642 Pan American Institute of Geography and History (PAIGH). Develops, coordinates, and disseminates geographical,cartographic, historical, geophysical and related scientific studies.

3643 pancake ice. Pieces of NEW ICE, usually approximately circular, about 30 cm to 3 m across, and with raised rims, dueto the pieces striking against each other as the result of WIND and SWELL.

3644 panchromatic (adj.). Sensitive to light of all colours, as a film or plate emulsion.

3645 panel. In cartography the completed assembly of pieces of film positives onto a grid or projection which is used as abase for compilation.

In photogrammetry an element of a target used for control station identification on aerial photography.

3646 pantograph. An instrument for copying a CHART, drawing, etc., to any desired SCALE within the limits of theinstrument.

3647 paper. For printing hydrographic charts, heavyweight, single layer paper is used. Such paper is generally made whollyor partly from rags and simulates hand-made paper. It is strong, moisture resistant and manufactured to withstandsurface erasure.

3648 parabola. An open curve all points of which are equidistant from a fixed point, called the focus, and a straight line.

3649 parachute drogue. See DROGUE.

3650 parallactic angle. That angle of the ASTRONOMICAL TRIANGLE at the CELESTIAL BODY; the angle between abody's HOUR CIRCLE and its VERTICAL CIRCLE. Also called position angle.

In PHOTOGRAMMETRY, the angle subtended by the eye of the observer at the object viewed. Also called angle ofconvergence or angular parallax.

3651 parallactic error. See ERROR.

3652 parallactic grid. See GRID.

3653 parallax. An apparent displacement of the position of a body with respect to a reference point or system, caused by ashift in the point of observation.

3654 parallax: absolute stereoscopic. In PHOTOGRAMMETRY, considering a pair of AERIAL PHOTOGRAPHS ofequal PRINCIPAL DISTANCE, the algebraic difference of the distances of the two IMAGES from their respectivePHOTOGRAPH NADIRS, measured in a horizontal plane and parallel to the AIR BASE. Also called horizontal paral-lax, linear parallax, or x-parallax.

3655 parallax: angular. See PARALLACTIC ANGLE.

3656 parallax: annual. The angle subtended at a CELESTIAL BODY by the radius of EARTH's ORBIT. Also called helio-centric parallax, or stellar parallax.

3657 parallax: diurnal. The difference in the apparent direction or position of a CELESTIAL BODY as observed from thecentre of the EARTH and a point on its surface. This varies with the body's ALTITUDE and distance from the EARTH.Also called geocentric parallax.

3658 parallax: equatorial horizontal. HORIZONTAL PARALLAX as observed at the EQUATOR, as it attains itsmaximum value.

169

3659 parallax: geocentric. See PARALLAX: DIURNAL.

3660 parallax: heliocentric. See PARALLAX: ANNUAL.

3661 parallax: horizontal. In ASTRONOMY, the GEOCENTRIC PARALLAX of a body on the observer's HORIZON.In PHOTOGRAMMETRY, PARALLAX: ABSOLUTE STEREOSCOPIC.

3662 parallax: instrumental (or instrument). That change which may result from the position in which an observer standswith respect to the FIDUCIAL MARKS on an instrument (personal parallax); or by the imperfect ADJUSTMENT ofthe optics of the instrument (optical parallax).

3663 parallax: linear. See PARALLAX: ABSOLUTE STEREOSCOPIC.

3664 parallax: optical. See PARALLAX: INSTRUMENTAL.

3665 parallax: personal. See PARALLAX: INSTRUMENTAL.

3666 parallax: solar. The EQUATORIAL HORIZONTAL PARALLAX of the SUN.

3667 parallax: vertical. See PARALLAX: Y-.

3668 parallax: X-. See PARALLAX: ABSOLUTE STEREOSCOPIC.

3669 parallax: Y-. In PHOTOGRAMMETRY, the difference between the perpendicular distances of the two IMAGES of apoint on a pair of photographs from the vertical plane containing the AIR BASE. The existence of y-parallax is anindication of TILT in either or both PHOTOGRAPHS and/or a difference in flight heights, and interferes with stereo-scopic examination of the pair. Also called vertical parallax or want of correspondence.

3670 parallax bar. See STEREOMETER.

3671 parallax correction. A CORRECTION due to PARALLAX, particularly that SEXTANT ALTITUDE correction dueto the difference between the apparent direction from a point on the surface of the EARTH to a CELESTIAL BODYand the apparent direction from the centre of the EARTH to the same body.

3672 parallax difference. In PHOTOGRAMMETRY, the difference in the ABSOLUTE STEREOSCOPIC PARALLAXESof two points imaged on a pair of PHOTOGRAPHS. Customarily used in determination of the difference inELEVATIONS of objects.

3673 parallax in altitude. GEOCENTRIC PARALLAX of a body at any ALTITUDE. The expression is used to distinguishthe PARALLAX at the given ALTITUDE from the HORIZONTAL PARALLAX when the body is in the HORIZON.

3674 parallax inequality. The variation in the RANGE OF TIDE or in the SPEED of TIDAL CURRENTS due to thecontinual change in the distance of the MOON from the EARTH.

3675 parallel. A circle (or approximation of a circle) on the surface of the EARTH, parallel to the EQUATOR andconnecting points of equal LATITUDE. Also called parallel of latitude.

3676 parallel: astronomical. A line on the surface of the EARTH which has the same ASTRONOMICAL LATITUDE atevery point. Because the DEFLECTION OF THE VERTICAL is not the same at all points on the EARTH, anastronomical parallel is an irregular line, not lying in a single plane.

3677 parallel: celestial. See PARALLEL OF DECLINATION.

3678 parallel: geodetic. A line on the SPHEROID which has the same GEODETIC LATITUDE at every point. A geodeticparallel, other than the EQUATOR, is not a GEODETIC LINE. In form, it is a SMALL CIRCLE whose plane isparallel with the plane of the GEODETIC EQUATOR.

3679 parallel: geographic(al). A line on the EARTH having the same LATITUDE at every point. The term is applicablealike to an ASTRONOMICAL PARALLEL or to a GEODETIC PARALLEL.

3680 parallel: ground. In PHOTOGRAMMETRY, the intersection of the plane of the PHOTOGRAPH with the plane ofreference of the GROUND. See also AXIS OF HOMOLOGY.

170

3681 parallel: isometric. In PHOTOGRAMMETRY, the intersecting line between the plane of a PHOTOGRAPH and ahorizontal plane having an equal perpendicular distance from the same PERSPECTIVE CENTRE.

3682 parallel: photograph. In PHOTOGRAMMETRY, the IMAGE on a PHOTOGRAPH of any horizontal line in theobject space which is perpendicular to the PRINCIPAL PLANE. All photograph parallels are perpendicular to thePRINCIPAL LINE. The photograph parallel passing through the PRINCIPAL POINT is the principal parallel and thatpassing through the ISOCENTRE is the ISOMETRIC PARALLEL.

3683 parallel: principal. See PARALLEL: PHOTOGRAPH.

3684 parallel: standard. A PARALLEL of latitude which is used as a control in the computation of a map PROJECTION.For a tangent cone, this is the PARALLEL of tangency. For a secant cone, the two PARALLELS of intersection are thestandards.

3685 parallel motion protractor. See PROTRACTOR.

3686 parallel of altitude. See ALMUCANTAR.

3687 parallel of declination. A circle of the CELESTIAL SPHERE parallel to the CELESTIAL EQUATOR. Also calledcelestial parallel.

3688 parallel of latitude. See PARALLEL.

3689 parallel plate. An optical disk with optically flat, parallel surfaces; used especially in optical MICROMETERS.

3690 parallel rulers (or rule). An instrument for transferring a line parallel to itself.

3691 parallel sailing. See SAILING.

3692 parallel sounding. A system of surveying in which the MOTHER VESSEL takes soundings while following a givencourse; at the same time an array of satellite boats takes soundings along parallel courses at a fixed distance from theMOTHER VESSEL. The satellite boats determine their position relative to the MOTHER VESSEL.

The term is also employed when a single vessel takes multiple lines of soundings with transducers arranged atregular intervals on outriggers.

3693 parameter. A quantity which remains constant within the limits of a given case or situation.

3694 parametric. Characterizing a quantity which is influenced by the behaviour of one or more parameters .

3695 parametric latitude. See LATITUDE.

3696 paranthelion. PHOTOMETEOR of the HALO family, consisting of white, round, luminous spots, having a diameterslightly greater than that of the SUN, which appears at 120° from it in the case of ordinary paranthelia, and at 90° in thecase of extraordinary paranthelia. The HEIGHT of the spots above the HORIZON is that of the SUN.

3697 parantiselena. PHOTOMETEOR of the HALO family, analogous to the PARANTHELION, the luminary being theMOON.

3698 paraselene. Optical phenomenon of the HALO family, similar to but less brilliant than the PARHELION, the luminarybeing the MOON.

3699 paraselenic circle. PHOTOMETEOR of the HALO family, analogous to the PARHELIC CIRCLE, the luminary beingthe MOON.

3700 parhelia associated with the 22° halo. PHOTOMETEOR of the HALO family, consisting of two luminous iridescentspots which appear on the PARHELIC CIRCLE close to the SMALL HALO.

3701 parhelia associated with the 46° halo. PHOTOMETEOR of the HALO family consisting of two luminous, iridescentspots which appear on the PARHELIC CIRCLE close to the LARGE HALO.

3702 parhelic circle. PHOTOMETEOR of the HALO family, consisting of a white, horizontal circle, situated at the sameangular height as the SUN.

3703 partial eclipse. See ECLIPSE.

171

3704 partial node. The point, line, or surface, in a STANDING WAVE system where some characteristic of the wave fieldhas a minimum AMPLITUDE differing from zero.

3705 partial tide. See TIDE.

3706 particle velocity. In ocean wave studies, the instantaneous VELOCITY of a water particle undergoing orbital motion.At the CREST, its direction is the same as the direction of progress of the WAVE, and at the TROUGH it is in theopposite direction.

3707 particular scale. See SCALE.

3708 pascal. The unit of pressure in the SI SYSTEM.

3709 pass. A break in a MOUNTAIN RANGE, permitting easier passage from one side of the range to the other; also calleda COL.

A navigable CHANNEL leading to a HARBOUR or RIVER. Sometimes called PASSAGE.A narrow connecting CHANNEL between two bodies of water.

3710 passage. A narrow navigable CHANNEL, especially one through REEFS or ISLANDS. Sometimes called a PASS, orin New England waters a HOLE.

3711 passage: meridian. See MERIDIAN TRANSIT.

3712 passive system. A system which records energy emitted or reflected but does not produce or transmit energy of its own.

3713 passometer. A pocket-size instrument which registers the number of steps taken by the pedestrian carrying it. SeePEDOMETER.

3714 pass point. In PHOTOGRAMMETRY, a point whose horizontal and/or vertical position is determined from PHOTO-GRAPHS by photogrammetrical methods and which is intended for use (as in the manner of a SUPPLEMENTALCONTROL POINT) in the ORIENTATION of other PHOTOGRAPHS.

3715 patch. In ice terminology, a collection of PACK ICE, less than 10 km across.

3716 patent log. See LOG.

3717 path. A line of movement; course taken, as the path of a METEOR.A line connecting a series of points in space and constituting a proposed or travelled route.

3718 pattern. See LATTICE.

3719 P-code. The Precise (or Protected) GPS code; a very long (about 1014 bit) sequence of pseudo-random binary biphasemodulations on the GPS carrier at a chip rate of 10.23 Mhz which does not repeat itself for about 267 days. Each one-week segment of the P-code is unique to one GPS satellite and is reset each week. See also C/A CODE.

3720 peak. A prominent elevation either pointed or of very limited extent across the SUMMIT. Also called pike.

3721 pebble. A small STONE worn smooth and rounded by the action of water, SAND, ICE, etc. ranging in diameterbetween 4 and 64 millimetres.

3722 pecked line. In CARTOGRAPHY, a SYMBOL consisting of a line broken at regular intervals.

3723 pedometer. A pocket-size instrument which registers in linear units the distance traversed by the pedestrian carrying it.See PASSOMETER.

3724 pelagic-abyssal sediments. Deep sea SEDIMENTS that are free of terrestrial material except for a small proportion ofvery fine CLAY.

3725 pelagic division. In the classification of the marine environment and its inhabitants, a primary division of the SEAwhich includes the whole mass of water. The division is made up of the neritic province which includes that watershallower than 200 METRES, and the oceanic province which includes that water deeper than 200 METRES. The otherprimary division of the SEA is the BENTHIC DIVISION.

172

3726 pelorus. A COMPASS CARD in the form of a metal plate mounted in GIMBALS to maintain a horizontal position andfitted with SIGHT-VANES for observing BEARINGS where direct use of the COMPASS for this purpose isimpracticable. Also called dumb compass or bearing plate. See also ALIDADE.

3727 pencil. In optics, a set of RAYS coming to or spreading out from a point. See LIGHT PENCIL.

3728 pencil beam. A BEAM in which the RADIANT ENERGY is concentrated in an approximately conical or cylindricalportion of space of relatively small diameter. This type beam is used for many revolving navigational LIGHTS.

3729 pendulum. A body so suspended from a fixed point as to swing freely to and from under the combined forces ofGRAVITY and MOMENTUM.

A vertical bar so supported from below by a stiff spring as to vibrate to and for under the combined action ofGRAVITY and the restoring force of the spring.

3730 pendulum: working. A PENDULUM which is used (swung) in a determination of the INTENSITY OF GRAVITY.

3731 pendulum sextant. See SEXTANT.

3732 peninsula. A piece of LAND that is almost an ISLAND, being nearly surrounded by water; by extension any piece ofLAND projecting into the SEA, so that the greater part of its BOUNDARY is COASTLINE.

3733 peninsula: submarine. An elevated portion of the SUBMARINE RELIEF resembling a PENINSULA. The opposite isSAC.

3734 perch. A staff placed on top of a BUOY, ROCK, or SHOAL as a MARK for navigators. A ball or cage is sometimesplaced at the top of the perch, as an identifying MARK.

A unit of length equal to 5.5 YARDS or 16.5 FEET. Also called ROD, POLE.

3735 percolation. The process by which water is forced by wave action through the interstices of the bottom sediment andhas a tendency to reduce WAVE HEIGHTS.

3736 perigean range (of tide). The average of all monthly TIDE RANGES occurring at the TIME of PERIGEE. It is largerthan the MEAN RANGE, where the TYPE OF TIDE is either semidiurnal or mixed, and is of no practical significancewhere the TYPE OF TIDE is diurnal.

3737 perigean tides. See TIDE.

3738 perigee. That orbital point nearest the EARTH when the EARTH is the centre of attraction, as opposed to APOGEE.

3739 perihelion. That orbital point nearest the SUN, when the SUN is the centre of attraction, as opposed to APHELION.

3740 period. The interval needed to complete a CYCLE.Any specified duration of time.

3741 period (of a navigational light). The TIME taken to exhibit one complete sequence of intervals of light and darkness.

3742 periodic current. See CURRENT.

3743 periodic error. See ERROR.

3744 periodic wave(s). See WAVE.

3745 peripheral equipment. In computer systems, any unit of equipment, distinct from the central unit, which may providethe system with outside communication.

3746 permafrost. A LAYER of SOIL or BEDROCK at a variable depth beneath the surface of the EARTH in which thetemperature has been below freezing continuously from a few to several thousands of years. Permafrost exists where thesummer heating fails to descend to the base of the LAYER of frozen ground.

3747 permanent current. See CURRENT.

173

3748 Permanent International Association of Navigation Congresses (PIANC). Fosters progress of inland and maritimewaterways, of ports and of coastal areas by research and information exchange.

3749 permanent magnetism. See MAGNETISM.

3750 permeability: magnetic. See MAGNETIC PERMEABILITY.

3751 personal correction. The CORRECTION due to PERSONAL ERROR.

3752 personal equation. The time interval between the sensory perception of a PHENOMENON and the motor reactionthereto. Personal equation may be either positive or negative, as an observer may anticipate the occurrence of an event,or wait until he actually sees it occur before making a record. This is a SYSTEMATIC ERROR, treated as of theconstant type. It is a PERSONAL ERROR, for which the term personal equation is reserved. It is of special significancein OBSERVATIONS of TIME, made to determine CHRONOMETER CORRECTION.

3753 personal error. See ERROR.

3754 personal parallax. See PARALLAX: INSTRUMENTAL.

3755 perspective centre. The point of origin or termination of bundles of PERSPECTIVE RAYS. The two such pointsusually associated with a survey photograph are the interior perspective centre and the exterior perspective centre. In aperfect lens-camera system, PERSPECTIVE RAYS from the interior perspective centre to the photographic IMAGESenclose the same angles as the corresponding RAYS from the exterior perspective centre to the objects photographed.In a LENS having DISTORTION this is true only for a particular zone of the PHOTOGRAPH. In a perfectly adjustedlens-camera system the exterior and interior perspective centres correspond, respectively, to the front and rear NODALPOINTS of the camera lens.

3756 perspective grid. See GRID.

3757 perspective plane. Any plane containing the PERSPECTIVE CENTRE. The intersection of a perspective plane and theGROUND will always appear as a straight line on an AERIAL PHOTOGRAPH.

3758 perspective projection. See PROJECTION.

3759 perspective ray. A line joining a PERSPECTIVE CENTRE and a point object.

3760 perturbation. Disturbance in the orbital PATH of a CELESTIAL BODY, natural or artificial, due to attraction ofanother body. See LUNAR INEQUALITY.

3761 Ph. A SCALE used for expressing the acidity or ALKALINITY of solutions.

3762 phantom bottom. A false BOTTOM indicated by an ECHO SOUNDER, some distance above the actual BOTTOM.Such an indication, quite common in the deeper parts of the OCEAN,is due to large quantities of small organisms. Alsocalled deep scattering layer (DSL) or false bottom.

3763 phase. The visible aspect of an object.The amount by which a CYCLE has progressed from a specified origin.In ASTRONOMY, any of the stages of variation in the illumination of the MOON or a PLANET. See PHASE(S)

OF THE MOON.In physics, the state of aggregation of a substance, for example, solid (ICE), liquid (water), or gas (vapour).

3764 phase angle. The phase difference of two periodically recurring PHENOMENA of the same FREQUENCY, expressedin angular measure.

The angle at a CELESTIAL BODY between the SUN and EARTH.

3765 phase inequality. Variations in the TIDES or TIDAL CURRENTS associated with changes in the PHASE OFTHE MOON.

3766 phase lag. Angular retardation of the maximum of a constituent of the observed TIDE behind the correspondingmaximum of the same constituent of the hypothetical EQUILIBRIUM TIDE. Also called tidal epoch.

3767 phase modulation. See MODULATION.

3768 phase of a navigational light. Each element of the sequence of intervals of light and darkness (e.g. a flash, anECLIPSE).

174

3769 phase(s) of the moon. The various appearances of the MOON during different parts of the SYNODICAL MONTH.The CYCLE begins with new moon or change of the MOON at CONJUNCTION. The visible part of the waxing moonincreases in size during the first half of the CYCLE until full moon appears at OPPOSITION, after which the visiblepart of the waning moon decreases for the remainder of the CYCLE. First quarter occurs when the waxing moon is ateast QUADRATURE, last quarter when the waning moon is at west QUADRATURE. From last quarter to new andfrom new to first quarter the MOON is crescent; from first quarter to full and from full to last quarter it is gibbous.

3770 phase velocity. VELOCITY, measured over a short time period, at which a particular WAVE CREST is propagated amedium.

3771 phenomenon (pl. phenomena). Any fact, circumstance, or experience that is apparent to the senses and that can bescientifically described or appraised; as, an ECLIPSE is a PHENOMENON of ASTRONOMY.

3772 phonometer. An instrument for measuring the intensity or FREQUENCY of SOUNDS.

3773 phosphorescence. The production of light without sensible heat.Emission of electromagnetic RADIATION by a substance as a result of previous absorption of RADIATION of

shorter WAVE LENGTH. In contrast to FLUORESCENCE, the emission may continue for a considerable time aftercessation of the existing IRRADIATION.

3774 phot. The unit of ILLUMINATION in the CGS system.

3775 photic (adj.). Of SEA WATER, that is penetrated or influenced by sunlight.

3776 photoalidade. A photogrammetric instrument having a telescopic ALIDADE, a plateholder, and a hinged ruling armmounted on a tripod frame. It is used for plotting lines of direction and measuring vertical angles to selected featuresappearing on OBLIQUE and TERRESTRIAL PHOTOGRAPHS.

3777 photo base. The length of the AIR BASE as represented on a PHOTOGRAPH.

3778 photocompose (v.t.). To combine distinct parts of chart original by photographic processes.

3779 photocontrol point. See CONTROL POINT.

3780 photodelineation. The DELINEATION of features on a PHOTOGRAPH. See also COMPILATION.

3781 photoelectric pilot. A device for automatic steering in conjunction with the MAGNETIC COMPASS. Its pinnacle unit,or directive element, consists of a high-grade MAGNETIC COMPASS of special construction, a simple light system,and a photoelectric cell, so arranged that the slightest deviation of the vessel from its COURSE is detected and relayedto the steering engine unit.

3782 photoengraving. A process by which PHOTOGRAPHS are reproduced on printing plates especially one in which theREPRODUCTION is in relief: opposed to PHOTOGRAVURE. A PLATE so made. A PRINT from such a PLATE.

3783 photogoniometer. An instrument for measuring angles from the true PERSPECTIVE CENTRE to points on a PHOTO-GRAPH.

3784 photogram. A survey photograph. Either of a pair of stereoscopic PHOTOGRAPHS.

3785 photogrammetric camera. See CAMERA.

3786 photogrammetric control. See CONTROL.

3787 photogrammetric extension. See PHOTOTRIANGULATION.

3788 photogrammetric survey. See SURVEY.

3789 photogrammetry. The science or art of obtaining reliable measurements from PHOTOGRAPHS.

3790 photogrammetry: aerial. PHOTOGRAMMETRY utilizing AERIAL PHOTOGRAPHS.

3791 photogrammetry: analytical (or analytic). PHOTOGRAMMETRY in which solutions are obtained by mathematicalmeans.

175

3792 photogrammetry: terrestrial. PHOTOGRAMMETRY utilizing TERRESTRIAL PHOTOGRAPHS.

3793 photograph. A general term for a positive or negative picture made with a CAMERA on sensitized material, orPRINTS from such a camera original.

3794 photograph: aerial. Any PHOTOGRAPH taken from the air. Sometimes called air photograph.

3795 photograph: composite. In AERIAL PHOTOGRAPHY, a PHOTOGRAPH made by assembling the separatePHOTOGRAPHS, made by the several LENSES of a MULTIPLE LENS CAMERA in simultaneous EXPOSURE, intothe equivalent of a PHOTOGRAPH taken with a single wide-angle lens.

3796 photograph: ground. See PHOTOGRAPH: TERRESTRIAL.

3797 photograph: high oblique. An OBLIQUE PHOTOGRAPH in which the APPARENT HORIZON is included withinthe field of view.

3798 photograph: horizontal. PHOTOGRAPH taken with the camera axis horizontal.

3799 photograph: low oblique. An OBLIQUE PHOTOGRAPH in which the APPARENT HORIZON is not includedwithin the field of view.

3800 photograph: multiple lens. A PHOTOGRAPH made with a MULTIPLE LENS CAMERA.

3801 photograph: oblique. A PHOTOGRAPH taken with the camera axis intentionally directed between the HORIZON-TAL and the VERTICAL.

3802 photograph: terrestrial. A PHOTOGRAPH taken by a CAMERA located on the ground. Sometimes called groundphotograph.

3803 photograph: vertical. An AERIAL PHOTOGRAPH made with the camera axis vertical (or as nearly vertical as practi-cable in an aircraft).

3804 photograph axes. See FIDUCIAL AXES.

3805 photograph centre. The centre of a PHOTOGRAPH as indicated by the IMAGES of the FIDUCIAL MARK orMARKS of the CAMERA. In a perfectly adjusted CAMERA the photograph centre and the PRINCIPAL POINT areidentical.

3806 photograph coordinates. See COORDINATES.

3807 photograph meridian. See MERIDIAN.

3808 photograph nadir. See NADIR.

3809 photograph parallel. See PARALLEL.

3810 photograph perpendicular. The perpendicular from the interior perspective centre to the plane of the photograph.

3811 photographic interpretation. The examination of photographic IMAGES for the purpose of identifying objects anddeducing their significance.

3812 photography. The art or process of producing IMAGES on sensitized material through the action of light.

3813 photography: aerial. The art, science, or process of taking AERIAL PHOTOGRAPHS.

3814 photogravure. A photoengraving process by which PHOTOGRAPHS or textual matters are reproduced on intaglioprinting plate or rolls from which they may be transferred to paper.

3815 photohead plotter. A device used to plot information by exposure of a photographic film.

3816 photoindex. A MOSAIC made by assembling individual PHOTOGRAPHS, with accompanying designations, into theirproper relative positions and copying the assembly photographically at a reduced SCALE.

3817 photolithograph. A lithograph produced by PHOTOLITHOGRAPHY. See LITHOGRAPHY.

176

3818 photolithography. That branch of lithographic printing in which PHOTOGRAPHY is employed for production of theIMAGE on the final printing surface. The original printing surface, lithographic stone, has been almost completelydisplaced by thin and flexible sheets of metal (zinc, aluminium, stainless steel, bimetallic PLATES, polymetallicPLATES).

3819 photomap. A photomosaic of a specified land area, which also contains marginal information, descriptive DATA, and areference grid and/or projection. See also MOSAIC.

3820 photometeor. Optical phenomenon of the ATMOSPHERE.

3821 photometer. An instrument used in measuring the intensity of light, especially in determining the relative intensity ofdifferent lights.

3822 photosphere. The visible surface of the SUN and direct source of almost all its RADIATION. It is a gaseous LAYERsome 400 km thick, below which the SUN becomes opaque.

3823 photosynthesis. The formation of carbohydrates in living plants from water and carbon dioxide, by the action ofsunlight on the CHLOROPHYLL.

3824 phototheodolite. A ground-survey instrument combining a THEODOLITE and a SURVEYING CAMERA in whichthe relationship between the camera axis and the LINE OF COLLIMATION of the THEODOLITE can be measured.

3825 phototopography. The science of SURVEYING in which the detail is plotted entirely from PHOTOGRAPHS taken atsuitable ground stations.

3826 phototriangulation. The process for the extension of HORIZONTAL and/or VERTICAL CONTROL whereby themeasurements of angles and/or distances on overlapping PHOTOGRAPHS are related into a spatial solution using theperspective principles of the PHOTOGRAPHS. Generally, this process involves using AERIAL PHOTOGRAPHS, andis called aerotriangulation, aerial triangulation, or photogrammetric extension.

3827 photozincography. The use of a zinc plate in PHOTOENGRAVING.

3828 pictomap. A map supplement on which the photographic imagery of a standard photomosaic has been converted intointerpretable colours and symbols by means of tonal masking techniques.

3829 picture control point. See CONTROL POINT.

3830 picture plane. A plane upon which a system of lines or RAYS from an object to form an IMAGE or picture can beprojected. In perspective drawing, the system of RAYS is understood to converge to a single point. In PHOTO-GRAMMETRY, the PHOTOGRAPH is the picture plane.

3831 piedmont (adj.). Situated or formed at the base of MOUNTAINS.

3832 pie graph. Circular SYMBOL divided into sectors to indicate proportions of a total value.

3833 pier. A long, narrow structure extending into the water to afford a berthing place for vessels, to serve as a promenade,etc. See also JETTY.

3834 pierhead. The outer end of a PIER. Also called wharfhead.

3835 piezoelectric effect. The PHENOMENON, exhibited by certain crystals, in which mechanical compression produces apotential difference between opposite crystal faces, or, an applied electric field produces corresponding changes indimensions.

3836 piezoelectricity. The property exhibited by some asymmetrical crystalline materials which when subjected to strain insuitable directions develop electric POLARIZATION proportional to the strain. Inverse piezoelectricity is the effect inwhich mechanical strain is produced in certain asymmetrical crystalline materials when subjected to an external electricfield; the strain is proportional to the electric field.

3837 piggot gun. A coring instrument for obtaining deep-sea BOTTOM SAMPLES. This instrument uses an explosivecharge to drive a metal bit into the BOTTOM at the instant the sharp end of the bit strikes the BOTTOM.

3838 pigmentation. A colouring of tissue by deposition of PIGMENTS.

177

3839 pigments. A colouring matter used as paint or dye or a natural colouring matter of a tissue.

3840 pile. A long heavy timber or section of steel, wood, concrete, etc., forced into the earth to serve as a support, as for aPIER, or to resist lateral pressure.

3841 piling. A group of piles set in a row.

3842 pillar buoy. A BUOY composed of a tall central structure on a broad flat base.

3843 pilot. 1. A person who directs the movements of a vessel through pilot waters, usually a person who has demonstratedextensive knowledge of channels, aids to navigation, dangers to navigation, etc., in a particular area and is licensed forthat area.

2. A book of sailing directions.

3844 pilot chart. See CHART.

3845 pilot station. The office or headquarters of pilots; the place where the services of a pilot may be obtained.

3846 pilot waters. Areas in which the services of a marine pilot are available. Waters in which NAVIGATION is by piloting.More usually called pilotage waters in British terminology.

3847 pilotage waters. See PILOT WATERS.

3848 ping. An acoustic pulse signal projected by an underwater TRANSDUCER.

3849 pinger. A battery powered acoustic device equipped with a TRANSDUCER that transmits SOUND WAVES. Whenthe pinger is attached to a wire and lowered into the water, the direct and bottom reflected sound can be monitored witha listening device. The difference between the arrival time of the DIRECT and REFLECTED WAVES is used tocompute the distance of the pinger from the ocean bottom.

3850 pingo. Small conical hills having a large central core of ice formed from the encroachment of permafrost and theresulting hydrostatic pressure.

3851 pingo-like features (PLF). Small conical undersea features having the geomorphological appearance of pingos.

3852 pinnacle. Any high tower or spire-shaped pillar of rock or coral, alone or cresting a SUMMIT. It may extend above thesurface of the water. It may or may not be a hazard to SURFACE NAVIGATION.

3853 pinnacled iceberg. An iceberg weathered in such manner as to produce spires or pinnacles. Also called pyramidaliceberg, irregular iceberg.

3854 pinpoint (v.t. and i.). To establish (POSITION) with great ACCURACY.

3855 pip. See BLIP.

3856 pipe. A hollow metal tube, of varying diameters and lengths, imbedded in the bottom in a manner similar to a pile. Pipesare often used as privately maintained aids to navigation and in the determination of beach or bottom sand migration(deposition or erosion).

3857 pipeline. A string of interconnected pipes used for the transport of matter, nowadays mainly oil or gas .

3858 pitch. The oscillations of a ship about the transverse axis, due to the bow and stern being raised or lowered on passingthrough successive crests and troughs of waves. Also called pitching.

The distance along the axis of a screw or other helix between consecutive threads or ribs.The distance a propeller would advance longitudinally, in one revolution if there were no slip.

3859 pitching. See PITCH.

3860 pivot. A spindle or pin by which a movable part of an instrument is supported so as to be free to turn. The pivot of aCOMPASS is usually a fixed point on which the NEEDLE hangs by a single jewelled cap.

3861 pivot point. After a ship has assumed its DRIFT ANGLE in a turn, the point on the centerline between the bow and theCENTRE OF GRAVITY at which the resultant of the velocities of rotation and transaction is directed along thecenterline. To an observer on board, the ship appears to rotate about this point.

178

3862 pixel. Contraction for "picture element". The smallest element resolvable by electronic raster devices such asSCANNER, DISPLAY, and PLOTTER. See REMOTE SENSING.

3863 plain. Any LAND with a flat or very slightly undulating surface.A flat, gently sloping or nearly level region of the SEA FLOOR, for example, ABYSSAL PLAIN.

3864 plan. An orthographic drawing on a horizontal plane, as of an instrument, a horizontal section, or a layout.A large scale map or chart of a small area.

3865 plane: horizontal. A plane perpendicular to the DIRECTION OF GRAVITY; any plane tangent to the GEOID orparallel to such a plane.

In SURVEYING, a plane perpendicular to the PLUMB LINE within which or on which angles and distances areobserved.

3866 plane: vertical. Any plane passing through a point on the EARTH and containing the ZENITH and NADIR of thatpoint.

In SURVEYING, a plane at right angles to a HORIZONTAL PLANE and within which angles and distances areobserved.

3867 plane coordinates. See COORDINATES: PLANE RECTANGULAR.

3868 plane rectangular coordinates. See COORDINATES.

3869 plane sailing. See SAILING.

3870 plane surveying. See SURVEYING.

3871 planet. A CELESTIAL BODY of the SOLAR SYSTEM, shining by reflected light and revolving around the SUN.

3872 planet(s): inferior. The PLANETS with ORBITS smaller than that of the EARTH: Mercury and Venus.

3873 planet(s): major. The four largest PLANETS (Jupiter, Saturn, Uranus and Neptune).

3874 planet(s): minor. See ASTEROID.

3875 planet(s): navigational. The four PLANETS commonly used for celestial observations: Venus, Mars, Jupiter, andSaturn.

3876 planet(s): superior. The PLANETS with ORBITS larger than that of the EARTH: Mars, Jupiter, Saturn, Uranus,Neptune, and Pluto.

3877 plane table. A field device for plotting the lines of a SURVEY directly from the OBSERVATIONS. It consistsessentially of a drawing board mounted on a TRIPOD, with a ruler on which a TELESCOPE or other sighting device ismounted. See ALIDADE. Also written as one word.

3878 plane table traverse. See TRAVERSE.

3879 planetary (adj.). Of a PLANET or the PLANETS. Like a PLANET. Terrestrial.

3880 planetoid. See ASTEROID.

3881 planform. The outline or shape of a body of water as determined by the STILL WATER LEVEL.

3882 planimeter. A mechanical integrator for measuring the area of a plane surface.

3883 planimetric map. See MAP.

3884 planimetry. The measurement of plane surfaces.The plan details of a MAP.

3885 planisphere. A MAP or CHART that is the PROJECTION of all or part of a sphere on a plane.A representation, on a plane, of the CELESTIAL SPHERE, especially one on a polar PROJECTION, with means

provided for making certain measurements such as ALTITUDE and AZIMUTH. See STAR FINDER.

3886 plankton. The passively drifting or weakly swimming organisms in marine and fresh waters.

179

3887 plan position indicator (PPI). A cathode-ray SCOPE on which SIGNALS appear in correct relation to each other, sothat the scope face presents a maplike representation of the area about the transmitter, the direction of a TARGET beingrepresented by the direction of its ECHO from a centre and range by its distance from that centre.

3888 planning chart. A chart designed for use in planning voyages or flight operations or investigating areas of marine oraviation activities.

3889 plat (U.S. terminology). A diagram drawn to SCALE showing land boundaries and subdivisions, and identification ofthe several units shown thereon, and including one or more certificates indicating due approval. A plat differs from aMAP in that it does not necessarily show additional cultural, drainage and relief features. See also MAP:CADASTRAL.

3890 plate. A thin, metal, plastic, or paper sheet, that carries the printing image and whose surface is treated to make only theimage areas ink-receptive.

3891 plateau. A flat or nearly flat area of considerable extent, dropping off abruptly on one or more sides; a TABLELAND.

3892 plate level. See LEVEL.

3893 platform. In geographical literature, a natural or artificial TERRACE; a flat elevated piece of GROUND; aTABLELAND, a PLATEAU.

In oceanographic terminology, any man-made structure (aircraft, ship, BUOY, or tower) from or on whichoceanographic instruments are suspended or installed.

Structures which are erected on or over the seabed and subsoil for the purpose of exploring for, developing,removing and transporting resources therefrom.

3894 platform: mineral development. A PLATFORM used in the exploitation of minerals.

3895 platform: offshore. A permanent offshore structure, either fixed or floating, used in the production of oil or natural gas.

3896 platform: piled. A platform with a steel jacket fixed by piles to the seabed.

3897 platform jacket. The section of a platform from base to deck level, on which deck modules are fitted.

3898 plot (v.t.). To draw lines and points to represent certain conditions graphically, as the various lines and points on aCHART or PLOTTING SHEET, representing the progress of a craft, a graphical solution of a problem, etc.

3899 plotter. An instrument used for plotting straight lines and measuring angles on a CHART or PLOTTING SHEET. SeePROTRACTOR.

In data processing it stands fora) Vector plotter : An electromechanical device for automatically drawing curves. The curves are generated by a

sequence of linear incremental pieces ("vectors") depending on the resolution provided by the device. For cartographicpurposes, high precision FLAT-BED PLOTTERS (drawing tables) are used.

b) Raster or electrostatic plotter : A sophisticated electronic device for producing pictures by means of rastertechniques. The picture is generated by lines of PIXELS drawn sequentially, typically with a resolution of 200-400pixels per inch. Raster plotters can produce complete pictures in short time irrespective of the complexity of thepicture's contents.

3900 plotting. See PLOT.

3901 plotting board. A stiff transparent polar coordinate plotting sheet for tracking other vessels and aircraft and solvingproblems involving RELATIVE MOVEMENT.

3902 plotting sheet. A blank CHART, usually on the MERCATOR PROJECTION, showing only the GRATICULE and aCOMPASS ROSE, so that the plotting sheet can be used for any LONGITUDE.

In hydrographic SURVEYING, a working sheet on which the main STATIONS of the SURVEY are plotted. Itforms the framework of the SURVEY and provides the basis for accurately locating and plotting all the detail of theSURVEY. See also LATTICE.

3903 plotting sheet: electronic. A CHART showing time difference or phase difference curves of a circular or a hyperbolicnavigational system. These LATTICE lines are used to accurately position the DATA collected during HYDROGRA-PHIC or OCEANOGRAPHIC SURVEYS.

180

3904 plumb bob. A conical device, usually of brass and suspended by a cord, by means of which a point can be projectedvertically into space over relative short distances. Also referred to as plummet.

3905 plumb line. The LINE OF FORCE in the geopotential field.The continuous curve to which the direction of the normal GRAVITY is everywhere tangential.

3906 plumb line: deflection of. See DEFLECTION OF THE VERTICAL.

3907 plumb point. See NADIR: PHOTOGRAPH.

3908 plummet. A SOUNDING LEAD. A PLUMB BOB.

3909 plunging breaker. See BREAKER.

3910 pluviograph. RAIN GAUGE which includes an arrangement for the time recording of the depth of water fromPRECIPITATION.

3911 pocket compass. See COMPASS.

3912 Poincaré wave. A long period gravity wave. The superposition of incident and reflected plane waves (Sverdrup Waves)such that the composite wave fronts are perpendicular to the coast with a set of nodal lines normal to the wave fronts.The term is used in tidal analysis to study ocean tides and tides in open seas and gulfs.

3913 point. One thirty-second of a circle, or 11 1/4 DEGREES. Also called COMPASS POINT when used in reference toCOMPASS directions.

The extreme end of a CAPE; or the outer end of any land area protruding into the water, usually less prominent thana CAPE.

In digital cartography, the 0-dimensional geometric primitive of an object that specifies location.

3914 pointing line. See COLLIMATION: LINE OF.

3915 point of Aries. See EQUINOX.

3916 point of arrival. See ARRIVAL: POINT OF.

3917 point of destination. See DESTINATION: POINT OF.

3918 point of origin. See COORDINATES: ORIGIN OF.

3919 point of symmetry. The point in the FOCAL PLANE of a CAMERA about which all lens DISTORTIONS aresymmetrical. If the LENS were perfectly mounted, the point of symmetry would coincide with the PRINCIPAL POINT.

3920 polar aurora. See AURORA.

3921 polar axis. See AXIS.

3922 polar bearing. See BEARING.

3923 polar chart. See CHART.

3924 polar circle. Either the ARCTIC CIRCLE (north polar circle) or the ANTARCTIC CIRCLE (south polar circle).

3925 polar coordinates. See COORDINATES.

3926 polar distance. ANGULAR DISTANCE from a CELESTIAL POLE; the arc of an HOUR CIRCLE between aCELESTIAL POLE, usually the ELEVATED POLE, and a point on the CELESTIAL SPHERE, measured from theCELESTIAL POLE through 180°. When the DECLINATION and LATITUDE are of the same name,CODECLINATION is the same as polar distance measured from the ELEVATED POLE.

3927 polar front. Quasi-permanent FRONT of great extent, of middle latitudes, which separates relatively cold polar air andrelatively warm tropical air, and on which WAVES are produced.

3928 polar ice. SEA ICE that is more than one year old (in contrast to WINTER ICE). The WMO code defines polar ice asany SEA ICE more than one year old and more than 3 metres thick.

181

3929 Polaris correction. A CORRECTION to be applied to the OBSERVED ALTITUDE of Polaris to obtain theLATITUDE.

3930 polarization. A condition of RADIANT ENERGY in which the VIBRATIONS take place in a regular manner, as in astraight line, circle or ellipse, in a plane perpendicular to the path of travel.

3931 polarization error. See ERROR.

3932 polar orthographic projection. See PROJECTION.

3933 polder. LAND reclaimed from the SEA or other body of water by the construction of an EMBANKMENT to restrainthe water. See DIKE.

3934 pole. Either of the two points of intersection of the surface of a sphere or SPHEROID and its AXIS.The origin of a system of POLAR COORDINATES.Either of the two MAGNETIC POLES of a MAGNET.A long slender rod, as of timer or metal. Such a rod used for SOUNDING is called SOUNDING POLE.

3935 pole: celestial. Either of the two points of intersection of the CELESTIAL SPHERE and the extended AXIS of theEARTH, labelled N or S to indicate either the north celestial pole or the south celestial pole.

3936 pole: depressed. The CELESTIAL POLE below the HORIZON, of opposite NAME to the LATITUDE.

3937 pole: elevated. The CELESTIAL POLE above the HORIZON, agreeing in NAME with the LATITUDE.

3938 pole: geographic(al). Either of the two points of intersection of the surface of the EARTH with its AXIS, labelled N orS to indicate either the north geographical pole or the south geographical pole.

3939 pole: magnetic. See MAGNETIC POLE.

3940 pole: terrestrial. One of the POLES of the EARTH. See POLE.

3941 pollution. The direct or indirect alteration of the physical, chemical, thermal, biological or radioactive properties of anypart of the environment in such a way as to create a hazard or potential hazard to the health, safety or welfare of anyliving species.

3942 polyconic projection. See PROJECTION.

3943 polygon. A non-selfintersecting, closed chain defining the boundary of an area.

3944 polynia. A water area enclosed in ICE, generally fast; this water area remains constant and usually has an oblong shape;sometimes limited to one side by the COAST.

Any enclosed sea water area in PACK ICE other than a LEAD, not large enough to be called OPEN WATER. If apolynia is found in the same region every year, for example, off the MOUTHS of big RIVERS, it is called a recurringpolynia. A temporary small clearing in PACK ICE which consists of small ICE FLOES and BRASH ICE in continuouslocal movement is called an unstable polynia; an opening which is flanked by large FLOES and therefore appears to berelatively stable is called a stable polynia. When frozen over, a polynia becomes an ice skylight from the point of viewof the submariner.

3945 polyvinyl corer. See CORER: HYDROPLASTIC.

3946 pond. A small body of still water of artificial formation, its BED being either hollowed out of the SOIL or formed byembanking and damming up a natural HOLLOW.

3947 pontoon. Floating structure, usually rectangular in shape which serves as landing, pierhead or bridge support.

3948 pool. A small body of still or standing water, permanent or temporary; chiefly one of natural formation.A deep or still place in a RIVER or STREAM.In ice terminology, any enclosed relatively small area in PACK ICE, drift ICE, other than a LEAD. See POLYNIA.

3949 porosity. The ratio of the aggregate volume of pore space in a ROCK or SEDIMENT to its total volume, usuallyexpressed as a percentage.

182

3950 port. A place provided with terminal and transfer facilities for loading and discharging cargo or passengers, usuallylocated in a HARBOUR.

The left side of a craft, facing forward. The opposite is STARBOARD.

3951 port authority. The entity responsible for administration and maintenance of harbour facilities.

3952 port handbook. A handbook for a specific port.

3953 position. DATA which define the location of a point with respect to a reference system. The COORDINATES whichdefine such a location. The place occupied by a point on the surface of the EARTH, or in space.

3954 position: adjusted. An adjusted value of the coordinate position of a point on the EARTH.

3955 position: apparent. In ASTRONOMY, the POSITION of a CELESTIAL BODY reduced to the centre of the EARTH.It differs from the position in which the object is observed as to the effects of REFRACTION, DIURNAL ABERRATI-ON, and DIURNAL PARALLAX.

3956 position: assumed. A point at which a craft is assumed to be located, particularly one used as preliminary toestablishing certain navigational DATA, as that point of the surface of the EARTH for which the computed ALTITUDEis determined in the solution of a celestial OBSERVATION.

3957 position: astronomical. A point on the EARTH whose COORDINATES have been determined as a result of OBSER-VATION of CELESTIAL BODIES. The expression is usually used in connection with POSITIONS on LANDdetermined with great ACCURACY for SURVEY purposes.

A point on the EARTH, defined in terms of ASTRONOMICAL LATITUDE and LONGITUDE.

3958 position: dead reckoning. The POSITION of a craft determined by DEAD RECKONING, or by advancing a previousPOSITION for COURSES and distances.

3959 position: detached. In HYDROGRAPHIC SURVEY, an expression indicating a POSITION taken, to locate ROCKS,floating AIDS TO NAVIGATION, least DEPTHS on SHOALS or other dangers or features of importance.

3960 position: estimated. The most probable POSITION of a craft determined from incomplete DATA or DATA ofquestionable ACCURACY. Such a POSITION might be determined by applying a correction to the DEADRECKONING POSITION.

3961 position: field. A POSITION computed while field work is in progress to determine the acceptability of theOBSERVATIONS or to provide a preliminary POSITION for other purposes.

3962 position: geodetic. A point on the EARTH, the COORDINATES of which have been determined byTRIANGULATION from an initial STATION whose location has been established as a result of ASTRONOMICALOBSERVATIONS. The COORDINATES of such a POSITION depend upon the REFERENCE SPHEROID used.

3963 position: geographic(al). The POSITION of a point on the surface of the EARTH expressed in terms of LATITUDEand LONGITUDE, either ASTRONOMICAL or GEODETIC.

A SUB-POINT.

3964 position: relative. A point defined with reference to another POSITION, either fixed or moving.

3965 position angle. See PARALLACTIC ANGLE.

3966 position angles. The two sextant angles observed on a SURVEY SHIP for determining its POSITION.

3967 position approximate. Of inexact POSITION. The expression is used principally on CHARTS to indicate that thePOSITION of a WRECK, SHOAL, etc., has not been accurately determined or does not remain fixed. Usually shown bythe abbreviation 'PA'.

3968 position circle. See CIRCLE OF POSITION.

3969 position dilution of precision (PDOP). See DILUTION OF PRECISION.

3970 position doubtful. Of uncertain POSITION. The expression is used principally on CHARTS to indicate that aWRECK, SHOAL, etc., has been reported in various POSITIONS and not definitely determined in any. Usually shownby the abbreviation 'PD'.

183

3971 positioning. The process of determination of a position.

3972 position line. See LINE OF POSITION.

3973 positive. In PHOTOGRAPHY, an IMAGE of the original object which corresponds to the same in the scheme of lightand shade.

3974 positive altitude. See ANGLE OF ELEVATION.

3975 positive gradient. In OCEANOGRAPHY, a positive rate of change with DEPTH.

3976 post. A small BEACON, more substantial than a PERCH, used for marking CHANNELS. See also PILE.

3977 potable water intake (PWI). A structure designed for the intake of drinking water. The intake is usually elevatedabove the bottom, supported and protected by a debris-screening structure (CRIB), a separately charted feature.

3978 potential density. In OCEANOGRAPHY, the DENSITY that a parcel of water would have if raised adiabatically to thesurface, that is, if determined from the parcel's 'in situ' SALINITY and POTENTIAL TEMPERATURE.

3979 potential temperature. In OCEANOGRAPHY, the temperature that a WATER SAMPLE would attain if raised adia-batically to the sea surface. For the deepest points of the OCEAN, which are just over 10 000 METRES, the adiabaticcooling would be less than 1.5° C.

3980 pound. A unit of mass, equal to 0.45359237 KILOGRAMS.

3981 power. See MAGNIFYING POWER.

3982 precautionary area. A routeing measure comprising an area within defined limits where ships must navigate withparticular caution and within which the direction of traffic flow may be recommended.

3983 precession. Change in the direction of the AXIS of ROTATION of a spinning body, as a GYROSCOPE, when actedupon by a torque. The direction of motion of the AXIS is such that it causes the direction of spin of the GYROSCOPEto tend to coincide with that of the impressed torque.

3984 precession of the equinoxes. The conical motion of the EARTH's AXIS about the vertical to the plane of theECLIPTIC, caused by the attractive force of the SUN, MOON, and other PLANETS on the equatorial protuberance ofthe EARTH.

3985 precipitation. PHENOMENON made up of an aggregate of aqueous particles, liquid or solid, crystallized oramorphous, which fall from a CLOUD or group of CLOUDS and reach the ground.

3986 precipitation intensity. Amount of PRECIPITATION collected in unit time interval. See RAINFALL AMOUNT.

3987 precision. The degree of refinement of a value not to be confused with ACCURACY, which is the degree ofconformance with the correct value.

3988 precision ephemeris. The ephemeris which describe the orbit of a navigational satellite precisely. They are of higherprecision than the broadcast ephemeris but not available in real time.

3989 predicted tide(s). See TIDE.

3990 preferred channel mark. A distinctive lateral mark used to indicate both the point at which a channel divides and thechannel that has been designated as preferred channel.

3991 preferred datum. A geodetic datum selected as a base for consolidation of local independent datums within ageographical area. Also called major datum.

3992 preliminary survey. See SURVEY: RECONNAISSANCE.

3993 press proof. A lithographed impression taken from among the first copies run on the press and used for chekingpurposes. Also called press pull.

3994 pressure: atmospheric. Pressure (force per unit area) exerted by the ATMOSPHERE, on any surface, by virtue of itsWEIGHT: it is numerically equal to the WEIGHT of a vertical column of air above the surface of unit base area,extending to the outer limit of the ATMOSPHERE.

184

3995 pressure: barometric. ATMOSPHERIC PRESSURE as indicated by a BAROMETER.

3996 pressure: hydrostatic. The pressure at a given DEPTH due to the WEIGHT of the water column above that DEPTH.

3997 pressure: sound. The instantaneous pressure at a point in a medium in the presence of a SOUND WAVE, minus thestatic pressure at that point.

3998 pressure-altitude. An ATMOSPHERIC PRESSURE expressed in terms of ALTITUDE which corresponds to thatpressure in the STANDARD ATMOSPHERE.

3999 pressure gauge. See GAUGE.

4000 pressure gradient. Vector, perpendicular to the isobaric line or surface, directed towards low pressure and of intensityequal to the rate of variation of the pressure with distance. Also called barometric gradient.

4001 pressure hydrophone. A HYDROPHONE in which the electric OUTPUT substantially corresponds to theinstantaneous SOUND PRESSURE of the impressed SOUND WAVE.

4002 pressure ice. A general term for FLOATING ICE which has been squeezed together and in places forced upwards,when it can also be described as RAFTED ICE, HUMMOCKED ICE, PRESSURE RIDGE, TENTED ICE.

4003 pressure plate. In PHOTOGRAPHY, a flat plate (usually of metal but frequently of glass or other substance) which, bymeans of mechanical force, presses film into contact with the focal-plane plate of a CAMERA.

4004 pressure ridge. A ridge or wall of HUMMOCKED ICE where one FLOE has been pressed against another.

4005 pressure wave. A short-period oscillation of pressure such as that associated with the propagation of SOUND throughthe ATMOSPHERE; a type of LONGITUDINAL WAVE.

4006 Price-Gurley current meter. A battery powered electromechanical CURRENT METER which measures current speedonly.

4007 pricker. A pointed tool used instead of a pencil for laying off finer and more accurate lines on a PLOTTING SHEET.

4008 primary circle. See PRIMARY GREAT CIRCLE.

4009 primary great circle. A GREAT CIRCLE used as the origin of measurement of a COORDINATE; particularly such acircle 90° from the POLES of a system of SPHERICAL COORDINATES, as the EQUATOR. Also called primary cir-cle, fundamental circle.

4010 primary radar. See RADAR.

4011 primary tide. See TIDE.

4012 primary tide station. See STATION: TIDE.

4013 prime meridian. See MERIDIAN.

4014 prime vertical. See PRIME VERTICAL CIRCLE.

4015 prime vertical circle. The VERTICAL CIRCLE through the east and west points of the HORIZON. Often shortened toprime vertical.

4016 priming of the tide(s). The acceleration in the TIMES of occurrence of HIGH and LOW WATER due to changes in therelative positions of the MOON and the SUN. The opposite effect is called LAGGING OF THE TIDES.

4017 principal axis. See AXIS OF LENS.

4018 principal distance. The perpendicular distance from the internal PERSPECTIVE CENTRE to the plane of a particularfinished NEGATIVE or PRINT. This distance is equal to the CALIBRATED FOCAL LENGTH corrected for both theenlargement or reduction ratio of the film or paper shrinkage or expansion and maintains the same perspective angles atthe internal PERSPECTIVE CENTRE to points on the finished NEGATIVE or PRINT, as existing in the takingCAMERA at the moment of EXPOSURE.

185

4019 principal line. In PHOTOGRAMMETRY, the trace of the PRINCIPAL PLANE upon the PHOTOGRAPH, (e.g., theline through the PRINCIPAL POINT and the PHOTOGRAPH NADIR).

4020 principal meridian. See MERIDIAN.

4021 principal parallel. See PARALLEL: PHOTOGRAPH.

4022 principal plane. In PHOTOGRAMMETRY, the vertical plane through the internal PERSPECTIVE CENTREcontaining the photograph perpendicular of a tilted PHOTOGRAPH.

4023 principal point. In PHOTOGRAMMETRY, the foot of the perpendicular from the interior PERSPECTIVE CENTREto the plane of the PHOTOGRAPH (i.e. the foot of the photograph perpendicular).

4024 principal scale. See SCALE.

4025 principal vertical circle. The VERTICAL CIRCLE through the north and south points of the HORIZON, coincidingwith the CELESTIAL MERIDIAN.

4026 print. In PHOTOGRAPHY, a copy made from a transparency by photographic means.

4027 print: blue. A nonreproducible blue image or outline usually printed photographically on paper.

4028 print: contact. A photographic PRINT made from either a NEGATIVE or a POSITIVE in contact with a sensitizedmaterial.

4029 print: ozalid. A photographic contact print developed by a dry diazo process. The process produces a POSITIVE froma positive IMAGE or a NEGATIVE from a negative IMAGE.

4030 print: ratio. A PRINT in which the SCALE has been changed from that of the NEGATIVE or transparency byprojection printing.

4031 printing plates. Any one-piece printing surface of any size, strength, or surface preparation, bearing an image mademanually, mechanically or photographically, specifically for the purpose of printing such an image onto some othermaterial.

4032 prismatic astrolabe. See ASTROLABE.

4033 probability. The likelihood of an event measured by the ratio of the favourable cases to the whole number of casespossible.

4034 probable error. See ERROR.

4035 probe. Any device inserted in an environment for the purpose of obtaining information about the environment.

4036 processor. See MICROPROCESSOR.

4037 production platform. A term used to indicate a permanent offshore structure equipped to control the flow of oil or gas.For charting purposes, the use of the term is extended to include all permanent platforms associated with oil or gasproduction, e.g., field terminal, drilling and accommodation platforms, and "booster" platforms located at intervalsalong some pipelines. It does not include entirely submarine structures. See also PLATFORM.

4038 profile. A graph or curve showing elevation or distribution of some property vs. distance along a line. The verticalSCALE may be greatly exaggerated with respect to the horizontal SCALE. An example of a profile is the graphic recordmade by a recording ECHO SOUNDER operating while a vessel is underway.

A vertical section of the surface of the GROUND, or of underlying STRATA, or both, along any fixed line.

4039 profiling: continuous seismic. Determination of the subsoil strata and geological structure by use of sound sourceswhich emit acoustic pulses to penetrate the seabed. Travel time between emission of pulses and reception is usuallymeasured in a towed streamer.

4040 program (or programme). The complete sequence of COMPUTER instructions and routines necessary to solve a dataprocessing problem.

4041 programming language. A language used to prepare computer programs, for example Fortran, Pascal and manyothers.

186

4042 progressive wave. See WAVE.

4043 progress sketch. A MAP or sketch showing work accomplished. In TRIANGULATION and TRAVERSE, each pointestablished is shown on the progress sketch as well as lines observed over and BASES measured. In a levelling survey,the progress sketch shows the route followed and the towns passed through, but not necessarily the locations of theBENCH MARKS.

4044 prohibited area. An area shown on CHARTS within which NAVIGATION and/or anchoring is prohibited.In aviation terminology, a specified area within the land areas of a state or territorial waters adjacent thereto over

which the flight of aircraft is prohibited.

4045 projection. The representation of a figure on a surface, either plane or curved, according to a definite plan. In aPERSPECTIVE PROJECTION this is done by means of projecting lines emanating from a single point, which may beinfinity.

In CARTOGRAPHY, any systematic arrangement of MERIDIANS and PARALLELS portraying the curved surfaceof the sphere or SPHEROID upon a plane. Also called map projection or chart projection.

In PHOTOGRAPHY, the reduction or enlargement of a POSITIVE or NEGATIVE, by projecting the IMAGEthrough a LENS.

4046 projection: azimuthal (or zenithal). A PROJECTION on which the AZIMUTHS or DIRECTIONS of all linesradiating from a central point or POLE are the same as the AZIMUTHS or DIRECTIONS of the corresponding lines onthe sphere.

4047 projection: azimuthal equidistant. An AZIMUTHAL PROJECTION on which straight lines radiating from the centreor pole of projection represent GREAT CIRCLES in their TRUE AZIMUTHS from the centre, and lengths along theselines are of exact SCALE. This PROJECTION is neither EQUAL AREA nor CONFORMAL.

4048 projection: azimuthal orthomorphic. See PROJECTION: STEREOGRAPHIC.

4049 projection: Cassini. A conventional PROJECTION constructed by computing the lengths of arcs along a selectedGEOGRAPHIC MERIDIAN and along a GREAT CIRCLE perpendicular to that MERIDIAN, and plotting these asRECTANGULAR COORDINATES on a plane.

4050 projection: conformal. A PROJECTION in which all angles around any point are correctly represented. In such aPROJECTION the SCALE is the same in all DIRECTIONS about any point. Very small shapes are correctly repre-sented, resulting in an orthomorphic projection. Hence, the terms of 'conformal' and 'orthomorphic' are usedsynonymously since neither characteristic can exist independently of the other.

4051 projection: conic (or conical). A PROJECTION produced by projecting the GEOGRAPHIC MERIDIANS andPARALLELS onto a cone which is tangent to (or intersects) the surface of a sphere, and then developing the cone into aplane.

4052 projection: cylindrical. A PROJECTION produced by projecting the GEOGRAPHICAL MERIDIANS andPARALLELS onto a cylinder which is tangent to (or intersects) the surface of a sphere, and then developing thecylinder into a plane.

4053 projection: equal area. A PROJECTION having a constant area scale. Such a PROJECTION is not conformal and isnot used for NAVIGATION. Also called equivalent projection.

4054 projection: equivalent. See PROJECTION: EQUAL AREA.

4055 projection: geometric. See PROJECTION: PERSPECTIVE.

4056 projection: gnomonic. A PERSPECTIVE PROJECTION on a plane tangent to the surface of a sphere having the pointof projection at the centre of the sphere. The PROJECTION is neither CONFORMAL nor EQUAL AREA. It is the onlyPROJECTION on which GREAT CIRCLES on the sphere are represented as straight lines. For this reason thisPROJECTION is used principally for CHARTS for GREAT CIRCLE SAILING.

4057 projection: inverse Mercator. The name given to the TRANSVERSE MERCATOR PROJECTION when the latter isextended right over both POLES. It is used for air charts of polar areas.

187

4058 projection: Lambert conformal conic. A CONFORMAL PROJECTION of the conical type, on which allGEOGRAPHIC MERIDIANS are represented by straight lines which meet in a common point outside the limits of theMAP, and the GEOGRAPHIC PARALLELS are represented by a series of arcs of circles having this common point fora centre. MERIDIANS and PARALLELS intersect at right angles, and angles on the EARTH are correctly representedon the PROJECTION. This PROJECTION may have one or two STANDARD PARALLELS along which the SCALEis held exact.

4059 projection: Mercator. A CONFORMAL PROJECTION of the cylindrical type. The EQUATOR is represented by astraight line true to SCALE; the GEOGRAPHIC MERIDIANS are represented by parallel straight lines perpendicularto the line representing the EQUATOR; they are spaced according to their distance apart at the EQUATOR. TheGEOGRAPHIC PARALLELS are represented by a second system of straight lines perpendicular to the family of linesrepresenting the MERIDIANS, and therefore parallel with the EQUATOR. CONFORMALITY is achieved bymathematical analysis, the spacing of the PARALLELS being increased with the increasing distance from theEQUATOR to conform with the expanding SCALE along the PARALLELS resulting from the MERIDIANS beingrepresented by parallel lines. Since RHUMB LINES appear as straight lines and DIRECTIONS can be measureddirectly, this PROJECTION is widely used in NAVIGATION.

4060 projection: meridional orthographic. An ORTHOGRAPHIC PROJECTION having the plane of the projectionparallel to the plane of some selected MERIDIAN; in this PROJECTION, the GEOGRAPHIC PARALLELS and theCENTRAL MERIDIAN are straight lines, the outer MERIDIAN is a full circle, and the other MERIDIANS are arcs ofellipses.

4061 projection: orthographic. A perspective azimuthal PROJECTION in which the projecting lines, emanating from apoint at infinity, are perpendicular to a tangent plane. This PROJECTION is used chiefly in NAUTICALASTRONOMY for interconverting COORDINATES of the CELESTIAL EQUATOR and HORIZON systems.

4062 projection: orthomorphic. See PROJECTION: CONFORMAL.

4063 projection: perspective. The representation of a figure on a surface, either plane or curved, by means of projectinglines emanating from a single point, which may be infinity. Also called geometric projection.

4064 projection: polar orthographic. An ORTHOGRAPHIC PROJECTION having the plane of the projectionperpendicular to the AXIS of ROTATION of the EARTH (parallel with the plane of the EQUATOR); in thisPROJECTION, the GEOGRAPHIC MERIDIANS are straight lines.

4065 projection: polyconic. A PROJECTION having the central GEOGRAPHIC MERIDIAN represented by a straight line,along which the spacing for lines representing the GEOGRAPHIC PARALLELS is proportional to the distances apartof the PARALLELS; the PARALLELS are represented by arcs of circles which are not concentric, but whose centreslie on the line representing the CENTRAL MERIDIAN, and whose radii are determined by the lengths of the elementsof cones which are tangent along the PARALLELS. All MERIDIANS except the central one are curved. ThisPROJECTION is neither CONFORMAL nor EQUAL AREA but it has been widely used for MAPS of small areasbecause of the ease with which it can be constructed.

4066 projection: skewed. Any standard PROJECTION used in MAP or CHART construction, which does not conform to ageneral north-south format with relation to the NEAT LINES of the MAP or CHART.

4067 projection: stereographic. A PERSPECTIVE PROJECTION having the point of PROJECTION at the opposite end ofthe diameter of the sphere from the point of tangency of the plane of projection. It is conformal and is the onlyAZIMUTHAL PROJECTION having that quality. When the centre of the PROJECTION is located at a POLE of thesphere, it is called a stereographic polar projection; when on the EQUATOR, a stereographic meridional projection;and when on some other selected PARALLEL of LATITUDE, a stereographic horizon projection. Also calledazimuthal orthomorphic projection.

4068 projection: stereographic horizontal. See PROJECTION: STEREOGRAPHIC.

4069 projection: stereographic meridional. See PROJECTION: STEREOGRAPHIC.

4070 projection: stereographic polar. See PROJECTION: STEREOGRAPHIC.

4071 projection: transverse cylindrical orthomorphic. See PROJECTION: TRANSVERSE MERCATOR.

188

4072 projection: transverse Mercator. A PROJECTION of the cylindrical type, being in principle equivalent to the regularMERCATOR PROJECTION turned (transversed) 90° in AZIMUTH. In this PROJECTION, the CENTRALMERIDIAN is represented by a straight line, corresponding to the line which represents the EQUATOR on the regularMERCATOR PROJECTION. Neither the GEOGRAPHICAL MERIDIANS, except the CENTRAL MERIDIAN, northe GEODETIC PARALLELS, except the EQUATOR (if shown), are represented by straight lines. It is aCONFORMAL PROJECTION. Also called transverse cylindrical orthomorphic projection.

4073 projection: Universal Transverse Mercator. See GRID: UNIVERSAL TRANSVERSE MERCATOR (UTM).

4074 projection: zenithal. See PROJECTION: AZIMUTHAL.

4075 projector. An optical instrument which throws the IMAGE of a NEGATIVE or PRINT upon a SCREEN or otherviewing surface, usually at a larger scale.

An underwater acoustic TRANSMITTER. See UNDERWATER SOUND PROJECTOR.

4076 projector: reflecting. In PHOTOGRAMMETRY, an instrument by means of which the IMAGE of an AERIALPHOTOGRAPH can be projected onto a MAP. By varying the position of the projector lens, the scale of the projectedIMAGE can be varied, and, by tilting the table top, compensation can be made for any tilt in the PHOTOGRAPH.

4077 prolate spheroid. See SPHEROID.

4078 prominent object. Objects which are easily identifiable, but do not justify being classed as conspicuous.

4079 promontory. A high point of LAND extending into a body of water. A CAPE.

4080 proof. In CARTOGRAPHY, an advanced copy of a MAP produced to check the design, register and/or to enableERRORS to be detected and corrected before final printing.

4081 proof plotting. An advanced copy of a map or chart produced by a PLOTTER to check the design and registration, andto detect and correct errors before final printing.

4082 propagated error. An error which occurs in one operation and contaminates subsequent operations.

4083 properties of sea water. See SEA WATER, SALINITY.

4084 proportional dividers. An instrument consisting in its simple form of two legs pointed at both ends and provided withan adjustable PIVOT, so that for any given pivot setting, the distance between one set of pointed ends always bears thesame ratio to the distance between the other set. A change in the PIVOT changes the ratio. The dividers are used intransferring measurements between CHARTS or other drawings which are not at the same SCALE.

4085 protected reversing thermometer. See THERMOMETER.

4086 proton precession magnetometer. See MAGNETOMETER: NUCLEAR PRECESSION.

4087 protractor. An instrument for measuring angles on a surface; an angular SCALE. In its most usual form it consists of acircle or part of one (usually a semicircle) graduated in DEGREES. See PROTRACTOR: THREE ARM andCOMPASS ROSE.

4088 protractor: Cust. Square or rectangular sheet of transparent xylonite engraved with a circle graduated in DEGREESand half DEGREES. Observed angles are drawn, in general, on the under side. See PROTRACTOR: THREE ARM.

4089 protractor: parallel motion. An instrument consisting essentially of a PROTRACTOR and one or more arms attachedto a parallel motion device, so that the movement of the arms is everywhere parallel. The protractor can be rotated andset at any position so that it can be oriented to a CHART. Also called universal drafting machine.

4090 protractor: three arm. An instrument consisting essentially of a circle graduated in DEGREES, to which is attachedone fixed arm and two arms pivoted at the centre and provided with clamps so that they can be set at any angle to thefixed arm, within the limits of the instrument. It is used for finding a ship's POSITION, when the angles between threefixed and known points are measured.

In British terminology, a perspex (or transparent plastic) device used when the MARKS are too close for the use of aSTATION POINTER. This instrument has replaced the CUST PROTRACTOR.

189

4091 province. In OCEANOGRAPHY and in a morphological sense, a region composed of a group of similar physiographicfeatures whose characteristics are markedly in contrast with surrounding areas (rarely used in marineCARTOGRAPHY).

4092 psephyte. A coarse fragmental ROCK (conglomerate) or DEPOSIT composed of rounded PEBBLES.

4093 psychrometer. Instrument used to measure the humidity of the ATMOSPHERE. It comprises two identical THERMO-METERS, the bulb of one of which is dry, while that of the other is wet and covered by a film of pure water or ICE.

4094 pseudorange. The time shift required to align (correlate) a replica of the GPS code generated in the receiver with thereceived GPS code, scaled into distance by the speed of light. This time shift is the difference between the time of signalreception (measured in the receiver time frame) and the time of emission (measured in the satellite time frame).

4095 puddle. An accumulation on the ICE of melt water, mainly due to melting SNOW, but in the more advanced stages alsoto the melting of ICE. Initial stage consists of patches of melted SNOW.

4096 pull. An impression from an individual printing PLATE.

4097 pulsar. A rotating neutron star emitting electromagnetic radiation in regular pulses related to its rotational period.

4098 pulse (or impulse). A variation of quantity whose value is normally constant (often zero), the variation beingcharacterized by a rise and a decay. A common example is a very short burst of electromagnetic energy. See WAVE:CONTINUOUS.

4099 pulse length. The time interval during which the amplitude of a pulse is at or greater than a specified value, usuallystated in terms of a fraction or percentage of the maximum value.

4100 pulse modulation. See MODULATION.

4101 pulse radio altimeter. See ALTIMETER: RADIO.

4102 pulse repetition rate. The rate at which recurrent PULSES are transmitted, usually expressed in PULSES perSECOND.

4103 pumice. An excessively cellular, glassy LAVA. It is very light and can float on water until it becomes waterlogged andsinks.

4104 punch register system. System involving the use of prepunched holes in the flat and plate and a set of plastic pins orbuttons which are used in registering succeeding flats, positioning the image on the plate, and positioning the plate onthe press. See also REGISTRATION.

4105 punched (or punch) card equipment. A system whose principal feature is that numbers are recorded on cards bymeans of holes punched in successive columns of a card, the row of the card indicating the digit.

4106 pycnocline. A vertical positive density GRADIENT in some LAYER of a body of water, which is appreciably greaterthan GRADIENTS above and below it.

190

Q

4107 Q-factor. The pressure coefficient of the unprotected THERMOMETER expressed in °C.

4108 QTG station. A RADIO STATION which is prepared to provide QTG service, that is to say, to transmit upon requestfrom a ship, a radiosignal, the BEARING of which can be taken by that ship.

4109 quadrant. A double-reflecting instrument for measuring angles. It is similar to a SEXTANT, but has an arc of 90°.Also a surveying or astronomical instrument composed of a graduated arc about 90° in length, equipped with a sightingdevice.

4110 quadrantal correctors. Masses of SOFT IRON placed near a MAGNETIC COMPASS to correct for QUADRANTALDEVIATION. Spherical quadrantal correctors are called quadrantal spheres.

4111 quadrantal deviation. See DEVIATION.

4112 quadrantal sphere. See QUADRANTAL CORRECTORS.

4113 quadrature. An ELONGATION of 90° usually specified as east or west in accordance with the direction of the bodyfrom the SUN. The MOON is at quadrature at first and last quarters. See PHASES OF THE MOON.

The situation of two periodic quantities differing by a quarter of a CYCLE.

4114 quadripod. A four-legged stand for TRIANGULATION, SIGNALS, etc.

4115 quality control. All procedures which ensure that the product meets certain standards and specifications.

4116 quality of the bottom. See BOTTOM: NATURE OF.

4117 quarantine anchorage. An area where a vessel anchors when satisfying quarantine regulations.

4118 quarantine buoy. A BUOY marking the location of a QUARANTINE ANCHORAGE.

4119 quarter-diurnal tide(s). See TIDE.

4120 quartering sea. WAVES moving in a direction approximately 45° from a vessel's HEADING, striking the vessel on thequarter.

4121 quarter point. The fourth of the distance from one POINT of the COMPASS to another, equivalent to about twoDEGREES and forty-nine MINUTES.

4122 quartz. Crystalline silica. In its most common form it is colourless and transparent, but it takes a large variety of formsof varying degrees of opaqueness and colour. It is the most common solid mineral. Part of the ocean bed is composed ofquartz.

4123 quartz clock. See CLOCK: CRYSTAL.

4124 quartz horizontal magnetometer. See MAGNETOMETER.

4125 quay. A WHARF approximately parallel to the SHORELINE and accommodating ships on one side only, the other sidebeing attached to the SHORE. It is usually of solid construction, as contrasted with the open pile construction usuallyused for PIERS.

4126 quayage. A comprehensive term embracing all the structures in a PORT alongside which vessels can lie.

4127 quenching. The great reduction in underwater sound transmission or reception resulting from absorption andSCATTERING of sound energy by air bubbles entrapped around the sonar dome. See ATTENUATION.

4128 quicksand. A loose mixture of SAND and water that yields to the pressure of heavy objects. Such objects are difficultto extract once they begin sinking.

4129 quintant. A double-reflecting instrument for measuring angles. It is similar to a SEXTANT, but has an arc of 72°.

4130 Q-wave. See WAVE: LOVE.

191

R

4131 race. Swiftly flowing water in a narrow CHANNEL or RIVER; also the CHANNEL itself which may be artificial as ina mill-race. Also a swift rush of water through a narrow CHANNEL in TIDAL WATERS and caused by the tidalmovement of the waters. See TIDE RACE.

4132 Racon. A RADAR BEACON which returns a coded SIGNAL which provides identification of the beacon as well asRANGE and BEARING. The RANGE and BEARING are indicated by the location of the first character received onthe radar PPI scope. The name 'racon' is derived from the words radar beacon.

4133 radar. A method, system or technique of using beamed, reflected, and timed RADIO WAVES for detecting, locating,or tracking objects, and for measuring ALTITUDES. The electronic equipment or apparatus used to generate, transmit,receive, and usually, to display radio scanning or locating waves; a radar set. The name 'radar' is derived from the wordsradio detecting and ranging.

4134 radar: primary. RADAR which transmits a SIGNAL and receives the incident energy reflected from an object todetect the object in contrast to secondary radar which receives PULSES from a TRANSPONDER triggered byPULSES transmitted from the RADAR.

4135 radar: secondary. See RADAR: PRIMARY.

4136 radar altimeter. See ALTIMETER: RADIO.

4137 radar beacon. A TRANSPONDER BEACON transmitting a coded SIGNAL on radar frequency, permitting an inter-rogating craft to determine the BEARING and RANGE of the TRANSPONDER. Also called RACON. See RAMARK.

4138 radar bearing. See BEARING.

4139 radar buoy. See BUOY.

4140 radar chain. An arrangement of radar sets in a chain usually used for vessel traffic control.

4141 radar chart. See CHART.

4142 radar conspicuous object. An object which returns a strong radar echo.

4143 radar dome. A dome shaped structure used to protect the antenna of a radar installation.

4144 radar surveillance station. A radar station established for traffic surveillance.

4145 radargrammetry. The process of obtaining reliable measurements in bi- and three-dimensional image processing bymeans of RADAR.

4146 radar guided tracks. Recommended TRACKS along which ships can be guided by coastal radar stations in the eventof bad VISIBILITY.

4147 radar reflector. A device capable of or intended for reflecting radar signals.

4148 radar-responder. See RADAR BEACON.

4149 radarsonde. Equipment used for determining high-level WINDS, by means of RADAR aimed at a TARGET carried bya free balloon.

4150 radar target. An object which reflects a sufficient amount of a radar signal to produce an echo signal on the radarscreen.

4151 radar view. A depiction (photographic reproduction or drawing) of appearance of LANDMARKS on the PPI scope ofa radar set at a particular geographic location, intended to facilitate the identification of the COAST. This can aid thenavigator to identify or match his own radar image of these same LANDMARKS.

192

4152 radial. A straight line extending outward from a centre.In PHOTOGRAMMETRY, a line or direction from the RADIAL CENTRE to any point on a PHOTOGRAPH. The

RADIAL CENTRE is assumed to be the PRINCIPAL POINT, unless otherwise designated (e.g., NADIR RADIAL).See also ISORADIAL.

4153 radial: nadir. A RADIAL from the nadir point. See NADIR: PHOTOGRAPH.

4154 radial centre. In PHOTOGRAMMETRY, the selected point on a PHOTOGRAPH from which RADIALS (directions)to various IMAGE POINTS are drawn or measured (i.e., the origin of RADIALS). The radial centre is either thePRINCIPAL POINT, the nadir point, the ISOCENTRE, or a SUBSTITUTE CENTRE.

4155 radial distortion. See under DISTORTION.

4156 radial plot. See RADIAL TRIANGULATION.

4157 radial triangulation. The aerotriangulation procedure, either graphical or analytical, in which directions from theRADIAL CENTRE, or approximate RADIAL CENTRE, of each overlapping PHOTOGRAPH are used for horizontal-control extension by the successive INTERSECTION and RESECTION of these direction lines. A radial triangulationalso is correctly called a radial plot or a minor-control plot. If made by analytical methods, it is called an analyticalradial triangulation. A radial triangulation is assumed to be graphical unless prefixed by the word analytical.

4158 radian. The unit of plane angles in the SI SYSTEM.

4159 radiant energy. See ENERGY.

4160 radiant flux. See FLUX.

4161 radiation. Emission or transfer of energy in the form of ELECTROMAGNETIC WAVES or particles.The energy itself is also termed RADIANT ENERGY.

4162 radiation fog. A major type of FOG, produced over a land area when radiational cooling reduces the air temperature toor below its DEW POINT.

4163 radio. Communication by ELECTROMAGNETIC WAVES, without a connecting wire.A RADIO RECEIVER.Sometimes called wireless particularly in British terminology.

4164 radio aids. An aid to navigation transmitting information by radio waves.

4165 radio acoustic ranging. Determining distance by a combination of RADIO and SOUND, the RADIO being used todetermine the instant of transmission or reception of the SOUND, and distance being determined by the TIME of transitof SOUND, usually in water. Also called radio acoustic sound ranging. See DISTANCE FINDING STATION, ECHORANGING, SONOBUOY.

4166 radio acoustic sound ranging. See RADIO ACOUSTIC RANGING.

4167 radio altimeter. See ALTIMETER.

4168 radiobeacon. A RADIO TRANSMITTER which emits a distinctive or characteristic SIGNAL used for the deter-mination of BEARINGS, COURSES or location. One intended to mark a specific location is called MARKERBEACON.

4169 radiobeacon buoy. See BUOY.

4170 radio bearing. See BEARING.

4171 radiocommunications. Any TELECOMMUNICATION by means of RADIO WAVES.

4172 radio compass. See RADIO DIRECTION FINDER.

4173 radio direction finder. Radio receiving equipment which determines the direction of arrival of a SIGNAL bymeasuring the ORIENTATION of the WAVE FRONT or of the magnetic or electric vector. Radio direction finders maybe either manual or automatic. Also called direction finder. Formerly called radio compass.

193

4174 radio direction finding (or finder) station. A RADIO STATION intended to determine only the direction of otherstations by means of transmission from the latter.

4175 radio fog signal. See FOG SIGNAL.

4176 radio frequency. See FREQUENCY.

4177 radio horizon. The line at which direct RAYS from a transmitting ANTENNA become tangent to the EARTH'ssurface. Its distance from the transmitting ANTENNA is greater than that of the visible HORIZON, and increases withdecreasing FREQUENCY.

4178 radiolocation. Determination of POSITION or of a LINE OF POSITION by means of radio equipment. SeeNAVIGATION: ELECTRONIC.

4179 radio mast. A tower, pole, or other structure for elevating an ANTENNA.

4180 radiometer. An instrument for measuring the intensity of RADIANT ENERGY.

4181 radio navigation. See NAVIGATION: ELECTRONIC.

4182 radio navigational warning. A radio-transmitted message affecting the safe NAVIGATION of vessels or aircraft.

4183 radio range. A radio facility the emissions of which are intended to provide a definite course guidance.

4184 radio range station. A radio navigation land station in the aeronautical radio navigation service providing radioEQUISIGNAL ZONES. In certain instances a radio range station may be placed aboard a ship.

4185 radio receiver. Equipment for receiving radio signals and converting them into usable form.

4186 radio signals: list of. See LIST OF RADIO SIGNALS.

4187 radiosonde. Instrument carried through the ATMOSPHERE, equipped with devices permitting one or several meteo-rological elements (pressure, temperature, humidity, etc.) to be determined, and provided with a RADIOTRANSMITTER for sending this information.

4188 radio sonobuoy. See SONOBUOY.

4189 radio spectrum. The RANGE of electromagnetic RADIATIONS useful for communication by RADIO (approximately10 kilohertz to 300,000 MEGAHERTZ).

4190 radio stars. Sources of RADIO WAVES existing in the universe. The position of many of the radio stars have beenplotted but their SIGNALS are generally very weak.

4191 radio station. A place equipped to transmit RADIO WAVES. Such a station may be either stationary or mobile, andmay also be provided with a RADIO RECEIVER. In British terminology, also called w/t station.

4192 radiotheodolite. Electronic THEODOLITE designed to give the direction in space of a RADIO TRANSMITTERcarried by a free balloon.

4193 radio time signal. A TIME SIGNAL sent by RADIO.

4194 radio transmitter. Equipment for generating radio signals.

4195 radio wave(s). See WAVE.

4196 radius: cruising. See CRUISING RADIUS.

4197 radius of visibility. See under VISIBILITY.

4198 radius vector. A straight line connecting a fixed REFERENCE POINT or centre with a second point, which may bemoving. In ASTRONOMY, the expression is usually used to refer to the straight line connecting a CELESTIAL BODYwith another which revolves around it, as a radius vector of the EARTH and MOON. See COORDINATES: PLANEPOLAR and COORDINATES: POLAR.

194

4199 radome. A dome, usually of glass reinforced plastic, housing a radar aerial. These domes are often prominent. The termis also used for domes or pods housing similar equipment in ships or on aircraft.

4200 rafted ice. Type of PRESSURE ICE formed by one FLOE overriding another.

4201 rafting. Pressure process whereby one piece of ICE overrides another. Most common in NEW and YOUNG ICE.

4202 rain. PRECIPITATION of liquid water particles in the form of drops of more than 0.5 mm diameter or of smaller wide-ly scattered drops.

4203 rainbow. Groups of concentric arcs with colours ranging from violet to red, produced on a "screen" of water drops (r-aindrops, droplets of DRIZZLE or FOG) in the ATMOSPHERE by LIGHT from the SUN or MOON.

4204 rainfall amount. Thickness of the layer of water which accumulates on a horizontal surface, as the result of one ormore falls of PRECIPITATION, in the absence of infiltration or evaporation, and if any part of the precipitation fallingas SNOW or ICE were melted. Also called amount of precipitation.

4205 rain gauge. See GAUGE.

4206 raise the land (v.t.). To sight LAND by approaching to the point where it appears above the HORIZON.

4207 ram. The underwater projection of ICE from an ICE FRONT, ICE WALL, ICEBERG, or FLOE.

4208 Ramark. A radar marker beacon which continuously transmits a SIGNAL appearing as a radial line on the PPI, the lineindicating the DIRECTION of the BEACON. Ramarks are intended primarily for marine use. The name 'ramark' isderived from the words radar marker.

4209 ramp. A sloping structure that can either be used, as a landing place, at variable water levels, for small vessels, landingships, or a ferry boat, or for hauling a cradle carrying a vessel.

An accumulation of SNOW that forms an inclined plane between land or land ice elements and SEA ICE or ICESHELF. Also called drift ice foot.

4210 random error. See ERROR: ACCIDENTAL.

4211 range. Two or more objects in line. Such objects are said to be in range. An observer having them in range is said to beon the range. See also TRANSIT.

Distance in a single direction or along a GREAT CIRCLE.The extreme distance at which an object or LIGHT can be seen, or a SIGNAL detected or used.The distance a craft can travel without refuelling, usually called CRUISING RADIUS.The difference in extreme values of a variable quantity.

4212 range: cruising. See CRUISING RADIUS.

4213 range: diurnal. The difference in HEIGHT between MEAN HIGHER HIGH WATER and MEAN LOWER LOWWATER. Also called great diurnal range.

4214 range: geographic(al). The greatest distance at which a LIGHT can be seen as a function of the curvature of theEARTH and the HEIGHTS of the light source and the observer.

4215 range: great tropic. The difference in HEIGHT between TROPIC HIGHER HIGH WATER and TROPIC LOWERLOW WATER.

4216 range: luminous. The greatest distance at which a LIGHT can be seen merely as a function of its LUMINOUS INTEN-SITY, the METEOROLOGICAL VISIBILITY, and the sensitivity of the observer's eye.

4217 range: magnetic. A RANGE oriented in a given magnetic direction and used to assist in determination of the deviationof a MAGNETIC COMPASS. See DEVIATION: MAGNETIC.

4218 range: nominal. The LUMINOUS RANGE of a LIGHT in a homogenous ATMOSPHERE in which theMETEOROLOGICAL VISIBILITY is 10 SEA MILES.

4219 range: small diurnal. The difference in HEIGHT between mean LOWER HIGH WATER and mean HIGHER LOWWATER.

195

4220 range: small tropic. The difference in HEIGHT between TROPIC LOWER HIGH WATER and TROPIC HIGHERLOW WATER.

4221 range finder. An optical instrument for measuring the distance to an object.

4222 range height indicator. A SCOPE which simultaneously indicates RANGE and HEIGHT of a RADAR TARGET.

4223 range light(s). See LIGHT.

4224 range of mountains. See MOUNTAIN RANGE.

4225 range of tide. The difference in HEIGHT between consecutive HIGH and LOW TIDES at a place. Also called tidal (ortide) range.

4226 range of visibility. The extreme distance at which an object or LIGHT can be seen. See RANGE: GEOGRAPHICAL,and RANGE: LUMINOUS.

4227 range rod. A simple SURVEYOR's rod, fitted with a sharp-pointed steel shoe. It is usually painted red and whitealternately, and used to line up points of a SURVEY.

4228 rapid(s). Portions of a STREAM with accelerated CURRENT where it descends rapidly but without a break in theSLOPE of the BED sufficient to form a waterfall. Usually used in the plural.

4229 raster data presentation. Method of digitally representing data of an image by a matrix-like scheme of PIXELs.

4230 rate. In ELECTRONIC NAVIGATION, the designation of PULSE REPETITION RATE for a pair of transmitting sta-tions, their SIGNALS, and the resulting LINES OF POSITION.

4231 rate station. See DRIFT STATION.

4232 rational horizon. See HORIZON: CELESTIAL.

4233 ratio of ranges. The ratio of the HEIGHT OF TIDE at the SUBORDINATE STATION to the HEIGHT OF TIDE atthe REFERENCE STATION.

4234 ratio of rise. The ratio of the HEIGHT OF TIDE at two places.

4235 ratio print. See PRINT.

4236 ravine. In general, any steep-sided VALLEY.

4237 ray. A BEAM of RADIANT ENERGY of infinitesimal cross-section, such as a LIGHT RAY.

4238 Rayleigh criterion. Used in tidal analysis. Only those harmonic constituents which are separated by at least a completeperiod from their neighbouring constituents over the length of data available should be included in the harmonic analysisof a given time series.

4239 reach. A straight section of a RIVER, especially a navigable RIVER between two bends. See also SEA REACH.An ARM OF THE SEA extending into the LAND.

4240 real image. See IMAGE.

4241 rear light. See LIGHT(S): RANGE.

4242 rear nodal point. See NODAL POINT.

4243 receiver. One who or that which receives anything, particularly a RADIO RECEIVER.

4244 recession of a shoreline. Landward movement of the SHORELINE. Also called retrogression of a beach.

4245 reciprocal bearing. See BEARING.

4246 reciprocal levelling. See LEVELLING.

4247 reckoning. In NAVIGATION, computation by which the POSITION of a ship is found. See DEAD RECKONING.

196

4248 recommended course. See TRACK: RECOMMENDED.

4249 recommended route. See ROUTE: RECOMMENDED.

4250 recommended track. See TRACK: RECOMMENDED.

4251 reconnaissance. In SURVEYING, a general examination or SURVEY of the main features, or certain specific features,of a region, usually as a preliminary to a more detailed SURVEY.

An examination of an area to gain specific information, as the weather conditions, extent and nature of ICE, etc.

4252 reconnaissance map. See MAP.

4253 reconnaissance survey. See SURVEY.

4254 recorder. The man on a survey party who records the observational DATA.A recording instrument.The part of an instrument or machine that records.

4255 recording anemometer. See ANEMOGRAPH.

4256 recording barometer. See BAROMETER.

4257 recoverable station. See STATION.

4258 recovery of station. In SURVEYING, the identifying and checking of an original STATION. This is considered asrecovered when its MARK (MONUMENT) is identified as authentic and proved to be occupying its original site.

4259 rectangular coordinates. See COORDINATES.

4260 rectification. The process of projecting a tilted or OBLIQUE PHOTOGRAPH onto an horizontal reference plane, theangular relationship between the PHOTOGRAPH and the plane being determined by ground methods.

4261 rectified altitude. See ALTITUDE: APPARENT.

4262 rectilinear current. See CURRENT: REVERSING.

4263 rectilinear stream. See CURRENT: REVERSING.

4264 rectoblique plotter. A mechanical device intended for RECTIFICATION of individual RAYS on tilted or OBLIQUEPHOTOGRAPHS.

4265 recurrence rate. See REPETITION RATE.

4266 red mud. See MUD.

4267 red sector. A sector of the CIRCLE OF VISIBILITY of a navigational LIGHT in which a red light is exhibited. Suchsectors are designated by their limiting BEARINGS, as observed at some point other than the LIGHT. Red sectors areoften located so that they warn of danger to vessels.

4268 red tide. A red or reddish-brown discoloration of surface waters, most frequently in coastal regions, caused byconcentration of certain microscopic organisms, particularly dinoflagellates. Toxins produced by the dinoflagellates cancause mass kills of fishes and other marine animals.

4269 reduced latitude. See LATITUDE: PARAMETRIC.

4270 reduction. The process of substituting for an observed value one derived therefrom.

4271 reduction of soundings. The CORRECTION of the observed DEPTHS, for the HEIGHT OF TIDE above or below theplane of reference at the TIME of SOUNDING. Usually the term reduction of soundings does not cover CORREC-TIONS other than those due to TIDE. See CORRECTION OF SOUNDINGS, LEAD LINE CORRECTION and TIDEREDUCER.

4272 reduction of tides. The processing of observed tidal data to obtain mean values of TIDAL CONSTANTS.

197

4273 reduction to centre. The amount which must be applied to a DIRECTION observed at an ECCENTRIC STATION, orto an ECCENTRIC SIGNAL, to reduce such DIRECTION to what it would be if there were no such ECCENTRICITY.

4274 reduction to sea level. A reduction applied to a measured horizontal length on the EARTH's surface to reduce it to thesurface of the SEA LEVEL DATUM.

4275 reduction to the meridian. The process of applying a CORRECTION to an ALTITUDE observed when a body is nearthe CELESTIAL MERIDIAN of the observer, to find the ALTITUDE at MERIDIAN TRANSIT. See ALTITUDE: EX-MERIDIAN.

4276 reed horn. A HORN that produces SOUND by means of a steel reed vibrated by air under pressure.

4277 reef. A mass of rock or coral which either reaches close to the sea surface or is exposed at low tide, posing a hazard toNAVIGATION.

4278 reef flat. A flat expanse of dead reef rock which is partly or entirely dry at LOW TIDE.

4279 reference direction. A DIRECTION used as a basis for comparison of other DIRECTIONS.

4280 reference ellipsoid. See REFERENCE SPHEROID.

4281 reference level. See DATUM: CHART and DATUM: VERTICAL CONTROL.

4282 reference line. Any line which can serve as a reference or base for the measurement of other quantities. Also calleddatum line.

4283 reference mark. In SURVEYING, a supplementary MARK of permanent character close to a STATION or to a BASETERMINAL, to which it is related by an accurately measured distance and DIRECTION, and/or a difference inELEVATION.

4284 reference object. A well-defined object selected as a starting point in the DIRECTION METHOD OFOBSERVATION.

4285 reference plane. See DATUM: CHART, and DATUM: VERTICAL CONTROL.

4286 reference point. Any point which can serve as a reference or base for the measurement of other quantities. Also calleddatum point.

4287 reference spheroid (or ellipsoid). A theoretical figure whose dimensions closely approach the dimensions of theGEOID. The exact dimensions are determined by various considerations of the section of the EARTH's surfaceconsidered. The SPHEROIDS of Bessel, Clarke, Delambre, Everest, Hayford, Helbert and others have been adopted asreference spheroids in geodetic work by different countries. Also called spheroid of reference, or ellipsoid of reference.See also SPHEROID: OBLATE.

4288 reference station. See STATION.

4289 reference tape. A BASE TAPE employed solely for use as a STANDARD of comparison. Also called standard tape.

4290 reflectance. The ratio of light given off by an object to the amount of light striking the object, expressed as percentage.

4291 reflected wave. See WAVE.

4292 reflecting projector. See PROJECTOR.

4293 reflecting telescope. See TELESCOPE.

4294 reflection. The process whereby a surface of discontinuity turns back a portion of the incident RADIATION into themedium through which the RADIATION approached. See SURFACE REFLECTION.

4295 reflection: angle of. See ANGLE OF REFLECTION.

4296 reflectivity. The ratio of the RADIANT ENERGY reflected by a surface to that incident upon it.

198

4297 reflector. A device capable of or intended for reflecting particles or RADIANT ENERGY. See RADAR REFLECTOR.

4298 refracted wave. See WAVE.

4299 refracting telescope. See TELESCOPE.

4300 refraction. The process in which the direction of energy propagation is changed as the result of a change in densitywithin the propagating medium, or as the energy passes through the INTERFACE representing a density discontinuitybetween two media.

4301 refraction: angle. See ANGLE OF REFRACTION.

4302 refraction: astronomical. The apparent displacement of an object that results from LIGHT RAYS from a sourceoutside the ATMOSPHERE being bent in passing the ATMOSPHERE. This results in all objects appearing to be higherabove the HORIZON than they actually are.

4303 refraction: atmospheric. Refraction resulting when a ray of radiant energy passes obliquely through the atmosphere. Itmay be called astronomical refraction if the ray enters the atmosphere from outer space, or terrestrial refraction if itemanates from a point on or near the surface of the EARTH.

4304 refraction: coastal. A small change in the direction of travel of a RADIO WAVE when it crosses a SHORELINEobliquely due to a difference in conducting and reflecting properties between land and water, and thus velocity.

4305 refraction: coefficient of. The ratio of the ANGLE OF REFRACTION at the point of observation to the angle at thecentre of the EARTH subtended by the arc connecting the point of observation and the observed point.

4306 refraction: index of. The ratio of the sine of the ANGLE OF INCIDENCE to the sine of the ANGLE OFREFRACTION.

4307 refraction correction. A CORRECTION due to REFRACTION, particularly such a CORRECTION to a SEXTANTALTITUDE,due to ATMOSPHERIC REFRACTION.

4308 refraction of light. The bending of LIGHT RAYS in passing from one transparent medium into another which has adifferent INDEX OF REFRACTION.

4309 refraction of water waves. The process by which the direction of a WAVE moving in SHALLOW WATER at anangle to the contours is changed. That part of the WAVE advancing in shallower water moves more slowly than theother part still advancing in deeper water, causing the WAVE CREST to bend toward alignment with the underwatercontours. Also called wave refraction.

The bending of WAVE CRESTS by CURRENTS.

4310 refuge. A place of safety for a vessel in danger.

4311 regime. In CLIMATOLOGY, term used to characterize the seasonal distribution of one or more elements at a givenplace.

4312 registration. Correct positioning of one component of a composite map image in relation to the other components.Achieved, for example, by punching sets of holes, having a fixed horizontal relationship to each other, in eachcomponent sheet and then attaching the components together using specially designed fasteners. See also PUNCHREGISTER SYSTEM.

4313 region A,B. Within the IALA Maritime Buoyage System there are two international buoyage regions, designated asRegion A and Region B, where lateral marks differ only in the colours of port and starboard hand marks.

4314 regression of the nodes. Precessional motion of a set of NODES. The expression is used principally with respect to theMOON, the NODES of which make a complete westerly REVOLUTION in approximately 18.6 years.

4315 regular error. See ERROR: SYSTEMATIC.

4316 relative accuracy. See ACCURACY: RELATIVE.

4317 relative aperture. See APERTURE: RELATIVE.

4318 relative bearing. See BEARING.

199

4319 relative humidity. The ratio of the actual vapor pressure to the vapor pressure corresponding to saturation at theprevailing temperature.

4320 relative motion. Motion of one object or body relative to another. The expression is usually used in connection withproblems involving motion of one craft, torpedo, or missile relative to another, the direction of such motion being calleddirection of relative movement, and the SPEED of such motion being called speed of relative movement or relativespeed. Usually called apparent motion when applied to the change of position of a CELESTIAL BODY as observedfrom the EARTH. Also called relative movement.

4321 relative orientation. See ORIENTATION.

4322 relative position. See POSITION.

4323 relative speed. See RELATIVE MOVEMENT.

4324 relative tilt. See TILT.

4325 relative wind. For a moving object, WIND VECTOR relative to this object. Also called apparent wind. SeeTRUE WIND.

4326 release. A device for holding or releasing a mechanism, particularly the device by which the TANGENT SCREW of aSEXTANT or surveying instrument is held in place or disengaged from the LIMB, or CIRCLE respectively. Inoceanography acoustic releases are used to sever deployed instruments from their anchoring.

4327 relief. The ELEVATIONS or the inequalities, collectively, of a land surface; represented on a MAP or CHART byCONTOURS, HYPSOMETRIC TINTS, SHADING, SPOT ELEVATIONS, HACHURES, etc.

4328 relief: submarine. Inequalities of the ocean bed, or their representation on a CHART.

4329 relief displacement. DISPLACEMENT of IMAGES radially inward or outward with respect to the PHOTOGRAPHNADIR, according as the ground objects are, respectively, below or above the ELEVATION of the GROUND NADIR.

4330 relief map. See MAP.

4331 relief model. A scaled representation in three dimensions of a section of the CRUST of the EARTH, or of anotherHEAVENLY BODY. A relief model primarily designed to display features on the surface of the EARTH is sometimesknown as a topographic model.

4332 remanence. See MAGNETIC RETENTIVITY.

4333 remote sensing. The measurement or acquisition of information of some property of an object or phenomenon by arecording device that is not in physical or infinite contact with the object or phenomenon under study. Sometimesrestricted to the practice of data collection in the wavelengths from ultraviolet to radio regions.

4334 remotely operated vehicle (ROV). An unmanned submersible craft which can be used either for observation purposesor to perform hydrographic surveys, or for working purposes. Also called "remotely controlled vehicle (RCV)".

4335 repeatability. See REPEATABLE ACCURACY.

4336 repeatable accuracy. In a navigation system, the measure of the accuracy with which the system permits the user toreturn to a position as defined only in terms of the coordinates peculiar to that system. For example, the distancespecified for the repeatable accuracy of the system such as LORAN-C is the distance between two LORAN-C positionsestablished using the same stations and time-difference readings at different times. The correlation between thegeographical coordinates and the system coordinates may or may not be known.

4337 repeater. A device for repeating at a distance the indications of an instrument or device. See COMPASS REPEATER,GYRO REPEATER, STEERING REPEATER.

4338 repeating instrument. See THEODOLITE: REPEATING.

4339 repeating theodolite. See THEODOLITE.

200

4340 repetition method of observation. A method of observing horizontal angles by which the OBSERVATIONS of eachangle are completed separately. See THEODOLITE: REPEATING.

4341 repetition rate. The rate at which recurrent SIGNALS are transmitted. Also called recurrence rate.

4342 reported danger. An object dangerous to NAVIGATION which is shown on a CHART but the existence of which hasnot been confirmed. Sometimes called VIGIA.

4343 representative fraction. See SCALE: NATURAL.

4344 reprint (of a chart) (U.S. terminology). An issue of a CHART the supply of which is approaching exhaustion. Thereprint is an exact duplicate of the current issue with no changes in printing or publication data.

4345 reproducible. An original drawing, photographic POSITIVE or NEGATIVE on stable base material to be used in theprocess of REPRODUCTION.

4346 reproduction. In CARTOGRAPHY, the processes involved in printing copies from an original drawing. The principalprocesses are PHOTOGRAPHY, LITHOGRAPHY (or ENGRAVING), and printing.

A printed copy of an original drawing, made by any of the processes of reproduction.

4347 repromat. Reproduction material, generally in the form of positive or negative copies on film or glass for each colourplate, from which a map or chart may be reproduced without redrafting.

4348 reseau. A NETWORK.In PHOTOGRAPHY, a glass plate on which is etched a NETWORK of fine lines. Sometimes used as a focal-plane

plate to provide a means of calibrating film DISTORTION.

4349 resection. The graphical or analytical determination of a POSITION as the intersection of at least three lines of knownrelative direction to corresponding points of known POSITION.

In PHOTOGRAMMETRY, the determination of the position and/or attitude of a CAMERA, or the PHOTOGRAPHtaken with that CAMERA, with respect to the exterior coordinate system.

4350 resection station. See STATION.

4351 reservoir. A place where anything is collected and stored, generally in large quantity; especially a POND, LAKE orBASIN, either natural or artificial, for the storage, regulation and control of water.

4352 residual. See ERROR: RESIDUAL.

4353 residual deviation. See DEVIATION.

4354 residual error. See ERROR.

4355 residual magnetism. See MAGNETISM.

4356 resolution. The separation by an optical system of parts of an object or of two or more objects close together. Thedegree of ability to make such a separation, called RESOLVING POWER, is expressed as the minimum distancebetween two objects that can be separated.

The degree of ability of a radar set to indicate separately the echoes of two targets in range and bearing.

4357 resolving power. A mathematical expression of DEFINITION in a RADAR or OPTICAL SYSTEM, usually stated asthe maximum number of lines per millimetre that can be seen as separate lines in the IMAGE.

4358 resonance. Re-enforcement or prolongation of any wave motion, such as SOUND, RADIO WAVES, etc., resultingwhen the NATURAL FREQUENCY of the body or system in VIBRATION is equal to that of an impressedVIBRATION.

In TIDES, the water movement resulting from the natural PERIOD of OSCILLATION of a body of water whichapproximates the PERIOD of one of the TIDE-PRODUCING FORCES.

4359 resonant frequency. See FREQUENCY.

4360 resource exploration. The search for economic deposits of minerals, ore, gas, oil or coal.

4361 responder. A transmitter, fitted to a submersible or on the seabed, which can be triggered by a hardwired externalcontrol signal to transmit an interrogation signal which is received by a TRANSDUCER or HYDROPHONE.

201

4362 responder beacon. See TRANSPONDER BEACON.

4363 response. For a device or system, the motion or other OUTPUT resulting from an excitation or stimulus under specialconditions.

4364 responsor. A RADIO RECEIVER which receives a reply from a TRANSPONDER and produces an OUTPUT suitablefor feeding to a display system. A responsor is usually combined in a single unit with an INTERROGATOR whichsends out the PULSE that triggers a TRANSPONDER, the combined unit being called an interrogator-responsor.

4365 restitution. The process of determining the true planimetric position of objects whose images appear on photographs.Restitution corrects for distortion resulting from both tilt and relief displacement.

4366 restricted area. A specified area designated by appropriate authority and shown on CHARTS, within whichNAVIGATION is restricted in accordance with certain specified conditions.

4367 resurvey. A retracing on the GROUND of the lines of an earlier SURVEY, in which all points of the earlier SURVEYthat are recovered are held fixed and used as a control. If too few points of the earlier SURVEY are recovered to satisfythe control requirements of the resurvey, a new survey may be made. A resurvey is related directly to an originalSURVEY although several resurveys may interpose between them.

4368 retardation. In ELECTRONIC NAVIGATION, the amount of delay in TIME or PHASE ANGLE introduced by theresistivity of the GROUND over which the SIGNAL is passing.

In tide terminology, DAILY RETARDATION.

4369 retentivity: magnetic. See MAGNETIC RETENTIVITY.

4370 reticle (or reticule). A system of wires, hairs, threads, etched lines, or the like, placed normal to the AXIS of aTELESCOPE at its principal FOCUS, by means of which the TELESCOPE is sighted on a STAR, SIGNAL, orTARGET, or by means of which appropriate readings are made on some SCALE, such as a LEVELLING or STADIAROD. See also CROSS WIRES, GRATICULE, DIAPHRAGM.

In PHOTOGRAMMETRY, a mark such as a cross or system of lines lying in the image plane of a viewingapparatus and used singly as a reference mark in certain types of monocular instruments or as one of a pair to form aFLOATING MARK as in certain types of STEREOSCOPES.

4371 reticle ring. The ring across which the system of wires, hairs, threads, or the like of a RETICLE are streched; or thering which supports the glass DIAPHRAGM when the RETICLE is a system of lines etched on glass.

4372 retired line of position. A LINE OF POSITION which has been moved backward along the COURSE line tocorrespond with a TIME previous to that at which the line was established. Called a transferred position line in Britishterminology.

4373 retrogression of a beach. See RECESSION OF A SHORELINE.

4374 reverberation. Continuation of RADIANT ENERGY, particularly SOUND by multiple REFLECTION.SOUND scattered towards the source, principally from the ocean surface (surface reverberation) or BOTTOM

(bottom reverberation), and from small scattering sources in the medium such as bubbles of air and suspended solidmatter (volume reverberation).

4375 reverse azimuth. See AZIMUTH: BACK.

4376 reversing current. See CURRENT.

4377 reversing thermometer. See THERMOMETER.

4378 reversing water bottle. See WATER BOTTLE.

4379 revetment. Facing of stone or other material, either permanent or temporary, placed along the edge of a stream tostabilize the bank and to protect it from the erosive action of the stream.

4380 revolution. Motion of a CELESTIAL BODY in its ORBIT; circular motion about an AXIS usually external to thebody. The terms revolution and ROTATION are often used interchangeably but, with reference to the motions of aCELESTIAL BODY, revolution refers to the motion in an ORBIT or about an AXIS external to the body, whileROTATION refers to motion about an AXIS within the body. Thus, the EARTH revolves about the SUN annually androtates about its AXIS daily.

202

4381 revolver. See FIX: CIRCULAR.

4382 revolving light. See LIGHT: ROTATING.

4383 rho-teta system. Positioning system measuring the distance (rho) and direction (teta) from an existing control station toa mobile or another point.

4384 rhumb. See RHUMB LINE.

4385 rhumb line. A line on the surface of the EARTH making the same oblique angle with all MERIDIANS; aLOXODROME spiralling toward the POLES in a constant true direction. PARALLELS and MERIDIANS, which alsomaintain constant true directions, may be considered special cases of the rhumb line. A rhumb line is a straight line on aMERCATOR PROJECTION. Sometimes shortened to rhumb.

4386 rhumb line sailing. See SAILING.

4387 ria. A long narrow INLET, with DEPTH gradually diminishing inward; a CREEK. Any broad RIVER opening into theOCEAN.

4388 ridge. a) A long, narrow ELEVATION with steep sides.b) A long, narrow ELEVATION often separating ocean basins.c) The major oceanic mountain systems of global extent.d) Region of the ATMOSPHERE in which pressure is high relative to the surrounding region at the same level. It is

represented on a WEATHER CHART by a system of nearly parallel ISOBARS or CONTOURS, of approximately U-shape, which are concave towards an ANTICYCLONE.

See OCEANIC RIDGE? TROUGH? WEDGE and PRESSURE RIDGE.

4389 rift. A cleft, fissure or chasm in the EARTH. See MEDIAN VALLEY.

4390 rift valley. See MEDIAN VALLEY.

4391 rig. A temporary, mobile structure, either fixed or floating, used in the exploration stages of oil and gas fields.

4392 right ascension. ANGULAR DISTANCE east of the vernal EQUINOX; the arc of the CELESTIAL EQUATOR, or theangle at the CELESTIAL POLE, between the HOUR CIRCLE of the vernal EQUINOX and the HOUR CIRCLE of apoint on the CELESTIAL SPHERE, measured eastward from the HOUR CIRCLE of the vernal EQUINOX through 24hours.

4393 right bank. Of a RIVER, the BANK on the right-hand side as one proceeds downstream.

4394 right reading image. An IMAGE which may be read or observed normally whether in NEGATIVE orPOSITIVE form.

4395 rill mark. A small groove, furrow or CHANNEL made in MUD or SAND on a BEACH by tiny STREAMS followingan outflowing TIDE.

4396 rip current. See CURRENT.

4397 ripple. The ruffling of the surface of water, hence a little curling WAVE or UNDULATION.A WAVE controlled to a significant degree by both surface tension and GRAVITY.

4398 ripple mark. Undulating surface features of various shapes produced in unconsolidated sediments by wave or currentaction.

4399 rip-rap. See GROIN.

4400 rips. A turbulent agitation of water generally caused by the interaction of CURRENTS and WIND; in nearshoreregions, rips may also be caused by CURRENTS flowing swiftly over an irregular BOTTOM.

4401 rise. A long, broad ELEVATION that rises gently and generally smoothly from the SEA FLOOR.

4402 rise (v.i.). Of a CELESTIAL BODY, to cross the visible HORIZON while ascending. The opposite is SET.

4403 rise of tide. The HEIGHT of HIGH WATER above DATUM. Also called tidal rise.

203

4404 rising tide. See TIDE.

4405 river. A relatively large natural STREAM of water.

4406 river discharge. The rate of FLOW of water past a point in a STREAM, expressed as volume per unit time (usuallycubic feet per second).

4407 river harbour. A harbour which lies on the banks of a river.

4408 river mouth. The exit or point of discharge of a river into the sea, a lake, or another river.

4409 river roadstead. A ROADSTEAD which lies in a river.

4410 rivulet. A small RIVER.

4411 road(s). See ROADSTEAD.

4412 roadstead. An area near the SHORE, where vessels can anchor in safety; usually a shallow INDENTATION in theCOAST. Also called road or roads. See OPEN ROADSTEAD.

4413 roaring forties. An expression often used to denote that belt in the OCEANS, between 40o and 50o south LATITUDE,where strong westerly WINDS prevail.

4414 Roberts radio current meter. An electro-mechanical CURRENT METER which measures current speed anddirection. This METER can be suspended below an anchored BUOY or ship which is equipped with a RADIOTRANSMITTER that transmits the current measurements to a ship or shore based monitor station.

4415 rock. Any formation of natural origin that constitutes an integral part of the LITHOSPHERE. The natural occurringmaterial that forms the firm, hard, and solid masses of the ocean floor.

An isolated rocky formation or a single large STONE, usually one constituting a danger to NAVIGATION. SeeROCK AWASH, SUNKEN ROCKS, PINNACLE and ROCK WHICH COVERS AND UNCOVERS.

4416 rock awash. A ROCK awash at CHART DATUM.

4417 rock borer. A member of any of several families of bivalves that live in cavities they bore in soft ROCK, concrete andother material.

4418 rocket. In hydrography or navigation, a pyrotechnic projectile used for signalling, or for life-saving purposes.

4419 rocket sounding. Determination of one or more upper-air meteorological elements by means of rocket-borneinstruments.

4420 rocket station. A LIFE SAVING STATION equipped with line-carrying rocket apparatus.

4421 rock flour. Finely ground rock particles, chiefly silt size, resulting from glacial ABRASION. A component of marineDEPOSITS off glacial stream mouths.

4422 rockweed. One of a group of marine plants, principally of an order (Fucales) of the brown ALGAE, mostly multi-branched and leathery, which grow attached to ROCKS in the INTERTIDAL ZONE by means of an organ called aholdfast. Also called wrack or fucus.

4423 rock which covers and uncovers. A ROCK of small area (PINNACLE) that COVERS AND UNCOVERS at variousstates of the TIDE.

4424 rocky area. An area with a rocky BOTTOM.

4425 rod. A unit of length equal to 5.5 YARDS or 16.5 FEET. Also called PERCH or POLE.A STADIA or LEVELLING ROD. See also SOUNDING POLE.

4426 rod: levelling. See LEVELLING ROD.

4427 rod: range. See RANGE ROD.

4428 rod: sounding. See SOUNDING POLE.

204

4429 rodded points. Points where the ROD was actually held, as opposed to points sketched in between.

4430 rodman. A person using a surveying ROD.

4431 Roentgen rays. See X-RAYS.

4432 roll (or rolling). The oscillation of a ship about the longitudinal axis.In PHOTOGRAMMETRY, a rotation of a CAMERA or a photograph-coordinate system about either the

photograph x axis or the exterior x axis.

4433 roller. An indefinite term, sometimes considered to denote one of a series of long-crested, large WAVES which roll inupon a COAST, as after a STORM.

Large BREAKERS on exposed COASTS formed by SWELL coming from a great distance.See COMBER.

4434 rollers: blind. See BLIND ROLLERS.

4435 roof prism. A type of prism in which the IMAGE is reverted by a roof, that is, two surfaces inclined at 90o toeach other.

4436 root mean square error. See ERROR: STANDARD.

4437 rose. Name given to a COMPASS CARD, or other DIAGRAM, having radiating lines.

4438 Rossby wave(s). Ignoring friction or depth changes; if a parcel of water with no initial relative vorticity (i.e. no rotation)is moved northward, as f (the Coriolis parameter) increases the parcel will gain negative relative vorticity and willcirculate clockwise. The Coriolis force will be greater on the poleward side of the parcel than on the equatorward sideand hence the parcel will be subjected to a net southward restoring force. This force will push the parcel south of thelatitude of zero relative vorticity overshoots and the circulation becomes counterclockwise. Due to the Coriolis variationthe parcel will now experience a northward restoring force. Thus the variation of f provides a restoring force (in thehorizontal plane) allowing oscillation to occur just as the effect of gravity does (vertically) for surface or internal waves.In reality Rossby waves are complicated by depth variations and frictional effects.

4439 rotary current. See CURRENT.

4440 rotary stream. See CURRENT: ROTARY.

4441 rotating light. See LIGHT.

4442 rotating loop radiobeacon. See ROTATING RADIOBEACON STATION.

4443 rotating prism. See DOVE PRISM.

4444 rotating radiobeacon station. A special type of RADIOBEACON station emitting a BEAM of WAVES to which auniform turning movement is given, the BEARING of the station being determined by means of an ordinary listeningRECEIVER and a STOP WATCH. Also referred to as rotating loop radiobeacon.

4445 rotation. Turning of a body about an AXIS within the body, as the daily rotation of the EARTH. See REVOLUTION.

4446 rotten ice. SEA ICE which has become honeycombed in the course of melting, and which is in an advanced state ofdisintegration.

4447 roundabout. A TRAFFIC SEPARATION SCHEME in which traffic moves in a counter-clockwise direction around aspecified point or zone.

4448 route: recommended. A route of undefined width, for the convenience of ships in transit, which is often marked bycentreline buoys.

4449 route (or routeing) chart. See CHART.

4450 routeing. A complex of measures concerning routes followed by ships and aiming at reducing the risk of casualties(TRAFFIC SEPARATION SCHEMES, DEEP-DRAUGHT ROUTES, areas to be avoided, etc.).

205

4451 routeing guide. A document designed to be used in conjunction with nautical charts and other nautical publications inareas where complex routeing systems have been established by the IMO.

4452 routeing system. Any system of one or more routes or routeing measures aimed at reducing the risk of casualties; itincludes traffic separation schemes, two-way routes, recommended tracks, areas to be avoided, inshore traffic zones,roundabouts, precautionary areas and deep-water routes.

4453 rubble. Fragments of hard SEA ICE, roughly spherical and up to 1,5 METRES (5 FEET) in diameter, resulting fromthe disintegration of larger ice formations. When afloat, commonly called BRASH ICE.

Loose angular rock fragment.

4454 rubidium vapour magnetometer. See MAGNETOMETER.

4455 rudite (or rudyte). Rock or sediment DEPOSITS composed of grains larger than 2 millimetres.

4456 ruin. A ruin is a structure in a decayed or deteriorated condition resulting from neglect or disuse, or a damaged structurein need of repair. A ruin is considered hazardous if it extends over or into navigable waters and thus represents a dangerto surface navigation.

4457 ruling. A mechanically produced series of equally-spaced parallel lines at a predetermined 'angle of ruling'. Used todistinguish particular areas of a MAP by providing a paler shade of the full printing colour.

4458 run. A BROOK, or small CREEK.The distance travelled by a craft during any given time interval, or since leaving a designated place.Complete performance of one PROGRAMME on a COMPUTER.

4459 run a line of soundings (v.t.) To obtain SOUNDINGS along a LINE OF SOUNDING, for surveying purposes.

4460 runnel. The smallest of natural STREAMS; a BROOK or RUN.A TROUGH or corrugation formed in the FORESHORE or in the BOTTOM, immediately offshore, formed by

WAVES or TIDAL CURRENTS.

4461 running fix. See FIX.

4462 running survey. See SURVEY.

4463 run of micrometer. See ERROR OF RUN.

4464 run-up. The rush of water up a structure on the breaking of a WAVE. The amount of run-up is the vertical HEIGHTabove STILL WATER LEVEL that the rush of water reaches. See UPRUSH.

4465 runway. A defined rectangular area, on a land aerodrome, prepared for the landing and take-off run of aircraft along itslength.

4466 runway light(s). See LIGHT.

206

S

4467 sac. An INDENTATION in the CONTOURS on a CHART showing SUBMARINE RELIEF which is analogous to aGULF on the surface. The opposite term is SUBMARINE PENINSULA.

4468 saddle. A COL or PASS or any land-form recalling in shape a saddle.A low part on a RIDGE or between SEAMOUNTS.

4469 safety aids. See AID TO NAVIGATION.

4470 safety fairway. An area within which permits are not granted for the erection of oil or gas related structures. The use ofa safety fairway is not usually mandatory, but is recommended.

4471 safety zone. The area around an offshore installation within which vessels are prohibited from entering withoutpermission. Special regulations protect installations within a safety zone and vessels of all nationalities are required torespect the zone.

4472 safe water mark. In the IALA Maritime Buoyage System a safe water mark indicates that there is navigable wateraround its position.

4473 sag correction (tape). See TAPE: SAG CORRECTION.

4474 sailing. A method of solving the various problems involving COURSE, distance, DIFFERENCE OF LATITUDE,DIFFERENCE OF LONGITUDE and DEPARTURE. The various methods are collectively spoken of as 'the sailings'.

4475 sailing: composite. A modification of GREAT-CIRCLE SAILING used when it is desired to limit the highestLATITUDE.

4476 sailing: great circle. Any SAILING when a GREAT CIRCLE TRACK is involved.

4477 sailing: Mercator. A mathematical solution of the plot as made on a MERCATOR CHART. It is similar to PLANESAILING, but uses MERIDIONAL DIFFERENCE and DIFFERENCE OF LONGITUDE in place of DIFFERENCEOF LATITUDE and DEPARTURE, respectively.

4478 sailing: meridian. Following a TRUE COURSE of 0° or 180°; SAILING along a MERIDIAN.

4479 sailing: middle-latitude. A method of converting DEPARTURE into DIFFERENCE OF LONGITUDE, or vice versa,when the true COURSE is not 90° or 270°, by assuming that such a COURSE is steered at the MIDDLE LATITUDE.

4480 sailing: parallel. A method of converting DEPARTURE into DIFFERENCE OF LONGITUDE, or vice versa, whenthe true COURSE is 90° or 270°.

4481 sailing: plane. A SAILING in which the EARTH or a small part of it is considered a plane.

4482 sailing: rhumb line. Any SAILING when a RHUMB LINE is involved.

4483 sailing: spherical. Any of the SAILINGS which consider the spherical or spheroidal shape of the EARTH.

4484 sailing: traverse. A method of determining the equivalent COURSE and distance made good by a craft following aTRACK consisting of a series of RHUMB LINES. The problem is customarily solved by the principles of PLANESAILING.

4485 sailing chart. See under CHART.

4486 sailing directions. Information published in book form describing COASTS, waters, CHANNELS, harbour facilities,etc., for use by mariners.

4487 Saint Elmo's fire. Faintly luminous electrical discharge in the ATMOSPHERE: this discharge, more or less continuousand of slight or moderate intensity, emanates from objects which present a high surface electric field, in particular frompoints and sharp edges.

4488 Saint Hilaire method. See MARCQ ST. HILAIRE METHOD.

207

4489 salina. A SALT MARSH or salt POND separated from the SEA but flooded by HIGH TIDES.

4490 salinity. A measure of the quantity of dissolved salts in SEA WATER. It is normally defined as the total amount ofdissolved solids in SEA WATER in parts per thousand (‰ ) by WEIGHT when all the carbonate has been converted tooxide, the bromide and iodide to chloride, and all organic matter is completely oxidized.

These qualifications result from the chemical difficulty in drying the salts in SEA WATER. In practice, salinity isnot determined directly but is computed from CHLORINITY, electrical conductivity, refractive index, or some otherproperty whose relationship to salinity is well established.

Because of the Law of Constancy of Proportions, the amount of CHLORINITY in a sea water sample is used toestablish the sample's salinity.

The relationship between chlorinity Cl and salinity S as set forth in Knudsen's Tables is S = 0.03 + 1.805 Cl.A joint committee of IAPO, UNESCO, ICES, and SCOR proposed the universal adoption of the following equation

for determining salinity from CHLORINITY: S = 1.80655 Cl. It was adopted by IAPO in 1963 and ICES in 1964.

4491 salinometer. Any device or instrument for determining SALINITY, especially one based on electrical conductivitymethods.

4492 sallying ship. Producing rolling motion of a vessel by the running in unison of a group from side to side. This is usuallydone to help float a vessel which is aground or to assist it to make HEADWAY when it is BESET by ICE.

4493 salt marsh. Flat, poorly drained coastal SWAMPS which are flooded by most HIGH TIDES.

4494 salt pans. Shallow POOLS of brackish water used for the natural evaporation of SEA WATER to obtain salt.

4495 sample. A representative part or single item from a larger whole or group especially produced for inspection or to giveevidence of quality.

4496 sampling. The process of taking samples.

4497 sand. Loose material consisting of small but easily distinguishable, separate grains, between 0.0625 and 2.000millimetres in diameter.

4498 sand dune. A RIDGE of SAND piled up by the action of WIND on sea coasts or in deserts.

4499 sanding. An irregular dot pattern used on some of the early HYDROGRAPHIC SURVEYS to accentuate the areabetween the HIGH- and LOW-WATER LINES.

4500 sandwave. A large wavelike sediment feature in very shallow water and composed of sand. The wavelength may reach100 meters; the amplitude is about 0.5 meter. Also called mega-ripple.

4501 sargasso (or sargassum). A certain type of SEAWEED, or more generally, a large floating mass of this SEAWEED.

4502 saros. The eclipse cycle of about 18 YEARS, almost the same length as 223 SYNODICAL MONTHS. At the end ofeach saros the SUN, MOON, and LINE OF NODES return to approximately the same relative positions, and anotherseries of ECLIPSES begins, closely resembling the series just completed.

4503 satellite. A relatively small CELESTIAL BODY revolving around a PLANET.Name sometimes given to the fictitious bodies assumed in the HARMONIC ANALYSIS OF TIDES.

4504 satellite: artificial. An object placed by man in ORBIT around the EARTH, or other CELESTIAL BODY. SeeNAVIGATIONAL (or NAVIGATION) SATELLITE.

4505 satellite: navigational (or navigation). See NAVIGATIONAL (or NAVIGATION) SATELLITE.

4506 satellite boat. A subsidiary boat used in PARALLEL SOUNDING.

4507 satellite geodesy. See GEODESY.

4508 satellite navigation. A positioning method using satellites.

4509 satellite sounding. SOUNDING effected by means of instruments placed on board an ARTIFICIAL SATELLITE.

4510 satellite station. See ECCENTRIC STATION.

208

4511 saturation. The condition existing when the greatest possible amount of anything has been reached, as a magneticsubstance which cannot be further magnetized, or an electronic AID TO NAVIGATION which is being used by all thecraft it can handle.

At given temperature and pressure, state of moist air whose mixing ratio is such that the moist air can co-exist inneutral EQUILIBRIUM with an associated condensed phase (liquid or solid) at the same temperature and pressure, thesurface of separation being plane.

4512 Savonius rotor current meter. A low-threshold current speed sensor composed of two cylindrical VANES disposed toform an S-shaped rotor responsive to a wide SPECTRUM of horizontal flow components.

4513 scale. A series of marks or GRADUATIONS at definite intervals.The ratio between the linear dimensions of a CHART, MAP, drawing, etc., and the actual dimensions represented. It

may be called chart scale or map scale when applied to a CHART or a MAP.In PHOTOGRAMMETRY, the ratio of a distance on a PHOTOGRAPH to a corresponding distance on the ground.

The scale of a PHOTOGRAPH varies from point to point because of DISPLACEMENTS caused by TILT andRELIEF; but it is usually taken as f/H where f is the PRINCIPAL DISTANCE of the CAMERA and H is the HEIGHTof the CAMERA above mean ground elevation.

4514 scale: bar. A graduated line on a MAP, PLAN, PHOTOGRAPH, or MOSAIC, by means of which actual grounddistances may be determined. Also called graphic scale or linear scale.

4515 scale: border. A SCALE drawn along the BORDER OF CHART.

4516 scale: equivalent. The relationship which a small distance on a MAP, CHART or graphic bears to the correspondingdistance on the EARTH, expressed as an equivalence.

4517 scale: fractional. See SCALE: NATURAL.

4518 scale: graphic. See SCALE: BAR.

4519 scale: large. A SCALE involving a relatively small reduction in size. A large scale CHART is one covering a smallarea. The opposite is SMALL SCALE. See SCALE: NATURAL.

4520 scale: latitude. The subdivided east and west borders of a MERCATOR CHART into DEGREES and MINUTES. Avariant of the BAR SCALE, since a MINUTE of LATITUDE is very nearly equal to a NAUTICAL MILE.

4521 scale: linear. See SCALE: BAR.

4522 scale: logarithmic. A SCALE graduated in the logarithms of uniformly-spaced consecutive numbers.

4523 scale: log factor. In HYDROGRAPHIC SURVEYS, a proportional DIAGRAM constructed for the SCALE of theSURVEY sheet by means of which the LOG FACTOR can be quickly determined.

4524 scale: natural. The ratio between the linear dimensions of a CHART, drawing, etc., and the actual linear dimensionsrepresented, expressed as a proportion. Occasionally called representative fraction, fractional scale or numerical scale.

4525 scale: numerical. See SCALE: NATURAL.

4526 scale parallel. Used in mercator projection charts to indicate the parallel at which the noted scale of the chart is exact.The scale parallel itself need not necessarily be depicted within the chart.

4527 scale: particular. The relation between an infinitesimal linear distance in any direction at any point on a mapPROJECTION and the corresponding linear distance on a GLOBE.

4528 scale: principal. The SCALE of a reduced or generating globe representing the sphere or SPHEROID defined by thefractional relation of their respective radii.

4529 scale: small. A SCALE involving a relatively large reduction in size. A small scale chart is one covering a large area.The opposite is LARGE SCALE. See SCALE: NATURAL.

4530 scale: speed. A graphic SCALE by means of which the rate of SPEED of a SURVEY SHIP, or the distance travelled ina given time can be determined quickly and accurately.

209

4531 scale error. The difference between the PRINCIPAL SCALE and the PARTICULAR SCALE resulting from projectionDISTORTION.

4532 scale factor. A multiplier for reducing a distance obtained from a MAP by computation or scaling to the actual distanceon the DATUM of the MAP.

A conventional modification which may be applied to the majority of map PROJECTIONS. This is the multipli-cation of the PRINCIPAL SCALE by some numerical constant which is slightly less than unity. This has the effect ofchanging all PARTICULAR SCALES by a corresponding amount and, since both maximum and minimum PAR-TICULAR SCALES are similarly modified, the special properties of the PROJECTION are unaffected. The effect is toproduce a better balance of positive and negative SCALE ERRORS over the mapped area.

4533 scale of hydrographic survey. That SCALE at which the POSITIONS of the SOUNDINGS in the area are plotted onthe HYDROGRAPHIC SURVEY SHEET. Suitable survey scales are selected in each case in accordance with thenature of the SURVEY and the average DEPTHS prevailing in the area.

4534 scan. See SCOPE.

4535 scanner. A device for directing a BEAM of RADIANT ENERGY successively over all points of a given region.In data processing a photo-electronic device for digital reproduction of pictures. The picture is scanned linewise and

decomposed into a matrix-like raster of individual pixels. In contrast to vectorial DIGITIZERS, the information tocoherent elements of the picture (lines, symbols) is lost by this rasterization process. To retrieve these elements therasterized picture has to be vectorized by pattern recognition methods and manual post-processing.

4536 scanning. Directing a BEAM of RADIANT ENERGY successively over all points of a given region.

4537 scar. A lofty, steep face of ROCK upon a mountain-side; a precipice; CLIFF.

4538 scarp. The steep face of a HILL.See also ESCARPMENT.

4539 scattering. In physics, the change in direction of a particle or WAVE, because of a collision with another particle orsystem.

In electromagnetism, diffusion of ELECTROMAGNETIC WAVES in a random manner by air masses in the upperATMOSPHERE. See also: TROPOSPHERIC SCATTERING.

4540 scend. Upward motion of a vessel, either upward motion of the bow and stern associated with pitching, or lifting of theentire vessel by WAVES or SWELL, when it may be called send.

4541 schist. A foliated metamorphic ROCK which can be split into thin flakes or flat lenticles. Schists are usually namedfrom the dominant mineral, e.g. mica schist.

4542 school. See SHOAL.

4543 Scientific Committee on Antarctic Research (SCAR). A committee established by ICSU to promote and coordinateantarctic research.

4544 scintillation. Rapid variations, often in the form of pulsations, of the brightness of the STARS or terrestrial lightsources.

4545 scope. The face of a CATHODE-RAY TUBE of electronic equipment. The various methods of presenting theinformation on a radar scope are usually designated by letter, as A-scope, B-scope, etc. Also called INDICATOR, scan,SCREEN, although the alphabetical designations are not generally used with SCREEN.

4546 scoria (pl. scoriae). Volcanic rock fragments usually of basic composition, characterized by marked vesicularity, darkcolour, high density, and a partly crystalline texture.

4547 scour. The action of a CURRENT or FLOW of water in clearing away MUD or other DEPOSIT; in civil engineering anartificial CURRENT or FLOW produced for this purpose.

4548 scree. A mass of DETRITUS, forming a precipitous, strong SLOPE upon a mountain-side. Also the material composingsuch a SLOPE. See also TALUS.

210

4549 screen. A device to shield or separate one part of an apparatus from other parts, or to separate the effects of one part onothers.

In electronic charting, a device connected to a COMPUTER displaying information either under computer control,or under user control, e.g. on a CATHODE-RAY TUBE. Devices allowing for high resolution are capable of displayingalso graphics.

4550 scribing. A method of preparing a MAP by cutting the lines into a prepared coating.

4551 sea. The great body of salt water in general, as opposed to LAND; OCEAN.One of the smaller divisions of the OCEANS.The state of the surface of the OCEAN with regard to WAVE or SWELL, as a calm sea. See CROSS SEA, HEAD

SEA, BEAM SEA, FOLLOWING SEA, QUARTERING SEA, SUGAR LOAF SEA.

4552 seabed sampler. A device used to obtain samples of the SEA FLOOR.

4553 seabed texture chart. A tracing showing the nature and texture of the BOTTOM, DEPTH CONTOURS and SEABEDfeatures including WRECKS and obstructions. Also called bottom texture tracing.

4554 seabed topography. See TOPOGRAPHY.

4555 sea bottom temperature. The temperature measured at or close to SEA FLOOR.

4556 seaboard. A general term for the rather extensive coastal region bordering the SEA.

4557 sea bed. See SEA FLOOR.

4558 sea (or lake) breeze. WIND of coastal regions, blowing by day from a large water surface (sea or lake) towards theLAND as a result of diurnal heating of the land surface.

4559 sea bottom. See SEA FLOOR.

4560 sea bottom sediment chart. See CHART: SEA BOTTOM SEDIMENT.

4561 sea buoy. See FAREWELL BUOY.

4562 sea channel. See CHANNEL.

4563 sea floor. The BOTTOM of the OCEAN when there is a generally smooth gentle GRADIENT. Also referred to as seabed, sea bottom.

4564 sea fog. See FOG.

4565 seagrass. Any grasslike marine ALGA. Eelgrass is one of the best known seagrasses.

4566 sea ice. Any form of ICE which has originated from SEA WATER. Generally, any ICE in the SEA.

4567 sea level. The height of the sea surface uninfluenced by wind waves and swell, which is frequently measured relative toa reference horizon. Observations of the sea level over a certain period are often evaluated to render minimum,maximum, and mean sea level.

4568 sea level datum. A determination of MEAN SEA LEVEL that has been adopted as a standard DATUM of HEIGHTSalthough it may differ from a later determination over a longer period of time.

4569 seamanship. A general term for the art by which vessels are handled.

4570 seamark. An AID TO NAVIGATION located with the express purpose of being visible from a distance toSEAWARD. Often erected in shoal water rather than on LAND.

4571 sea mile. The length of one MINUTE of arc, measured along the MERIDIAN in the LATITUDE of the POSITION; itslength varies both with the LATITUDE and with the FIGURE OF THE EARTH in use.

Used in NAVIGATION to measure distances on sea charts using the MERCATOR PROJECTION. Not to beconfused with NAUTICAL MILE or INTERNATIONAL NAUTICAL MILE.

4572 sea moat. See MOAT.

211

4573 seamount (or peak). An isolated or comparatively isolated ELEVATION rising 1 000 METRES or more from the SEAFLOOR and of limited extent across the SUMMIT.

4574 seamount chain. Several SEAMOUNTS in a line with bases separated by a relatively flat SEA FLOOR.

4575 seamount group. Several closely spaced SEAMOUNTS not in a line.

4576 seamount range. Several SEAMOUNTS having connected bases and aligned along a RIDGE or RISE.

4577 seaport. A PORT on or near the SEA and readily accessible to seagoing vessels.

4578 sea reach. The straight part of a RIVER between the last bend and the SEA.

4579 sea scarp. See ESCARPMENT.

4580 seashore. The SHORE of a SEA or OCEAN.

4581 seasonal current. See CURRENT.

4582 sea state. Numerical scale of the average surface WAVE HEIGHT as defined by the WMO code. Also called state ofthe sea.

4583 sea valley. See VALLEY.

4584 sea wall. An EMBANKMENT or wall for protection against WAVES or tidal action along a SHORE or WATERFRONT.

4585 seaward (adj. and adv.). Away from the LAND; toward the SEA.

4586 sea water. The water of the SEAS, distinguished from fresh water by its appreciable SALINITY. The degree of theSALINITY greatly affects the water's physical characteristics.

4587 sea water: colour of. The apparent colour of the surface layers of the SEA caused by the REFLECTION of certaincomponents of the visible light spectrum coupled with the effects of dissolved material, concentration of PLANKTON,DETRITUS, or other matter. Colour of oceanic water varies from deep blue to yellow and is expressed by numbervalues which are a variation of the FOREL SCALE. Plankton concentrations may cause a temporary appearance of red,white, green, or other colours. Also called water colour. See also DISCOLOURED WATER.

4588 seaway. A moderately rough SEA. Used chiefly in the expression 'in a seaway'.HEADWAY of a vessel.The SEA as a route of travel from one place to another; a shipping LANE.

4589 seaweed. Any macroscopic marine ALGA or SEAGRASS.

4590 Secchi disc. A white, black, or varicoloured disc, 30 centimetres in diameter, used to measure water transparency(clarity). The disc is lowered in the water and the DEPTH (in METRES) at which it disappears from sight is averagedwith the DEPTH at which it reappears. This average value is used to represent sea water transparency.

4591 second. The unit of TIME in the SI system.A sixtieth part of a MINUTE, an angle or an arc.

4592 secondary port. See STATION: SUBORDINATE.

4593 secondary radar. See RADAR: PRIMARY.

4594 secondary station. See STATION: SUPPLEMENTARY.

4595 secondary tide(s). See TIDE.

4596 secondary tide station. See STATION: TIDE.

4597 second nodal point. See NODAL POINT.

4598 second order triangulation. See TRIANGULATION CLASSIFICATION.

212

4599 section. Process of locating and coordinating a point from at least two existing control stations by observing horizontaldirections from the control stations to the point. Also called intersection.

4600 sector scan sonar. A horizontal aperture SONAR used to detect obstructions ahead of the vessel by mechanical orelectronic scanning in the horizontal plane.

4601 secular change. An increase or decrease of intensity and/or change of direction of the total MAGNETIC FIELD over aperiod of many years.

4602 sediment. Particulate organic and inorganic matter which accumulates in a loose unconsolidated form. It may bechemically precipitated from solution, secreted by organisms, or transported by air, ICE, WIND, or water and deposited.

4603 sediment(s): bottom. In general all sedimentary material regardless of origin found on or in the submarine BOTTOM,including ballast or other material dumped into the SEA by man. More specifically it is limited to unconsolidatedmineral and organic material forming the sea bottom, not including CORAL REEFS or BEDROCKS.

4604 sediment survey. A survey to determine the nature and distribution of types of sea bottom SEDIMENTS.

4605 sedimentary (adj.). Formed by the deposition of SEDIMENT.

4606 sedimentary rocks. ROCKS formed by the accumulation of SEDIMENT in water (aqueous DEPOSITS) or from air(eolian DEPOSITS). The SEDIMENTS may consist of rock fragments or particles of various sizes (conglomerate,sandstone, shale); of the remains or products of animals or plants (certain LIMESTONES and coal); of the product ofchemical action or of evaporation (salt, gypsum, etc.); or of mixture of these materials.

4607 sedimentation. The process of breakup and separation of particles from the parent ROCK, their transportation,deposition, and consolidation into another ROCK.

4608 sediment core. See CORE.

4609 sedimentology. The science concerned with the description, classification, origin, and interpretation of sediments andsedimentary rock.

4610 sediment trap. A device used to measure the rate and amount of SEDIMENTATION in a location.

4611 seiche. A STANDING WAVE oscillation of an enclosed or semi-enclosed water body that continues, pendulumfashion, after the cessation of the originating force, which may have been either seismic, atmospheric, or wave induced.

4612 seismic reflection. The study of the shallower internal structure of the earth by generating acoustic waves andregistering their reflection from subsurface layers.

4613 seismic refraction. The study of the shallower internal structure of the earth by generating acoustic waves andregistering their return at increasing distances from the acoustic source, thus detecting waves that have been transmittedalong deeper layers of greater elasticity and rigidity .

4614 seismic sea wave. See TSUNAMI.

4615 seismograph. An instrument that records the direction, intensity and TIME of EARTHQUAKES.

4616 seismology. The science and study of EARTHQUAKES, and their causes and effects and related PHENOMENA.

4617 selectivity. The degree of decrease in the RESPONSE of a resonant device with departure from RESONANCE. Selec-tivity determines the ability of a RADIO RECEIVER to differentiate between SIGNALS of different carrierFREQUENCY.

4618 selenotrope. A device used in geodetic SURVEYING for reflecting the MOON's RAYS to a distant point, to aid inlong-distance OBSERVATIONS.

4619 self-aligning level. See LEVELLING INSTRUMENT: SELF-ALIGNING LEVEL.

4620 semaphore. Any apparatus for signalling.

213

4621 semicircular deviation. See DEVIATION.

4622 semidiameter. Half the angle at the observer subtended by the visible disk of a CELESTIAL BODY. SEXTANTALTITUDES of the SUN and MOON should be corrected for semidiameter unless the centre is observed, as with aBUBBLE SEXTANT.

4623 semidiameter correction. A CORRECTION due to SEMIDIAMETER, particularly that sextant altitude correctionresulting from OBSERVATION of the upper or lower LIMB of a CELESTIAL BODY, rather than the centre ofthat body.

4624 semidiurnal current. See CURRENT.

4625 semidiurnal tide. See TIDE.

4626 semi-enclosed sea. See ENCLOSED SEA.

4627 semimajor axis. One-half of the longest diameter of an ellipse.

4628 semiminor axis. One-half of the shortest diameter of an ellipse.

4629 send. See SCEND.

4630 sensibility of magnetic compass. The ability of a magnetic compass card to align itself with the MAGNETICMERIDIAN after deflection.

4631 sensibility (or sensitivity) of spirit level. The ACCURACY and PRECISION which a SPIRIT LEVEL is capable ofproducing. Sensibility depends on the radius of curvature of its longitudinal section; the longer the radius, the moresensitive the level.

4632 sensible horizon. See HORIZON.

4633 sensitive bubble. See LEVEL: SPIRIT.

4634 sensitivity of electronic equipment. The ability of electronic equipment to amplify a SIGNAL, measured by theminimum strength of signal INPUT capable of causing a desired value of OUTPUT. The lower the INPUT signal for agiven OUTPUT, the higher the sensitivity.

4635 sensitivity of spirit level. See SENSIBILITY OF SPIRIT LEVEL.

4636 separation effect. In ECHO SOUNDING instruments with separate transmitting and receiving units, the effect causedby such separation. This introduces an ERROR depending on the distance between the acoustic units and varying withthe DEPTH of water. Such ERROR makes the registered DEPTH greater than the actual DEPTH. Separation alsocauses a LAG in registration of the transmitted SIGNAL.

4637 separation line. See SEPARATION ZONE.

4638 separation zone (or line). A zone or line separating the traffic LANES in which ships are proceeding in opposite, ornearly opposite directions; or separating a traffic lane from the adjacent sea area; or separating traffic lanes designatedfor particular classes of ships proceeding in the same direction.

4639 sequence of current. The order of occurrence of the TIDAL CURRENT strengths of a day, with special reference towhether the greater FLOOD immediately precedes or follows the greater EBB.

4640 sequence of tide. The order in which the TIDES of a day occur, with special reference to whether the HIGHER HIGHWATER immediately precedes or follows the LOWER LOW WATER.

4641 serial temperatures. The OBSERVATIONS required to determine the vertical distribution of temperature andSALINITY with DEPTH at any one place in the OCEAN.

4642 set. The direction toward which a CURRENT flows.

4643 set (v.i.). Of a CELESTIAL BODY, to cross the APPARENT HORIZON while descending. The opposite is RISE.To establish, as to set a COURSE.

214

4644 set of observations. A set of two or more OBSERVATIONS at different settings of the apparatus made in order toeliminate certain ERRORS which, for the apparatus concerned, are inherent in a single OBSERVATION.

4645 settlement. The general lowering in level of a moving vessel, relative to what its level would be were it motionless.Settlement is due to the regional depression of the surface of the water in which the ship moves. It is not an increase indisplacement. Settlement is a factor to be reckoned in ECHO SOUNDING.

4646 sexagesimal. An angular division whereby a circle is divided in 360 equal parts called degrees. A degree is divided in60 minutes and one minute in 60 seconds.

4647 sextant. A double-reflecting instrument for measuring angles, primarily ALTITUDES of CELESTIAL BODIES. Thesextant has an arc of 60°, a sixth of a circle, from which it derives its name, and a RANGE of 120°. In modern practicethe term also applies to a similar instrument, regardless of its RANGE. See also OCTANT, QUADRANT andQUINTANT.

4648 sextant: air. A SEXTANT designed primarily for AIR NAVIGATION. Air sextants are generally provided with someform of ARTIFICIAL HORIZON.

4649 sextant: bubble. A SEXTANT with a bubble or SPIRIT LEVEL to indicate the horizontal.

4650 sextant: gyro. A SEXTANT provided with a GYROSCOPE to indicate the horizontal.

4651 sextant: hydrographic. See SEXTANT: SOUNDING.

4652 sextant: marine. A SEXTANT designed primarily for MARINE NAVIGATION.

4653 sextant: pendulum. A SEXTANT provided with a PENDULUM to indicate the horizontal.

4654 sextant: sounding (British terminology). A simplified and more robustly constructed version of the marine sextantintended primarily for use in HYDROGRAPHIC SURVEY. It has no SHADES, a wide angle low magnificationtelescope and is generally graduated in MINUTES of arc. It may be fitted with either a VERNIER or a MICROMETERreading device. Both mirrors are silvered all over to enable angles up to 180° to be measured. Some sounding sextantshave a 90° prism attachment which reflects the normal direct LINE OF SIGHT 90° to the left. Called surveying sextant,or hydrographic sextant in U.S. terminology.

4655 sextant: surveying. See SEXTANT: SOUNDING.

4656 sextant adjustment. The process of checking the ACCURACY of a SEXTANT and removing or reducing its ERROR.

4657 sextant altitude. See under ALTITUDE.

4658 sextant altitude correction. Any of several CORRECTIONS applied to a SEXTANT ALTITUDE in the process ofconverting it to TRUE ALTITUDE.

4659 sextant chart. See CIRCLE SHEET.

4660 sextant error. See ERROR.

4661 sextometer method. A substitute for PLANE TABLE TRAVERSE for short distances in unimportant areas where theuse of the PLANE TABLE is impracticable and the survey party operates in a LAUNCH. AZIMUTH is carried forwardby means of angles accurately measured with a SEXTANT. Distance is determined by measuring, also with theSEXTANT, the small horizontal angle between fixed TARGETS on a special rod held horizontally. The angle may beconverted into a distance by means of a HYPSOGRAPH.

4662 shade. A coloured glass that can be moved into the LINE OF SIGHT of an optical instrument, such as a SEXTANT, toreduce the intensity of light reaching the eye. Also called shade glass. See HORIZON SHADE, and INDEX SHADE.

4663 shaded relief. See ILLUMINATED RELIEF.

4664 shade glass. See SHADE.

4665 shading. Gradations of light, colour, or thickness of lines; or indications of shadows. Shading of lines is sometimes usedon CHARTS to produce the effect of HEIGHT or DEPTH. See also HILL SHADING.

215

4666 shadow pin. A small rod or pin used to cast a shadow on an instrument, such as a MAGNETIC COMPASS or SUNCOMPASS, to determine the direction of the luminary; a GNOMON.

4667 shadow zone. A region into which very little sound energy penetrates.

4668 shallow (adj.). Having little DEPTH.

4669 shallow (n.). An area composed of unconsolidated material where the DEPTH of water is relatively slight. It may be ahazard to SURFACE NAVIGATION.

4670 shallows. An indefinite term applied to expanses of SHOAL or SHALLOW WATER.

4671 shallow water. Commonly, water of such a DEPTH that SURFACE WAVES are noticeably affected by bottom topog-raphy. It is customary to consider water of DEPTHS less than half the surface WAVE LENGTH as shallow water.

4672 shallow water constituent. A short-period harmonic term introduced into the formula of ASTRONOMICAL TIDEconstituents to take account of the change in the form of a TIDE WAVE resulting from SHALLOW WATER conditi-ons. Shallow water constituents include OVERTIDES and COMPOUND TIDES. See HARMONIC CONSTITUENT.

4673 shallow water corrections. Quantities to be applied to a TIDE PREDICTION to correct for the QUARTERDIURNAL TIDES.

4674 shallow water tide(s). See TIDE.

4675 shallow water wave. See WAVE: TRANSITIONAL WATER.

4676 shear strength. The internal resistance of a body to shear stress.

4677 shelf. Geologically an area adjacent to a continent or around an island and extending from the low-water line to thedepth at which there is usually a marked increase of slope towards greater depth. See also CONTINENTAL SHELF.

4678 shelf break. See SHELF EDGE.

4679 shelf edge (or shelf break). The line along which there is a marked increase of SLOPE at the outer margin of aCONTINENTAL (or ISLAND) SHELF.

4680 shell. The hard outside covering of an animal. Part of the ocean bed is composed of numerous shells of marine animals.

4681 shelving. A gently sloping area.

4682 shimmer. Apparent fluttering of objects at the EARTH's surface, when they are viewed in an almost horizontaldirection above strongly heated surfaces.

4683 shingle. Rounded, often flat waterworn rock fragments larger than approximately 16 millimetres.

4684 ship of opportunity. An expression designating ships of commercial lines employed in gathering oceanographicDATA.

4685 ship-to-shore triangulation. See TRIANGULATION.

4686 shipyard. A place where ships are built or repaired.

4687 shoal (adj.). SHALLOW.

4688 shoal (n.). An offshore hazard to NAVIGATION on which there is a DEPTH of ten FATHOMS or twenty METRES orless, composed of unconsolidated material, except CORAL or ROCK. See REEF.

A great number of fish or aquatic animals swimming in company or considered as a group. Also called school.

4689 shoal (v.i.). To proceed from a greater to a lesser DEPTH.To cause to become less deep.

216

4690 shoaling effect. The alteration of a WAVE proceeding from deep water to SHALLOW WATER.

4691 shoot (v.t.). To observe the ALTITUDE of (a CELESTIAL BODY).

4692 shore. The narrow strip of LAND in immediate contact with any body of water including the area between HIGH andLOW WATER lines.

4693 shoreface. The narrow zone seaward from the low tide shoreline, permanently covered by water, over which the beachsands and gravels actively oscillate with changing wave conditions.

4694 shore lead. A LEAD between PACK ICE and the SHORE, or between PACK ICE and a narrow fringe of FAST ICE.

4695 shoreline. The line where SHORE and water meet. Although the terminology of COASTS and SHORES is ratherconfused, shoreline and coastline are generally used as synonymous.

4696 shoreline map. Shoreline maps are the graphic representation of planetable and photogrammetric surveys. The mapscontain graphic data relating to the shoreline, alongshore natural and man-made features, and a narrow zone of naturaland man-made features inland from the shoreline. The original sources of a shoreline map are ground survey data andphotographs. Utilizing these sources, photogrammetric map compilation techniques, and instruments, cartographersgenerate shoreline maps, overlays, and associated data. The data are primarily generated to support nautical chartmaintenance, new nautical chart construction, and hydrographic survey operations.

4697 shore station. See FIXED STATION.

4698 short baseline acoustic system. An underwater acoustic position fixing system comprising a minimum of threehydrophones on the hull of a surface vessel. Three ranges are measured to a beacon either mounted on the seabed or asubmersible to obtain a three-dimensional position fix.

4699 short range navigation. See NAVIGATION.

4700 short wave. See WAVE.

4701 shower. PRECIPITATION, often short-lived and heavy, falling from convective CLOUDS; the drops or solid particlesin showers are usually bigger than the corresponding elements in other types of PRECIPITATION. Showers arecharacterized by their sudden beginning and ending, generally by large and rapid changes of intensity, and, mostfrequently, by the appearance of the sky; namely, rapid alternations of dark, menacing CLOUDS (cumulonimbus) andof CLEARANCES of short duration.

4702 shuga. An accumulation of spongy white ice lumps, a few centimetres across; the lumps are formed from grease ice orslush and sometimes from anchor ice rising to the surface.

4703 shutter. In PHOTOGRAPHY, the mechanism of a CAMERA which controls the length of time the EMULSION isexposed.

4704 shutter: between-the-lens. A SHUTTER located between the lens elements of a CAMERA; usually consisting of thinmetal leaves which open and close or revolve to make the EXPOSURE.

4705 shutter: focal-plane. A SHUTTER located near the FOCAL PLANE; usually consisting of a curtain with a slot whichis pulled across the FOCAL PLANE to make the EXPOSURE.

4706 shutter: louver. A SHUTTER consisting of a number of thin metal strips or louvers which operate like a venetian blindto make the EXPOSURE; usually located just in front of or just behind the LENS.

4707 side echo(es). See ECHO(ES): FALSE.

4708 side lap. See OVERLAP.

4709 side-looking airborne radar (SLAR). A radar system using a stabilized antenna orientated at right angles to theaircraft flight path.

4710 side scan sonar. A form of active sonar in which fixed acoustic beams are directed into the water perpendicularly to thedirection of travel to scan the bottom and generate a record of the bottom configuration.

217

4711 side scan sonar mosaic. An assembly of side scan sonar images whose edges have been cut or torn and matched toform a continuous side scan sonar image.

4712 sidereal (adj.). Of or pertaining to the STARS.

4713 sidereal chronometer. See CHRONOMETER.

4714 sidereal day. See DAY.

4715 sidereal hour angle. See HOUR ANGLE.

4716 sidereal month. See MONTH.

4717 sidereal time. See TIME.

4718 sidereal year. See YEAR.

4719 side shot. A reading or measurement from a survey station to locate a point which is not intended to be used as a basefor the extension of the SURVEY. A side shot is usually made for the purpose of determining the POSITION of someobject which is to be shown on the MAP.

4720 siemens. The unit of conductance in the SI SYSTEM.

4721 sight. OBSERVATION of the ALTITUDE, and sometimes also the AZIMUTH of a CELESTIAL BODY for a LINEOF POSITION, or the DATA obtained by such OBSERVATION.

Any of various devices used to aid the eyes in lining up an optical instrument on its OBJECTIVE.

4722 sight line. See COLLIMATION: LINE OF.

4723 sight (or sighting) vane. See VANE.

4724 sigma-t. (Symbol s t). A conveniently abbreviated value of the DENSITY of a sea water sample of TEMPERATURE tand SALINITY S:

st = (rho(S,t)-1) * 103

where rho(S,t) is the value of the sea water density in CGS units at standard ATMOSPHERIC PRESSURE. If, forexample, rho(S,t) = 1.02648, then st = 26.48.

4725 signal. As applied to electronics, any transmitted electrical IMPULSE.That which conveys intelligence in any form of communication, such as a TIME SIGNAL, a PIP on the SCOPE of

electronic equipment, or an object marking the location of a surveying STATION.

4726 signal: eccentric. See ECCENTRIC SIGNAL.

4727 signal: fog. See FOG SIGNAL.

4728 signal: hydrographic. Any object, existing or specially erected, for the CONTROL of HYDROGRAPHIC SURVEY.See also SOUNDING MARK.

4729 signal: longitude. A sign indicating a time event, observable at different stations, and used in comparing LOCALTIMES of those stations, and determining the difference of their LONGITUDES.

4730 signal: luminous. In SURVEYING, a SIGNAL consisting of HELIOTROPE or HELIOSTAT by day, andSELENOTROPE or SIGNAL LAMP by night.

4731 signal: sound. See SOUND SIGNAL.

4732 signal: survey. A natural or artificial object or structure whose horizontal and sometimes vertical POSITION isobtained by surveying methods. Survey signals are given special designation according to the kind of SURVEY inwhich they are determined, or which they may later serve.

4733 signal: tidal current. A SIGNAL or message conveying information on condition of TIDAL CURRENT in the area inquestion.

4734 signal: tide. A SIGNAL or message conveying information on tidal conditions in the area in question.

218

4735 signal: time. An accurate SIGNAL marking a specified TIME or time interval. It is used primarily for determiningERRORS of TIMEPIECES. Such SIGNALS are usually sent from an observatory by RADIO or telegraph, but visualSIGNALS are used at some PORTS.

4736 signal: topographic. Any object, existing or specially erected, used for the CONTROL of TOPOGRAPHIC SURVEY.

4737 signal: triangulation. A rigid structure erected close or over a TRIANGULATION STATION. Also any object, naturalor artificial, whose POSITION is obtained in a triangulation survey. The term may be applied to a structure whosePOSITION is determined by TRIANGULATION, but whose primary purpose is to serve later in a HYDROGRAPHICor TOPOGRAPHIC SURVEY, when it may become known as a HYDROGRAPHIC or TOPOGRAPHIC SIGNAL.

4738 signal: tripod. A three-legged TRIANGULATION SIGNAL. It is usually constructed in such a manner that an obser-ver can set up his THEODOLITE and make OBSERVATIONS without moving any of the structure.

4739 signal: water. A HYDROGRAPHIC SIGNAL erected in SHALLOW WATER.

4740 signal: weather. A visual SIGNAL displayed to indicate a WEATHER FORECAST.

4741 signal lamp. See LAMP.

4742 signal station. A place on SHORE from which SIGNALS are made to ships at sea.

4743 signi. Abbreviation for Signalisation de Navigation Intérieure. A system of buoyage used in certain inland waters in theNetherlands.

4744 sill. The low part of the RIDGE or RISE separating ocean basins from one another or from the adjacent SEA FLOOR.

4745 sill depth. The greatest depth over a sill.

4746 silt. An unconsolidated SEDIMENT whose particles range in size from 0.0039 to 0.0625 millimetres in diameter(between CLAY and SAND size).

4747 siltation. The deposition or accumulation of silt that is suspended in a body of water.

4748 simplex. A method in which TELECOMMUNICATION between two stations takes place in one direction at a time.

4749 simultaneous altitudes. ALTITUDES of two or more CELESTIAL BODIES observed at the same TIME.

4750 simultaneous comparison of depths. The comparison of echo sounding depths with simultaneous direct verticalmeasurements of DEPTH. Simultaneous comparisons are periodically made to determine the INSTRUMENTALERROR of ECHO SOUNDERS.

4751 single buoy mooring (SBM). A large mooring buoy used by tankers to load and unload in port approaches or inoffshore oil and gas fields.

4752 single day tide. See TIDE: DIURNAL.

4753 single point mooring (SPM). A mooring structure used by tankers to load and unload in port approaches or in offshoreoil and gas fields. The size of the structure can vary between a large mooring buoy and a manned floating structure.

4754 sinker. WEIGHT used to sink fishing line or SOUNDING LINE.

4755 sinking. A downward movement of surface water generally caused by converging CURRENTS or when water massbecomes more dense than the surrounding water. See UPWELLING, and CONVERGENCE.

An apparent lowering of distant terrestrial objects by abnormal ATMOSPHERIC REFRACTION. Because ofsinking, objects normally visible at or near the HORIZON sometimes disappear below the HORIZON. The opposite isLOOMING.

4756 siren. A type of FOG SIGNAL apparatus which produces SOUND by virtue of the passage of air through slots or holesin a revolving disk.

219

4757 SI System. See SYSTÈME INTERNATIONAL.

4758 sketch map. See MAP.

4759 sketch survey. See SURVEY.

4760 skew convergence. The angle, on a SKEWED PROJECTION, between TRUE NORTH and skew north.

4761 skewed projection. See PROJECTION.

4762 sky compass. See COMPASS.

4763 sky wave. See WAVE.

4764 sky-wave correction. The CORRECTION to be applied to the time difference reading of received SKY WAVES toconvert it to an equivalent GROUND WAVE reading.

4765 slack tide. See SLACK WATER.

4766 slack water. The interval when the SPEED of the TIDAL CURRENT is very weak or zero; usually refers to the periodof reversal between EBB and FLOOD currents. Also called slack tide.

4767 slant distance. The straight-line distance from one point to another, as contrasted with GROUND DISTANCE. Thisexpression is customarily used only when the straight line connecting the two points lies above the surface of theEARTH. Also called slant range.

4768 slant range. See SLANT DISTANCE.

4769 slave. A SLAVE STATION.

4770 slave station. In a radio navigation system, the transmitting station controlled or triggered by the SIGNAL receivedfrom the MASTER STATION. Often shortened to slave.

4771 slide: dark. A thin PLATE (usually metal or fibre, rigid or flexible) which, after insertion in a camera magazine,renders it light-tight.

4772 slide (or sliding) rule. A device used for performing rapid computations involving multiplication, division, proportion,roots, and powers. It has a body like a ruler, but with a sliding section. Both body and sliding section are graduated withnumbers according to LOGARITHMIC SCALES.

4773 slime. Soft, fine, oozy MUD or other substance of similar consistency.

4774 slip. See DOCK.

4775 slipway. The prepared and usually reinforced inclined surface on which keel- and bilge-blocks are laid for supporting avessel under construction.

4776 slope. An inclined surface or line.The degree of INCLINATION to the horizontal. Usually expressed as a ratio.

4777 slope and height factor. In the measurement of distances with Tellurometer, a factor to be reckoned when reducingdistances to the SPHEROID. It takes into account the mean HEIGHT of the stations (master and remote units) and theSLOPE of the line.

4778 slope angle. The angle between a SLOPE and the horizontal.

4779 slope correction (tape). See TAPE: GRADE CORRECTION.

4780 slotted-template triangulation. In PHOTOGRAMMETRY, a graphical RADIAL TRIANGULATION using slottedTEMPLATES.

4781 slough. A minor marshland or tidal WATERWAY which usually connects other tidal areas; often more or lessequivalent to a BAYOU.

Quagmire, SWAMP, miry place.

220

4782 sludge. Spongy whitish ice lumps, a few centimetres across; they consist of SLUSH, of snow slush and sometimes ofspongy ice lumps formed on the BOTTOM of the SEA and emerging on the surface.

4783 sluice. Sliding gate or other contrivance for changing the level of a body of water by controlling FLOW into or out of it.

4784 slush. An accumulation of ice crystals which remain separate or only slightly frozen together. It forms a thin layer andgives the sea surface a greyish or leaden-tinted colour. With light WINDS no RIPPLES appear on the surface.

4785 small circle. See CIRCLE.

4786 small correction (British terminology). Minor corrections made to the chart plates. They have been discontinued since1954.

4787 small craft chart. Chart designed for easy reference and plotting in limited spaces. It portrays regular nautical chartdetail and other specific details of special interest to the small craft operator, such as enlargements of harbours; tide,current and weather information; rules of the road information; locations of marine facilities; anchorages; courses; anddistances.

4788 small diurnal range. See RANGE.

4789 small halo. See HALO.

4790 small scale. See SCALE.

4791 small tropic range. See RANGE.

4792 smoothing. Averaging of data in space or time, designed to compensate for random errors or fluctuations of a scalesmaller than that presumed significant to a specific problem.

4793 smooth sheet (U.S. terminology). See FAIR CHART.

4794 snag. A tree or branch embedded in a river or lake bottom and not visible on the surface, forming thereby a hazard toboats.

4795 snapper. A bottom sampling device used to collect a small amount (less than 1 pint) of material from the ocean floor. Ithas metal jaws that snap shut when the device touches the BOTTOM. Also called clamshell snapper.

4796 snapper: mud. A small clamshell SNAPPER which is attached to the bottom of SOUNDING LEAD by means of ahole drilled in the LEAD.

4797 snow. PRECIPITATION of ice crystals, most of which are branched (sometimes starshaped).

4798 snowstorm. STORM of blowing SNOW.

4799 soft iron. Iron or steel which is easily magnetized by induction, but loses its MAGNETISM when the MAGNETICFIELD is removed. The opposite is HARD IRON.

4800 software. All programs which can be used on a computer system. In particular the system software (i.e. theOPERATING SYSTEM) consists of all programs which are necessary for the proper functioning of the computer. Theapplication software consists of all programs developed for special user applications.

4801 soil. The EARTH or GROUND; the face or surface of the EARTH.The GROUND with respect to its composition, quality, etc., or as the source of vegetation.

4802 solar apex. See APEX.

4803 solar cell. A cell using the solar light or other light energy for conversion into electric energy.

4804 solar day. See DAY.

4805 solar eclipse. See ECLIPSE.

4806 solar parallax. See PARALLAX.

221

4807 solar radiation. RADIATION emitted by the SUN.

4808 solar system. The SUN and other CELESTIAL BODIES within its gravitational influence, including PLANETS,PLANETOIDS, SATELLITES, COMETS, and METEORS.

4809 solar tide. See TIDE.

4810 solar time. See TIME.

4811 solar year. See YEAR: TROPICAL.

4812 solenoid. A coil of wire, often helical in form, which becomes an electromagnet when an electric current is passedthrough it.

4813 solid. Matter of stable shape, not liquid or fluid.

4814 solitary wave. See WAVE.

4815 solstice. One of the two points of the ECLIPTIC farthest from the CELESTIAL EQUATOR; one of the two points onthe CELESTIAL SPHERE occupied by the SUN at maximum north or south DECLINATION. That in the northernhemisphere is called the summer solstice or first point of Cancer, and that in the southern hemisphere, the wintersolstice or first point of Capricorn. Also called solsticial point.

That instant at which the SUN reaches one of the solstices, about June 21 (summer solstice) or December 22 (wintersolstice).

4816 solsticial colure. See COLURE: SOLSTICIAL.

4817 solsticial point. See SOLSTICE.

4818 solsticial tide(s). See TIDE.

4819 sonar. A system of determining distance of an underwater object by measuring the interval of time betweentransmission of an underwater sonic or ultrasonic SIGNAL and the return of its ECHO. Direction may also bedetermined by noting the direction of transmission of the SIGNAL. The name sonar is derived from the words soundnavigation and ranging. See ECHO RANGING.

4820 sonargram. The analog echo record produced by the side scan sonar recording device on special chemically-treatedpaper.

4821 sonar target. An object which reflects a sufficient amount of a sonar signal to produce a detectable echo signal at thesonar equipment.

4822 sonic bearing. See BEARING.

4823 sonic depth finder. See ECHO SOUNDER.

4824 sonic frequency. See AUDIOFREQUENCY.

4825 sonic line of position. A LINE OF POSITION determined by means of SOUND WAVES. Also called acoustic line ofposition.

4826 sonic navigation. See NAVIGATION.

4827 sonic wave. See WAVE: SOUND.

4828 sono-buoy. A BUOY with equipment for automatically transmitting a radio signal when triggered by an underwaterSOUND SIGNAL.

4829 sono-radio buoy. See SONOBUOY.

4830 sound. A relatively long ARM OF THE SEA forming a CHANNEL between an ISLAND and a MAINLAND orconnecting two larger bodies of water, as a SEA and the OCEAN, or two parts of the same body but usually wider andmore extensive than a STRAIT.

A vibratory disturbance in air or some other elastic medium, capable of being heard by the human ear, and thus ofany FREQUENCY between about 20 and 20,000 CYCLES PER SECOND.

222

4831 sound (v.i.). To measure the DEPTH of the water.

4832 sound buoy. See BUOY.

4833 sound channel. The region in the water column where SOUND VELOCITY first decreases to a minimum value withDEPTH and then increases in value as a result of pressure. Above the minimum value SOUND RAYS are bentdownward, and below the minimum value SOUND RAYS are bent upward; the RAYS are thus trapped in this CHAN-NEL. SOUND travelling in a deep channel can be detected thousands of MILES from the sound source.

4834 sounder. A machine or apparatus for SOUNDING. An ECHO SOUNDER.

4835 sounder: echo. See ECHO SOUNDER.

4836 sounding. Measured or charted DEPTH of water, or the measurement of such a DEPTH.In METEOROLOGY, determination of one or several upper-air meteorological elements by means of instruments

carried up by balloon, aircraft, kite, glider, ROCKET, SATELLITE, etc.

4837 sounding: acoustic. See ECHO SOUNDING.

4838 sounding: danger. A minimum SOUNDING chosen for a vessel of specific DRAFT in a given area to indicate thelimit of safe NAVIGATION.

4839 sounding(s): detached. Those SOUNDINGS, including least DEPTHS on SHOALS, whose POSITIONS wereaccurately located. See POSITION: DETACHED.

4840 sounding: doubtful. A DEPTH shown on a CHART over a SHOAL, a ROCK, etc., that may be less than thatindicated.

4841 sounding: drift. A method of finding the least DEPTH in a shoal area by letting the boat drift along a series of closelyspaced, previously run parallel SOUNDING LINES, while the LEADSMAN sounds or 'feels' along the BOTTOM. Theoperation is repeated until the SHOAL is found or the area is covered.

4842 sounding: echo. See ECHO SOUNDING.

4843 sounding: fixed angle. A method consisting in sounding along the arcs of circles subtended by two objects whosePOSITION is known. For each arc, an angle remains constant (fixed angle) and the second angle, between another pairof objects, defines the POSITION of the vessel along the arc. By varying the fixed angle successively by equal amounts,a series of LINES OF SOUNDING covering the area systematically can be run. See FIXED ANGLE PLOT.

4844 sounding: formation. SOUNDING carried out by two or more ships in formation searching for a reported danger.

4845 sounding: hand lead. SOUNDING with HAND LEAD. Hand lead soundings are usually taken from a slowly movingvessel.

4846 sounding: line of. See LINE OF SOUNDING.

4847 sounding(s): minus (U.S. terminology). SOUNDINGS that reduce to HEIGHT above the SOUNDING DATUM (planeof reference) when corrected for HEIGHT OF TIDE. Minus soundings are shown on the SMOOTH SHEET precededby a minus sign.

4848 sounding(s): no bottom (U.S. terminology). SOUNDINGS where the BOTTOM was not reached because the generalDEPTHS were too great for the method of measurement.

4849 sounding: wire. SOUNDING with a SOUNDING MACHINE. To obtain a vertical wire sounding the ship or boat mustbe stopped while the wire is running out.

4850 sounding(s): zero (U.S. terminology). SOUNDINGS that reduce to HEIGHTS above the SOUNDING DATUM butshown on the SMOOTH SHEET as zero soundings regardless of HEIGHT. A practice now discontinued. SeeSOUNDING: MINUS.

4851 sounding apparatus. Any apparatus designed for use in measuring the DEPTH of water.

4852 sounding board. See FIELD BOARD.

223

4853 sounding book. See SOUNDING RECORD.

4854 sounding bottle. A strong metal container fitted with non-return valves for taking WATER SAMPLES at greatDEPTHS. See also WATER BOTTLE.

4855 sounding datum. See DATUM.

4856 sounding interval. The interval between successive SOUNDINGS along a LINE OF SOUNDING. See DENSITY OFSOUNDINGS.

4857 sounding lead. A LEAD used for determining DEPTH of water. Also referred to as plummet.

4858 sounding line. A LINE OF SOUNDING.A LEAD LINE.

4859 sounding machine. An instrument for measuring DEPTH of water, consisting essentially of a reel of wire to one end ofwhich is attached a WEIGHT which carries a device for recording the DEPTH. A crank or motor is provided for reelingin the wire.

4860 sounding mark (British terminology). A MARK erected for CONTROL of SOUNDING; its POSITION is determinedby plotting using INTERSECTIONS from sextant FIXES.

4861 sounding pole. A graduated pole or rod used for SOUNDING in SHALLOW WATER. Also called sounding rod.

4862 sounding record. Bound record book in which all of the DATA taken on a HYDROGRAPHIC SURVEY are entered,and become part of the permanent records of the SURVEY. Also called sounding book.

4863 sounding rod. See SOUNDING POLE.

4864 sounding selection. 1. Water depths numerically displayed on a CHART.2. The process of choosing individual water depths from a hydrographic survey in the compilation of a chart.

4865 sounding tube. A glass tube of small diameter, used with certain types of SOUNDING MACHINES in determiningDEPTH.

4866 sounding wire. A wire used with a SOUNDING MACHINE in determining DEPTH.

4867 sound intensity. See INTENSITY.

4868 sound pressure. See PRESSURE.

4869 sound ray. A line perpendicular at each of its points to the fronts of a sound WAVE.

4870 sound signal. Any SOUND produced to convey intelligence, as a FOG SIGNAL.

4871 sound signal: submarine. A SOUND SIGNAL transmitted through water.

4872 sound velocity. The rate of motion at which sound energy moves through a medium.The velocity of sound in SEA WATER is a function of temperature, SALINITY, and the changes in pressure

associated with changes in DEPTH. An increase in any of these factors tends to increase the VELOCITY.

4873 sound wave. See WAVE.

4874 source. The point where a RIVER rises.

4875 source diagram. A small scale map or index placed on a chart to indicate the coverage area, and textual description, ofthe source data, used in construction of the chart. Also called compilation diagram.

4876 south celestial pole. See POLE: CELESTIAL.

4877 south geographical pole. See POLE: GEOGRAPHICAL.

4878 southing. The distance a craft makes good to the south. The opposite is NORTHING.

4879 south magnetic pole. See MAGNETIC POLE.

224

4880 space coordinates. See COORDINATES.

4881 space modulation. See MODULATION.

4882 spar buoy. A BUOY made of a tapered log or metal shaped like a tapered log, and secured so as to float in anapproximately vertical position.

4883 sparker. An ECHO SOUNDER which uses an electrical spark discharge as the sound source. A RECORDER producesa CHART which represents a vertical cross section of the geological structure beneath the water bottom.

4884 spatial (adj.). Pertaining to or limited by space.

4885 special mark. In the IALA Maritime Buoyage System a special mark indicates an area or feature referred to in nauticaldocuments; it is not primarily intended to assist navigation.

4886 special purpose map. See MAP.

4887 specific gravity. The ratio of the density of a substance at a given temperature and the density of some other substanceused as a STANDARD. For liquids and solids, the STANDARD assumed is the density of distilled water at 4°C.

4888 spectroscope. An optical instrument for forming SPECTRA. It is very useful in studying the characteristics ofCELESTIAL BODIES.

4889 spectrum (pl. spectra). A series of IMAGES formed when a BEAM of RADIANT ENERGY is separated into itsvarious WAVE LENGTH components, as when a BEAM of white light is refracted and dispersed by a prism.

The entire RANGE of electromagnetic RADIATIONS, or any part of it used for a specific purpose, as the RADIOSPECTRUM (10 kilohertz to 300,000 MEGAHERTZ).

4890 specular reflection. The type of reflection, in optics or microwave theory, characteristic of a highly polished planesurface from which all rays are reflected at an angle equal to the angle of incidence.

4891 speed. Rate of motion. The terms speed and VELOCITY are often used interchangeably, but speed is a scalar, havingmagnitude only, while VELOCITY is a vector quantity, having both magnitude and direction. Speed may either be theship's speed through water, or the SPEED MADE GOOD over ground.

4892 speed: angular. Change of direction per unit time. Also called angular rate or angular velocity.

4893 speed: ground. See GROUND SPEED.

4894 speed: linear. Rate of motion in a straight line.

4895 speed: relative. See RELATIVE MOTION.

4896 speed line. A LINE OF POSITION approximately perpendicular to the COURSE.

4897 speed made good. The actual velocity of the ship along a course measured in relation to the ocean bottom or to fixedobjects ashore.

4898 speed of lens. See APERTURE: RELATIVE.

4899 speed of propagation. The speed with which waves of energy travel through or along a medium other than a speciallyconstructed path.

4900 speed of relative movement. See RELATIVE MOTION.

4901 speed of sound. The SPEED OF PROPAGATION of sound waves .

4902 speedometer. An ODOMETER recording the SPEED of a craft by a process of differentiation.

4903 speed scale. See SCALE.

4904 sphere. A curved surface all points of which are equidistant from a fixed point within, called the centre.

4905 spherical aberration. See ABERRATION OF LIGHT.

225

4906 spherical buoy. See BUOY.

4907 spherical coordinates. See COORDINATES.

4908 spherical excess. The amount by which the sum of the three angles of a spherical triangle exceeds 180°.

4909 spherical sailing. See SAILING.

4910 spherical wave. See WAVE.

4911 spheroid. An ellipsoid; a figure resembling a sphere. Also called ellipsoid or ELLIPSOID OF REVOLUTION, fromthe fact that it can be formed by revolving an ellipse about one of its axes. In GEODESY, this term is frequently used tomean REFERENCE SPHEROID. See also SPHEROID: OBLATE and SPHEROID: PROLATE.

4912 spheroid: oblate. An ELLIPSOID OF REVOLUTION, the minor AXIS of which is the AXIS of REVOLUTION. TheEARTH is approximately an oblate spheroid.

4913 spheroid: prolate. An ELLIPSOID OF REVOLUTION, the major AXIS of which is the AXIS of REVOLUTION.

4914 spheroid of reference. See REFERENCE SPHEROID.

4915 spicule. A minute needlelike or multiradiate calcareous or siliceous body in sponges, radiolarians, primitive chitons, andechinoderms. They frequently are identified in marine sediment samples.

4916 spilling breaker. See BREAKER.

4917 spindle. A spar serving as a BEACON.A slender pin or rod, as one constituting part of a machine.

4918 spire. A pointed structure extending above a building. The spire is seldom less than two-thirds of the entire height andits lines are rarely broken by stages or other features. The term is not applied to short pyramid-shaped structures risingfrom a tower or belfry.

4919 spirit bubble. See LEVEL: SPIRIT.

4920 spirit compass. See COMPASS: LIQUID.

4921 spirit level. See LEVEL.

4922 spirit level axis. See AXIS OF SPIRIT LEVEL.

4923 spirit levelling. See LEVELLING.

4924 spit. A small point of land or narrow SHOAL projecting into a body of water from the SHORE.

4925 splines. Flexible rulers or interpolation algorithm used to draw smooth curves between points when constructing e.g.hyperbolic lattices.

4926 split fix. See FIX.

4927 split line of sounding. A LINE OF SOUNDING run to fill a void when the maximum spacing of principal lines isexceeded.

4928 spoil. MUD, SAND, SILT or other DEPOSIT obtained from the BOTTOM of a CHANNEL or HARBOUR bydredging.

4929 spoil banks. Submerged accumulations of dumped material dredged from CHANNELS or HARBOURS.

4930 spoil ground. A sea area where dredged material is deposited. Also called dumping ground.

4931 spoil ground buoy. A BUOY marking the limit of a SPOIL GROUND.

4932 spot elevation. A point on a MAP or CHART whose ELEVATION is noted. It is usually indicated by a dotaccompanied by a number indicating the vertical distance of the point from the reference DATUM. Spot elevations areused principally to indicate points higher than their surroundings. Also called spot height.

226

4933 spot height. See SPOT ELEVATION.

4934 spout. Phenomenon consisting of an often violent WHIRLWIND, revealed by the presence of a cloud column orinverted cloud cone (funnel cloud), protruding from the base of a cumulonimbus and of a 'bush' composed of waterdrops raised from the surface of the SEA or of DUST, SAND, or litter, raised from the GROUND.

4935 spray. Ensemble of water droplets torn by the WIND from the surface of an extensive body of water, generally from theCRESTS of WAVES, and carried up a short distance into the air.

4936 spring. SPRING TIDE.A natural issue of water or other substances from the EARTH. One on the BOTTOM of the SEA is called a subma-

rine spring.

4937 spring high water. See MEAN HIGH WATER SPRINGS.

4938 spring low water. See MEAN LOW WATER SPRINGS.

4939 spring range. The mean semidiurnal RANGE OF TIDE when SPRING TIDES are occurring. The mean difference inHEIGHT between MEAN HIGH WATER SPRINGS and MEAN LOW WATER SPRINGS. Also called mean springrange.

4940 spring rise. See MEAN SPRING RISE.

4941 spring tide(s). See TIDE.

4942 spur. A subordinate ELEVATION, RIDGE or RISE projecting outward from a larger feature.

4943 squall. Atmospheric phenomenon characterized by a very large variation of wind speed: it begins suddenly, has aduration of the order of minutes, and decreases rather suddenly in SPEED. It is often accompanied by a SHOWER orTHUNDERSTORM.

4944 squall line. Fictitious moving line, sometimes of considerable extent, along which squall phenomena occur.

4945 squares: method of. A method of transferring one SURVEY to another of different SCALE. Also called squaringdown or squaring up depending on whether the SCALE is being decreased or increased.

4946 squaring down. See SQUARES: METHOD OF.

4947 squaring up. See SQUARES: METHOD OF.

4948 squat. For a ship underway, the change of level of the bow and stern from the still water condition in response to theelevation and depression of the water level about the hull resulting from the bow and stern wave systems.

4949 stability. Property of the state of rest or continuous movement of a system such that any disturbance introduced into thisstate decreases. In METEOROLOGY, the term is often used as a synonym of STATIC STABILITY.

4950 stability: static. State of hydrostatic equilibrium of the ATMOSPHERE in which a particle of air moved from its initiallevel undergoes a hydrostatic force which tends to restore it to this level. Also called hydrostatic stability.

4951 stabilization: gyroscopic. See GYROSCOPIC STABILIZATION.

4952 stabilizer: marine. A device installed on ships to reduce rolling.

4953 stadia. A graduated rod used in the determination of distance by observing the intercept on the rod subtending a smallknown angle at the point of observation. In practice the angle is usually defined by two fixed lines in the RETICLE of aTELESCOPE. Also called stadia rod.

4954 stadia constant. The constant which is multiplied by the STADIA INTERVAL to obtain the length of a sight inMETRES.

4955 stadia interval. The length of rod subtended between the top and the bottom CROSS WIRES in the LEVELLINGINSTRUMENT as seen projected against the face of the LEVELLING ROD.

4956 stadia lines. Two or more horizontal wires fixed in the DIAPHRAGM at equal distances from the AXIS ofTELESCOPES, of THEODOLITES and LEVELS. Also called stadia wires.

227

4957 stadia rod. See STADIA.

4958 stadia wires. See STADIA LINES.

4959 stadimeter. An instrument used to measure the distance from the observer to a more-or-less distant object, such as avessel, when the HEIGHT of a specified part of the latter (e.g. the mast) is known. It is in effect a hand-held RANGEFINDER.

4960 stake. An elongated wood or metal pole embedded in the bottom to serve as a marker or support for fish nets.

4961 stand. See STAND OF TIDE.

4962 standard. An exact value (a physical entity or an abstract concept) established and defined by authority, custom, orcommon consent, to serve as a reference, model, or rule in measuring quantities or qualities, establishing practices orprocedures, or evaluating results.

In chart construction, a master copy of a CHART upon which are marked all corrections affecting the CHARTbetween printings (U.S. terminology).

4963 standard atmosphere. See ATMOSPHERE.

4964 standard chronometer. See CHRONOMETER.

4965 standard circle sheet. See CIRCLE SHEET.

4966 standard compass. See COMPASS.

4967 standard depth. See DEPTH.

4968 standard deviation. See ERROR: STANDARD.

4969 standard error. See ERROR.

4970 standard meridian. See MERIDIAN.

4971 standard parallel. See PARALLEL.

4972 standard port. See STATION: REFERENCE.

4973 standard sea water. See NORMAL WATER.

4974 standard station. See STATION: REFERENCE.

4975 standard tape. See REFERENCE TAPE.

4976 standard time. See TIME.

4977 standardization. The comparison of an instrument or device with a STANDARD to determine the value of theinstrument or device in terms of an adopted unit. See CALIBRATION.

4978 standing wave. See WAVE.

4979 stand of tide. The condition at HIGH TIDE or LOW TIDE when there is no change in the HEIGHT of the water. It maybe called high water stand if it occurs at the TIME of HIGH TIDE, and low water stand if it occurs at LOW TIDE.Also called stand.

4980 standpipe. A tall cylindrical structure in a waterworks system, whose height is several times greater than its diameter. Itextends from the ground and may be supported by a skeleton-type framework.

4981 star. CELESTIAL BODIES which appear as points of light due to their great distance from the EARTH. They areself-luminous whereas PLANETS shine only by reflection light.

4982 starboard. The right side of a craft facing forward. The opposite is PORT.

4983 star chart. See CHART.

228

4984 star finder. A device to facilitate the identification of STARS. Sometimes called a star identifier. See PLANISPHERE.

4985 star globe. A small globe representing the CELESTIAL SPHERE, on which the apparent positions of the STARS areindicated. It is usually provided with graduated arcs and a suitable mount for determining the approximate ALTITUDEand AZIMUTH of the STARS, to serve as a STAR FINDER. Also called celestial globe.

4986 star identifier. See STAR FINDER.

4987 starred lines of sounding. LINES OF SOUNDING which are 'starred' round a ROCK or ISLET.

4988 state of sea scale. Numerical SCALE of average WAVE HEIGHT in which the numbers increase with the WAVEHEIGHT.

4989 state of sky. State of the ATMOSPHERE in respect of the amount, genus, height, etc. of the CLOUDS which arepresent.

4990 state of the sea. See SEA STATE.

4991 static instability. See INSTABILITY.

4992 static stability. See STABILITY.

4993 station. Generally, a permanent or temporary location where scientific OBSERVATIONS and measurements are made.In SURVEYING, a point whose POSITION has been (or is to be) determined. A station may be a marked station

(i.e., a point more or less permanently marked for recovery) or an unmarked station, one which is not recoverable. Alsocalled survey station.

4994 station: astronomical. A point on the EARTH whose POSITION has been determined by OBSERVATIONS ofCELESTIAL BODIES.

4995 station: control. See CONTROL.

4996 station: eccentric. See ECCENTRIC STATION.

4997 station: electronic navigation. A STATION emitting radio signals which are used in electronic navigational systems.

4998 station: intersection. In SURVEYING, an object whose horizontal POSITION is determined by OBSERVATIONSfrom other STATIONS, no OBSERVATIONS being made at the object itself. Where the object is observed from onlytwo stations, the position is termed a no check position as there is no proof that such OBSERVATIONS are free fromBLUNDERS. Intersection stations are either objects which would be difficult to OCCUPY with an instrument, orSURVEY SIGNALS whose POSITIONS can be determined with sufficient ACCURACY without being occupied. Alsocalled intersected point.

4999 station: Laplace. A TRIANGULATION or TRAVERSE STATION at which a LAPLACE AZIMUTH is determined.At a Laplace station both ASTRONOMICAL LONGITUDE and ASTRONOMICAL AZIMUTH are determined.

5000 station: 'lost'. See STATION: UNRECOVERABLE.

5001 station: magnetic. The geographic location at which any set of MAGNETIC OBSERVATIONS was taken.

5002 station: main scheme. A STATION through which the main computations and ADJUSTMENTS of the survey data arecarried and serve for the continued extension of the SURVEY.

5003 station: oceanographic. The geographic location at which any set of oceanographic observations are gathered. TheSTATION may be occupied by a mobile PLATFORM (a ship) or a fixed platform such as a tower.

5004 station: recoverable. A STATION artificially marked by well defined natural or artificial objects, whoseGEOGRAPHIC POSITION has been accurately determined and which has been described.

229

5005 station: reference. A place where tide or tidal current constants have been determined from OBSERVATIONS, andwhich is used as a STANDARD for the comparison of simultaneous OBSERVATIONS at a SUBORDINATESTATION. It is also a place for which independent daily predictions are given in the tide or tidal current tables, fromwhich corresponding predictions are obtained for other locations by means of differences or factors. Also calledstandard station and standard port (British terminology).

5006 station: resection. A STATION located by RESECTION methods.

5007 station: satellite. See ECCENTRIC STATION.

5008 station: secondary. See STATION: SUPPLEMENTARY.

5009 station: subordinate. One of the places for which tide or tidal current predictions are determined by applying aCORRECTION to the predictions of a REFERENCE STATION.

A tide or tidal current station at which a short series of OBSERVATIONS was made and reduced by comparisonwith simultaneous OBSERVATIONS at a REFERENCE STATION.

Called secondary port in British terminology. See STATION: TIDE.

5010 station: subsidiary. A STATION established to overcome some local obstacle to the progress of a SURVEY, and notto determine position data for the station point.

5011 station: supplementary. An auxiliary STATION, established to increase the number of CONTROL STATIONS in agiven area, or to place a STATION in a desired location where it is impracticable or unnecessary to establish a principalSTATION. Also called secondary station.

5012 station: survey. See STATION.

5013 station: tide. A place where TIDAL OBSERVATIONS are obtained. It is a primary tide station when continuousOBSERVATIONS are available for a sufficient number of years to determine the characteristic tide features for thelocality. A secondary tide station is operated during a short period of time to obtain DATA for a specific purpose.

5014 station: topographic. A definite point on the EARTH, whose GEOGRAPHIC POSITION has been determined by gra-phic methods, usually by PLANE TABLE TRAVERSE or graphic TRIANGULATION. Also a point located by geode-tic methods but with less than third-order ACCURACY.

5015 station: traverse. A definite point on the EARTH whose POSITION has been determined from another known point bylinear measurements controlled in AZIMUTH by angular measurements.

5016 station: triangulation. A recoverable point on the EARTH, whose GEOGRAPHIC POSITION has been determined byangular methods with geodetic instruments. A triangulation station is a selected point, which has been marked with aSTATION MARK, or it is a conspicuous natural or artificial object. Also called trigonometric station or triangulationpoint.

5017 station: trigonometric. See STATION: TRIANGULATION.

5018 station: unrecoverable. A TRIANGULATION STATION that cannot be found and cannot be included in theTRIANGULATION to connect it with a new DATUM. Sometimes called 'lost' station.

5019 stationary wave. See WAVE: STANDING.

5020 stationary wave theory. A theory which assumes that the basic tide motion in the open OCEAN consists of a system ofSTANDING WAVE oscillations; PROGRESSIVE WAVES are of secondary importance except where the TIDEadvances into tributary waters.

5021 station buoy. A BUOY used to mark the approximate POSITION of an important BUOY or LIGHTSHIP should it becarried away or temporarily removed. Also called MARKER BUOY, watch buoy.

5022 station error. See DEFLECTION OF THE VERTICAL.

5023 station mark (or marker). A buried mark-stone, a metal plug or disk, or any kind of permanent MARK intended toprovide a means of re-identifying main CONTROL STATIONS. See SURFACE MARK andUNDERGROUND MARK.

230

5024 station pointer (British terminology). A metal instrument with three legs, a circle graduated in degrees and 1/2 degrees,and a VERNIER or MICROMETER for setting the angles. See also PROTRACTOR: THREE ARM.

5025 statoscope. A highly sensitive BAROMETER or ALTIMETER for measuring slight variations in pressure orALTITUDE.

5026 statute mile. A unit of distance equal to 1,760 YARDS or 5,280 FEET (1,609.35 m). This MILE is generally used onLAND, and is sometimes called land mile.

5027 steel tape. See TAPE.

5028 steep-to (adj.). Precipitous. The term is applied particularly to a SHORE, BANK or SHOAL that descends steeply to alower level.

5029 steerageway. The condition wherein the ship has sufficient way on that it will respond to rudder movements to maintaindesired COURSE.

5030 steering compass. See COMPASS.

5031 steering repeater. A COMPASS REPEATER by which a craft is steered. See also COMPASS: STEERING.

5032 stellar parallax. See PARALLAX: ANNUAL.

5033 stellite. A trade name for an alloy of chromium, cobalt and tungsten. Stellite mirrors, free of imperfection andpractically indestructible, are frequently used in hydrographic SEXTANTS.

See SEXTANT: SOUNDING.

5034 steradian. The unit of a solid angle in the SI system.

5035 stereocomparator. In PHOTOGRAMMETRY, a stereoscopic instrument for measuring PARALLAX; usuallyincludes a means of measuring PHOTOGRAPH COORDINATES of IMAGE POINTS.

5036 stereocompilation. See COMPILATION.

5037 stereogram. A STEREOSCOPIC PAIR of photographs or drawings correctly oriented and mounted or projected forstereoscopic viewing.

5038 stereographic horizon projection. See PROJECTION: STEREOGRAPHIC.

5039 stereographic meridional projection. See PROJECTION: STEREOGRAPHIC.

5040 stereographic polar projection. See PROJECTION: STEREOGRAPHIC.

5041 stereographic projection. See PROJECTION.

5042 stereometer. A measuring device comprising a micrometer movement by means of which the separation of two INDEXMARKS can be changed to measure PARALLAX DIFFERENCE on a STEREOSCOPIC PAIR of photographs. Alsocalled parallax bar.

5043 stereometric camera. See CAMERA.

5044 stereomodel. See STEREOSCOPIC IMAGE.

5045 stereopair. See STEREOSCOPIC PAIR.

5046 stereophotogrammetry. PHOTOGRAMMETRY utilizing stereoscopic equipment and methods.

5047 stereoscope. A binocular optical instrument for helping an observer to view PHOTOGRAPHS, or DIAGRAMS, toobtain the mental impression of a three-dimensional model.

5048 stereoscopic fusion. The mental process which combines the two perspective views to give an impression of a three-dimensional model.

5049 stereoscopic image. The mental impression of a three-dimensional model which results from viewing two overlappingperspective views. Also called stereoscopic model or stereomodel.

231

5050 stereoscopic model. See STEREOSCOPIC IMAGE.

5051 stereoscopic pair. In PHOTOGRAMMETRY, two PHOTOGRAPHS of the same area taken from different AIRSTATIONS so as to afford STEREOSCOPIC VISION. Frequently called a stereopair.

5052 stereoscopic plotting instrument. An instrument for compiling a MAP or obtaining spatial solutions by observation ofSTEREOSCOPIC IMAGES formed by STEREOSCOPIC PAIRS. See COMPILATION.

5053 stereoscopic vision. The particular application of BINOCULAR VISION which enables the observer to obtain theimpression of depth, usually by means of two different perspectives of an object, as two PHOTOGRAPHS taken fromdifferent AIR STATIONS.

5054 stereoscopy. The science and art which deals with the use of BINOCULAR VISION for observation of a pair ofoverlapping PHOTOGRAPHS or other perspective views, and with the methods by which such viewing is produced.

5055 stereotriangulation. A triangulation procedure that uses a STEREOSCOPIC PLOTTING INSTRUMENT to obtain thesuccessive ORIENTATION of the STEREOSCOPIC PAIRS of photographs into a continuous strip. The spatial solu-tion for the extension of horizontal and/or vertical CONTROL using these STRIP COORDINATES may be obtained byeither graphical or computational procedures. Also called bridging.

5056 sternway. Making way through the water in a direction opposite to the HEADING.

5057 still water level. The LEVEL that the sea surface would assume in the absence of WIND WAVES; not to be confusedwith MEAN SEA LEVEL or HALF-TIDE LEVEL.

5058 stippling. Graduation of SHADING by numerous separate touches. Shallow areas on CHARTS, for instance, aresometimes indicated by numerous dots decreasing in density as the DEPTH increases.

5059 stone. A general term for ROCK and rock fragments ranging in size from PEBBLES and GRAVEL to BOULDERS orlarge rock masses.

5060 stooping. Apparent decrease in the vertical dimension of an object near the HORIZON, due to large inequality ofATMOSPHERIC REFRACTION in the LINE OF SIGHT to the top and bottom of the object. The opposite is TOWE-RING.

5061 stop. See APERTURE STOP.

5062 stop watch. See WATCH.

5063 storage. In a COMPUTER, a device in which DATA can be inserted and preserved, and from which DATA can beretrieved. Common storage devices are magnetic tapes, drums, disks and cores.

5064 storm. WIND with a SPEED between 44 and 50 KNOTS (Beaufort scale wind force 10).NOTE: The term 'storm' has been substituted for the term 'whole gale' in U.S. Weather Bureau Warnings. This term

refers to wind speeds of 48-63 KNOTS.

5065 storm high water. See HIGH WATER.

5066 storm surge. A RISE above normal water level on the OPEN COAST due only to the action of WIND stress on thewater surface. Storm surge resulting from a HURRICANE or other intense STORM also includes the RISE in level dueto ATMOSPHERIC PRESSURE reduction as well as that due to wind stress. A storm surge is more severe when itoccurs in conjunction with a HIGH TIDE. Also called storm tide, storm wave, tidal wave.

5067 storm tide. See STORM SURGE.

5068 storm warning. See WARNING.

5069 storm wave. See WAVE.

5070 straight-edge. A bar of wood, metal, etc. with one edge accurately straight for drawing straight lines in cartographicapplications.

5071 straight line. Mathematically the line of shortest distance between two points in a specified space or on a specifiedsurface.

232

5072 strait (also straits). A PASSAGE connecting two larger bodies of water.

5073 straits used for international navigation. Geographically, a narrow passage between two land masses or islands orgroup of islands connecting two larger sea areas.

5074 strand. The portion of the SEASHORE between high and low water line.

5075 strand (v.t. & i.). To run aground. The term strand usually refers to a serious grounding, while the term GROUNDrefers to any grounding, however slight.

5076 stranded. 1. The terms "stranded" and "sunken" apply exclusively to items that once were afloat but which are nowresting on the bottom. Stranded items project above the sounding datum while sunken items do not project above thesounding datum. These terms apply most often to wrecks. Masts, funnels, and other extensions of wreck superstructureshould be disregarded when applying the above definition; i.e., such features may project above the sounding datum andstill have the wreck classified as sunken.

2. The grounding of a vessel so that it is not soon refloated; a serious grounding.

5077 strath. A broad elongated DEPRESSION with relatively steep walls located on a CONTINENTAL SHELF. The longi-tudinal profile of the floor is gently undulating with greatest DEPTHS often found in the inshore portion.

5078 stratification. The state of a fluid that consists of two or more horizontal layers arranged according to their density.

5079 stratocumulus. See CLOUD GENERA.

5080 stratopause. Top of the INVERSION LAYER in the upper STRATOSPHERE, at about 50-55 km.

5081 stratosphere. Region of the ATMOSPHERE, situated between the TROPOPAUSE and the STRATOPAUSE, in whichthe temperature generally increases with HEIGHT.

From an imperfect analogy with the ATMOSPHERE, the term is sometimes applied in OCEANOGRAPHY to thenearly uniform masses of cold, deep and bottom water of middle and low LATITUDES. This LAYER is separated fromthe TROPOSPHERE by the THERMOCLINE.

5082 stratum (pl. strata). A single sedimentary bed or LAYER of generally homogenous ROCK, independent of thickness.

5083 stratus. See CLOUD GENERA.

5084 strays. In ECHO SOUNDING, false indications occasionally appearing on the DIAL or FATHOGRAM of an ECHOSOUNDER. They may be caused by the motion of the vessel through the water, by acoustic or electric NOISES in theship, or by electric NOISES in the echo sounding equipment. Strays may be mistaken for the true echo or may be ofsuch a nature as to prevent the identification of the true echo.

5085 stream. Any RIVER, BROOK, RIVULET or course of running water.A steady CURRENT in the SEA or in a RIVER, especially the middle or most rapid part of a TIDE or CURRENT.A steady FLOW of a fluid, small solid particles, or RADIANT ENERGY.

5086 stream (v.t.). To place overboard and secure, as to stream a LOG, or stream a sea anchor.

5087 stream: ebb. See EBB STREAM.

5088 stream: flood. See FLOOD STREAM.

5089 stream: ingoing. See FLOOD STREAM.

5090 stream: outgoing. See EBB STREAM.

5091 stream: rectilinear. See CURRENT: REVERSING.

5092 stream: rotary. See CURRENT: ROTARY.

5093 stream: tidal. See CURRENT: TIDAL.

5094 stream current. See CURRENT.

233

5095 streamer. A string of hydrophones towed behind a ship.

5096 streamline. A LINE OF FLOW.In METEOROLOGY, line envelope in space of the tangents to the instantaneous wind directions at a given time.

Also written as two words.See also TRAJECTORY.

5097 streamline flow. See LAMINAR FLOW.

5098 strength of current. The PHASE of a TIDAL CURRENT at which the SPEED is a maximum; also the SPEED at thistime.

5099 strength of figure. In GEODESY, the comparative PRECISION of computed lengths in a TRIANGULATION NET asdetermined by the size of the angles, the number of conditions to be satisfied, and the distribution of BASES and pointsof fixed POSITION.

5100 striding level. See LEVEL.

5101 strip. In ice terminology, a long narrow area of PACK ICE, about 1 km or less in width, usually composed of smallfragments detached from the main mass of ICE, and run together under the influence of WIND, SWELL or CURRENT.

5102 strip coordinates. See COORDINATES.

5103 strip film (or stripping film). A film which has the emulsion coated on an extremely thin membrane which is in turnsupported on a normal-thickness film base. The emulsion and the membrane can be stripped (separated) from the baseafter exposure and processing. The thin image so obtained can be mounted face down on a new support for lateral imagereversal or used for combining with, or inserting in, other image elements.

5104 stripes (of colour). Areas of contrasting colour separated from one another by straight lines, and used as distinguishingcharacteristic for navigation marks. In British terminology horizontal stripes are denoted as bands.

5105 strong gale. Wind with a speed between 41 and 47 KNOTS (Beaufort scale wind force 9).

5106 structure. The term "structure" shall include, without limitation, any pier, wharf, dolphin, weir, boom, breakwater,bulkhead, revetment, riprap, jetty, permanent mooring structure, power transmission line, permanently moored floatingvessel, piling, aid to navigation, or any other obstacle or obstruction.

5107 stylus. A pointer that is operated by placing it in a display space or a tablet.

5108 subastral point. See SUBSTELLAR POINT.

5109 sub-bottom profiler. A form of active, low frequency sonar in which acoustic beams penetrate the BOTTOM. Arecorder produces a chart which represents a cross section of the geological structure of the subbottom. See also: ECHOSOUNDER.

5110 sub-celestial point. See SUB-POINT.

5111 sublunar point. The GEOGRAPHICAL POSITION of the MOON. That point on the surface of the EARTH at whichthe MOON is in the ZENITH at a specified TIME. See SUB-POINT.

5112 submarine bell. A bell whose SIGNAL is transmitted through water.

5113 submarine cable. See CABLE.

5114 submarine canyon. See CANYON.

5115 submarine geology. See GEOLOGY.

5116 submarine navigation. See NAVIGATION.

5117 submarine oscillator. See OSCILLATOR.

5118 submarine peninsula. See PENINSULA.

234

5119 submarine pipeline. See PIPELINE.

5120 submarine relief. See RELIEF.

5121 submarine ridge. An elongated elevation of the sea floor, with either irregular or relatively smooth topography andsteep sides which constitutes a natural prolongation of land territory.

5122 submarine sentry. A form of underwater KITE towed at a predetermined constant DEPTH in search of ELEVATIONSof the BOTTOM. The KITE rises to the surface upon encountering an OBSTRUCTION.

5123 submarine sound channel. A water layer with minimum sound velocity and high concentration of sound energy inwhich sound waves are propagated without reflection from sea surface or bottom.

5124 submarine sound signal. See SOUND SIGNAL.

5125 submarine spring. See SPRING.

5126 submarine valley. See VALLEY.

5127 submarine volcano. A seabed volcano, submerged at the chart sounding datum, which may or may not be active.

5128 submerged (adj.). Under water; not showing above water. The opposite is UNCOVERED.

5129 submerged rock. A rock covered at the chart sounding datum and considered to be potentially dangerous to navigation.

5130 subordinate station. See STATION.

5131 subpermanent magnetism. See MAGNETISM: PERMANENT.

5132 subplan. An INSET on a SURVEY SHEET used to extend the survey coverage shown on the sheet, or to show smallcongested areas at enlarged SCALES.

5133 sub-point. That point on the surface of the EARTH at which a particular CELESTIAL BODY is in the ZENITH at aspecified TIME. Also called 'geographical position of a celestial body'. Called sub-celestial point in Canadianterminology. See SUBLUNAR POINT, SUBSOLAR POINT, SUBSTELLAR POINT.

5134 sub-sea completion. See WELL: PRODUCTION.

5135 subsidiary station. See STATION.

5136 subsoil. All naturally occurring matter lying beneath the sea-bed or deep ocean floor.

5137 subsolar point. The GEOGRAPHICAL POSITION of the SUN; that point on the EARTH at which the SUN is in theZENITH at a specified TIME.

5138 subsonic (adj.). Designating or of SPEEDS that are less than that of SOUND.

5139 substellar point. The GEOGRAPHICAL POSITION of a STAR; that point on the surface of the EARTH at which theSTAR is in the ZENITH at a specified TIME. Also called subastral point.

5140 substitute centre. A point which, because of its ease of identification on overlapping PHOTOGRAPHS, is used insteadof the PRINCIPAL POINT as a RADIAL CENTRE.

5141 substitute map. See MAP.

5142 subsurface current. See CURRENT.

5143 subsurface mark. See UNDERGROUND MARK.

5144 subsurface sound channel. The SUBMARINE SOUND CHANNEL which is adjacent to the water surface.

5145 subtense bar. A horizontally held bar of precisely determined length, used to measure distances by observing the angleit subtends at the distance to be measured.

235

5146 subtense method. A procedure by which distance measurements are obtained by use of a SUBTENSE BAR.

5147 sue. The vertical amount which a vessel aground dries due to the fall of the TIDE.

5148 sugar loaf. A descriptive term for a conical HILL.

5149 sugar loaf sea. Waves that rise into sugar loaf shapes, with little WIND, possibly resulting from intersecting WAVES.

5150 summer solstice. See SOLSTICE.

5151 summer time. See TIME: DAYLIGHT SAVING.

5152 summit. The highest point, part or ELEVATION; top or APEX.

5153 Sumner line. A CELESTIAL LINE OF POSITION, particularly one established by the SUMNER METHOD. Namedafter Captain Thomas H. Sumner, the discoverer of the LINE OF POSITION by celestial observation.

5154 Sumner method. The establishing of a LINE OF POSITION from the OBSERVATION of the ALTITUDE of aCELESTIAL BODY by assuming two LATITUDES (or LONGITUDES) and calculating the LONGITUDES (orLATITUDES) through which the LINE OF POSITION passes.

5155 sun. The luminous CELESTIAL BODY at the centre of the SOLAR SYSTEM, around which other CELESTIALBODIES revolve.

5156 sun: apparent. The actual SUN as it appears in the sky. Also called true sun.

5157 sun: mean. A fictitious SUN conceived to move eastward along the CELESTIAL EQUATOR at a rate that provides auniform measure of TIME equal to the average APPARENT TIME. It is used as a reference for reckoning MEANTIME, ZONE TIME, etc.

5158 sun: true. See SUN: APPARENT.

5159 sun compass. See COMPASS.

5160 sundial. An instrument that indicates TIME by the position of the shadow of a pointer or GNOMON cast by the SUNon the face of a DIAL marked in HOURS.

5161 sunken rock. A ROCK potentially dangerous to SURFACE NAVIGATION, the SUMMIT of which is below the lowerlimit of the zone for ROCK AWASH.

5162 sunrise. The crossing of the APPARENT HORIZON by the upper LIMB of the ascending SUN.

5163 sunset. The crossing of the APPARENT HORIZON by the upper LIMB of the descending SUN.

5164 sunshine recorder. See HELIOGRAPH.

5165 sunspot. Any of the dark spots sometimes seen at the surface of the SUN; they are believed to have some connectionwith magnetic disturbances on EARTH.

5166 sunspot cycle. The time interval (11.2 YEARS) during which the number of SUNSPOTS progresses from a minimumto a maximum and decreases again to a minimum.

5167 super high frequency (SHF). See FREQUENCY: RADIO.

5168 superior conjunction. See CONJUNCTION.

5169 superior mirage. See MIRAGE.

5170 superior planet(s). See PLANET(S): SUPERIOR.

5171 superior transit. See MERIDIAN TRANSIT.

5172 superjacent waters. The waters lying immediately above the sea-bed or deep ocean floor up to the surface.

236

5173 super-short baseline acoustic system. A variation of the short baseline system with three sensors incorporated in asingle transducer.

5174 supersonic (adj.). Designating or of a SPEED greater than the SPEED of SOUND.

5175 supplemental control. See CONTROL.

5176 supplemental control point. A photoimage point for which an ELEVATION or a horizontal POSITION, or both, is tobe, or has been determined. See also CONTROL POINT.

5177 supplementary station. See STATION.

5178 surf. Collective term for BREAKERS.The wave activity in the area between the SHORELINE and the outermost limit of BREAKERS.

5179 surface boundary layer. See GROUND LAYER.

5180 surface current. See CURRENT.

5181 surface duct. A zone immediately below the sea surface where SOUND RAYS are refracted toward the surface andthen reflected. They are refracted because the SOUND VELOCITY at some DEPTH near the surface is greater than atthe surface. The RAYS alternately are refracted and reflected along the duct to considerable distances from the soundsource.

5182 surface mark. A STATION MARK used in addition to the UNDERGROUND MARK, over which it is plumbed.

5183 surface navigation. See NAVIGATION.

5184 surface of position. A surface on some point of which a craft is located.

5185 surface reflection. The return of SOUND RAYS to depth after striking the sea surface.

5186 surface temperature. Temperature of the water of the surface layer of the SEA (or LAKE, RIVER, etc.).

5187 surface wave. See WAVE.

5188 surf zone. The area between the outermost BREAKER and the limit of wave UPRUSH.

5189 surge. A ship's bodily motion forward and backward along the longitudinal axis, caused by the force of the sea actingalternately on the bow and stern.

The name applied to wave motion with a period intermediate between that of the ordinary WIND WAVE and that ofthe TIDE, from about 1/2 to 60 minutes.Horizontal oscillation of water with comparative short period accompanying a SEICHE.

5190 surge (v.i.). To rise and fall, as a vessel at anchor.To vary abruptly and momentarily in amount, as an electric current in a circuit.

5191 surge: negative. An occasion when the level of the sea is lower than the predicted level. It is caused by meteorologicalconditions.

5192 surge: positive. An occasion when the level of the sea is higher than the predicted level. It is caused by meteorologicalconditions.

5193 surging breaker. See BREAKER.

5194 surround. See MARGIN.

5195 survey. The orderly process of determining DATA relating to the physical or chemical characteristics of the EARTH.The act or operation of making measurements for determining the RELATIVE POSITION of points on, above or

beneath the EARTH's surface.The result of such operations.An organization for making surveys.

237

5196 survey: aerial. A SURVEY using AERIAL PHOTOGRAPHS as part of the surveying operation; also the taking ofAERIAL PHOTOGRAPHS for surveying purposes.

5197 survey: cadastral. A SURVEY relating to land boundaries and subdivisions, made to create units suitable for transferor to define the limitations of title. Also called land survey.

5198 survey: coastal. HYDROGRAPHIC SURVEY of coastal area including COASTLINING.

5199 survey: exploratory. A SURVEY executed for the purpose of obtaining general information concerning areas aboutwhich such information was not, previously, a matter of record.

5200 survey: geodetic. A SURVEY in which the figure and size of the EARTH is considered. It is applicable for large areasand long lines and is used for the precise location of basic points suitable for controlling other SURVEYS.

5201 survey: geologic(al). A SURVEY or investigation of the character and structure of the EARTH, of the physical changeswhich the EARTH'S CRUST has undergone or is undergoing, and of the causes producing those changes.

5202 survey: graphic control. A skeleton SURVEY made by PLANE TABLE for the purpose of (a) locating theHYDROGRAPHIC SIGNALS, (b) locating additional CONTROL from AERIAL SURVEYS and clarification ofindefinite detail on PHOTOGRAPHS, of (c) a combination of the two.

5203 survey: gravimetric. A SURVEY made to determine the ACCELERATION OF GRAVITY at various places on theEARTH's surface.

5204 survey: ground. A SURVEY made by ground methods, as distinguished from an AERIAL SURVEY. A ground surveymay or may not include the use of PHOTOGRAPHS.

5205 survey: harbour. HYDROGRAPHIC SURVEY of a harbour area.

5206 survey: hydrographic. A SURVEY having for its principal purpose the determination of DATA relating to bodies ofwater. A hydrographic survey may consist of the determination of one or several of the following classes of DATA:DEPTH of water; configuration and NATURE OF THE BOTTOM; directions and force of CURRENTS; HEIGHTSand TIMES of TIDES and water stages; and location of topographic features and fixed objects for survey and navigationpurposes.

5207 survey: land. See SURVEY: CADASTRAL.

5208 survey: large scale. A HYDROGRAPHIC SURVEY at a LARGE SCALE. Large scale surveys are usually intended tofurnish detailed information for dredging, or other types of harbour improvement.

5209 survey: magnetic. A SURVEY conducted to measure the strength and direction of the EARTH'S MAGNETIC FIELDat specific points on or near the surface of the EARTH.

5210 survey: magnetometer. A SURVEY wherein the EARTH'S MAGNETIC FIELD is mapped by the use of aMAGNETOMETER.

5211 survey: new. See RESURVEY.

5212 survey: oceanographic. A study or examination of any physical, chemical, biological, geological or geophysicalcondition in the OCEAN, or any part of it.

An expedition to gather DATA, samples or information to conduct such studies or examination.

5213 survey: offshore. HYDROGRAPHIC SURVEYS conducted in areas beyond a certain distance from the shore, e.g. inthe HIGH SEAS.

5214 survey: photogrammetric. A SURVEY utilizing either TERRESTRIAL or AERIAL PHOTOGRAPHS.

5215 survey: preliminary. See SURVEY: RECONNAISSANCE.

5216 survey: reconnaissance. A hasty preliminary SURVEY of a region made to provide some advance information regar-ding the area which may be useful, pending the execution of more complete SURVEYS. Also called preliminarysurvey.

238

5217 survey: revisory. Hydrographic surveys conducted for the revision of charts including checking depths, aids tonavigation, existence of land and water features, and any changes or additions such as new features constructed that areimportant to the mariner. A revisory survey is not considered to be a detailed resurvey.

5218 survey: running. A HYDROGRAPHIC SURVEY in which the greater part of the work is done from the ship steamingalong the COAST. A LINE OF SOUNDING is run on the COURSE MADE GOOD and any additional information of ahydrographic nature which can be obtained whilst under way, is collected.

5219 survey: sketch. A HYDROGRAPHIC SURVEY made (due to lack of time or facilities) to a lower degree ofACCURACY and detail than the chosen SCALE would normally indicate.

5220 survey: sonar. HYDROGRAPHIC SURVEY using a SONAR.

5221 survey: tagline. A LARGE SCALE SURVEY made by employing a TAGLINE. An accurate method sometimesadopted in river and harbour work.

5222 survey: topographic. A SURVEY which has for its major purpose the determination of the configuration (RELIEF) ofthe surface of the EARTH and the location of natural and artificial objects thereon.

5223 survey: wire-drag. A HYDROGRAPHIC SURVEY made utilizing a WIRE DRAG. In areas of rocky BOTTOM orwhere submerged obstacles such as WRECKS are present, a wire-drag survey represents the most practical way ofmaking sure that all OBSTRUCTIONS or dangers have been found and least DEPTHS over them obtained. Also calledwire drag sweep.

5224 survey buoy. See BUOY.

5225 survey boat. A boat used for hydrographic surveys.

5226 surveying. Specifically, the science or art of making such measurements as are necessary to determine the RELATIVEPOSITION of points above, on, or beneath the surface of the EARTH, or to establish such points. Generally, the art ofmaking a SURVEY.

5227 surveying: plane. A branch of the art of SURVEYING in which the surface of the EARTH is considered a planesurface. In plane surveying, curvature of the EARTH is neglected, and computations are made using the formulae ofplane geometry and plane trigonometry.

5228 surveying (or mapping) camera. See CAMERA.

5229 surveying sextant. See SEXTANT: SOUNDING.

5230 survey mark. An object placed at the site of a station to identify the surveyed location of that station. In particular, anobject whose coordinates are used for control in a geodetic network. Also called MARKER, geodetic marker, andMONUMENT.

5231 survey net. See LEVEL NET, TRIANGULATION NET.

5232 surveyor. A person engaged in SURVEYING.

5233 surveyor's compass. See COMPASS.

5234 survey sheet. See HYDROGRAPHIC SURVEY SHEET.

5235 survey (or surveying) ship (or vessel). A ship or vessel specially equipped for carrying out HYDROGRAPHIC and/orOCEANOGRAPHIC SURVEY.

5236 survey signal. See SIGNAL.

5237 survey station. See STATION.

5238 survey traverse. See TRAVERSE.

5239 Swallow float. A tubular BUOY, usually made of aluminium, that can be adjusted to remain at a selected density levelto drift with the motion of that water mass. The FLOAT is tracked by shipboard listening devices and current velocitiescan be determined.

239

5240 swamp. An area of spongy land saturated with water. It may have a shallow covering of water, usually with aconsiderable amount of vegetation appearing above the surface.

5241 swash. A narrow CHANNEL or SOUND within a sand bank, or between a sand bank and the SHORE. Also calledswashway.

A BAR over which the SEA washes.The rush of water up onto a BEACH following the breaking of a WAVE.

5242 swash channel. A CHANNEL across a BANK, or among SHOALS.

5243 swash mark. The thin wavy line of fine SAND, mica scales, bits of SEAWEED, etc., left by the UPRUSH when itrecedes from its upward limit of movement on the BEACH.

5244 swashway. See SWASH.

5245 swath(e). The strip or lane on the ground scanned by a multi-beam sounder when the survey vessel proceeds along itscourse.

5246 swath(e) sounding system. A multi-beam system which is capable of obtaining a lane of soundings from a single ship'strack.

5247 sway. The side-to-side bodily motion of a ship, independent of ROLLING, caused by uniform pressure being exerted allalong one side of the hull.

5248 sweep (v.t. & i.). To tow a line or object below the surface, to determine the least depth in an area or to insure that agiven area is free from navigational dangers to a certain depth; or the removal of such dangers.

5249 sweeping. The process of towing a line or object below the surface, to determine whether an area is free from isolatedsubmerged dangers to vessels and to determine the position of any dangers that exist, or to determine the least depth ofan area.

5250 swell. Ocean waves which have travelled out of their generating area. Swell characteristically exhibits a more regularand longer PERIOD and has flatter CRESTS than WAVES within their FETCH.

Rising of the water of a RIVER above its usual LEVEL.Gently rising GROUND or a rounded HILL above the surrounding GROUND.

5251 swing. In PHOTOGRAMMETRY, a rotation of a PHOTOGRAPH in its own plane about its camera axis.

5252 swinger. See FIX: CIRCULAR.

5253 swinging buoy. See COMPASS ADJUSTMENT BUOY.

5254 swinging ship. The process of placing a vessel on various HEADINGS and comparing MAGNETIC COMPASS rea-dings with the corresponding magnetic DIRECTIONS, to determine DEVIATION. This usually follows COMPASSCOMPENSATION (or ADJUSTMENT), and is done to obtain information for making a DEVIATION TABLE. Alsocalled compass calibration, or calibration of compass. See COMPENSATION OF MAGNETIC COMPASS.

5255 swirl error. See ERROR.

5256 symbol. A CHARACTER, letter, or similar graphic representation used on a CHART, MAP, HYDROGRAPHICSURVEY SHEET, or DIAGRAM to represent some object, quantity, characteristic, etc.

5257 synodical month. See MONTH.

5258 synodic (or synodical) period. The interval of time between any planetary configuration of a CELESTIAL BODY,with respect to the SUN, and the next successive same configuration of that body, as from INFERIOR CONJUNCTIONto INFERIOR CONJUNCTION.

5259 synoptic analysis. Sea WEATHER ANALYSIS.

5260 synoptic chart (or map). See CHART (or MAP): WEATHER.

5261 synoptic forecast. WEATHER FORECAST based on SYNOPTIC OBSERVATION.

240

5262 synoptic hour. Hour, expressed in terms of UT, at which, by international agreement, METEOROLOGICALOBSERVATIONS are made simultaneously throughout the globe.

5263 synoptic meteorology. Branch of METEOROLOGY which is concerned with the study of METEOROLOGICALELEMENTS in space. It is based on the analysis of the SYNOPTIC CHARTS on which are plotted SYNOPTICOBSERVATIONS for the purpose of the WEATHER ANALYSIS or WEATHER FORECAST.

5264 synoptic observation. METEOROLOGICAL OBSERVATION made at the same time (SYNOPTIC HOUR) atnumerous stations to obtain general representation of the state of the atmosphere at the given time.

5265 synthetic aperture radar (SAR). A RADAR with a synthetic aperture antenna which is composed of a large numberof elementary transducing elements. The signals are electronically combined into a resulting signal equivalent to that ofa single antenna of a given aperture in a given direction.

5266 syntony. The situation of two or more oscillating circuits having the same RESONANT FREQUENCY.

5267 systematic error. See ERROR.

5268 system development. The development of a program system comprising analysis, design, programming, testing, andimplementation.

5269 systems of sounding lines. The predetermined lines that the SURVEY VESSEL is to follow for the best developmentof the DEPTH CONTOURS in an area.

5270 Système International (SI System). System of metric measures consisting of seven basic units which are METRE,KILOGRAMME, SECOND, AMPERE, Kelvin, MOLE and CANDELA.

5271 syzygy. In ASTRONOMY, either of two opposing points in the ORBIT of a PLANET or SATELLITE, especially ofthe MOON, at which it is in CONJUNCTION with or in OPPOSITION to the SUN.

5272 syzygy tide. See TIDE.

241

T

5273 table. An orderly, condensed arrangement of numerical or other information, usually in parallel vertical columns. SeeCONVERSION TABLE, CURRENT TABLES, TIDE TABLES, TRAVERSE TABLE.

5274 tableland. An elevated region of LAND with a generally level surface of large or considerable extent; a lofty PLAIN; aPLATEAU.

5275 tablemount. A SEAMOUNT having a comparatively smooth flat top. Also called guyot.

5276 tabular berg. A flat-topped ICEBERG. Most tabular bergs form by breaking from an ICE SHELF and show horizontalbanding.

5277 tabulation. Orderly arrangement as in a TABLE.

5278 tachometer. A device that indicates or measures the revolutions per MINUTE of a revolving shaft or the VELOCITYof a machine.

5279 tachymeter (or tacheometer). A surveying instrument designed for use in the rapid determination of distance,DIRECTION, and difference of ELEVATION from a single OBSERVATION, using a short base which may be anintegral part of the instrument.

5280 tachymetry. A method of SURVEYING for the rapid determination of distance, DIRECTION, and relative ELEVATI-ON of a point with respect to the instrument station by a single OBSERVATION on a rod or other object at the point.The stadia method of SURVEYING is an example of tachymetry.

5281 tach stave (levelling staff). A staff with a length of about 2m with subdivisions. It is used in combination with alevelling instrument or theodolite.

5282 taffrail. The after rail at the stern of a vessel.

5283 taffrail log. See LOG.

5284 tagline. A line, either marked at equal intervals or run over a registered sheave, used in LARGE-SCALE SURVEYS totake equally spaced SOUNDINGS at predetermined distances from the CONTROL STATIONS.

5285 tagline survey. See SURVEY.

5286 tail wind. WIND which blows in the same direction as that in which an object is moving, with respect to the EARTH'ssurface.

5287 talus. A SLOPE. A sloping mass of DETRITUS lying at the base of a CLIFF or the like, and consisting of materialwhich has fallen from its face; also the SLOPE or inclination of the surface of such a mass.

5288 talweg (or thalweg). The line joining the lowest points of a VALLEY throughout its length. Sometimes called valleyline.

5289 tandem control. In HYDROGRAPHIC SURVEY, a system of obtaining two or more adjacent SOUNDING LINESrun simultaneously and controlled by one ship, the other ships, or LAUNCHES being merely referenced to it.

5290 tank. A fixed structure for storing liquids.

5291 tangent screw. A slow motion screw of a surveying instrument or SEXTANT.

5292 tape. In SURVEYING, a ribbon of steel, INVAR, or other suitable material on which GRADUATIONS are placed forthe measurement of lengths or distances. See also BASE TAPE (or WIRE).

5293 tape: alignment correction. A CORRECTION applied to the measured length of a line to allow for the TAPE notbeing held exactly in a vertical plane containing the line.

5294 tape: base. See BASE TAPE (or WIRE).

5295 tape: catenary correction. See TAPE: SAG CORRECTION.

242

5296 tape: grade correction. A CORRECTION applied to a distance measured on a SLOPE to reduce it to a horizontaldistance between the vertical lines through its end points. Also termed slope correction or correction for inclination oftape.

5297 tape: invar. Any survey TAPE made of INVAR.

5298 tape: length correction. The difference between the nominal length of a TAPE and its effective length under conditionsof STANDARDIZATION.

5299 tape: reference. See REFERENCE TAPE.

5300 tape: sag correction. The difference between the effective length of a TAPE (or part of a TAPE) when supportedcontinuously throughout its length and when supported at a limited number of independent points. Also called catenarycorrection.

5301 tape: standard. See REFERENCE TAPE.

5302 tape: steel. Any survey TAPE made of steel.

5303 tape: temperature correction. The quantity applied to the nominal length of a TAPE to allow for a change in itseffective length because of its being used at a temperature other than that for which its standard length is given.

5304 tape: tension correction. The CORRECTION applied to the nominal length of a TAPE to allow for a change ineffective length because of its being used at a tension other than that for which its standard length is known.

5305 tape correction(s). CORRECTIONS applied to a distance measured with a TAPE to eliminate ERRORS caused by thephysical condition of the TAPE, and to the manner in which it is used.

5306 tape stretcher. A mechanical device which facilitates holding a TAPE at a prescribed tension and in a prescribedposition.

5307 tape thermometer. See under THERMOMETER.

5308 taping. The operation of measuring a distance on the GROUND using a TAPE or ribbon of metal or other material.Also called chaining.

5309 target. Any object, point, etc., toward which something is directed.An object which reflects a sufficient amount of a radiated SIGNAL to produce an echo signal on detection

equipment. See RADAR TARGET, SONAR TARGET.The distinctive marking or instrumentation of a ground point to aid in its identification on a PHOTOGRAPH. In

PHOTOGRAMMETRY, target designates a material marking so arranged and placed on the GROUND as to form adistinctive pattern over a geodetic or other control point marker, on a property corner or line, or at the position of anidentifying point above an underground facility or feature. A target is also the image pattern on AERIALPHOTOGRAPHS of the actual MARK placed on the GROUND prior to PHOTOGRAPHY.

5310 target: false. See ECHO: FALSE.

5311 tartan plots. LATTICES resulting from the enlargement of small parts of Two Range and hyperbolic LATTICES, usedfor work which has to be carried out on a SCALE larger than that being used for the main SURVEY.

5312 taut wire measuring gear. An apparatus designed for measuring horizontal distances at sea. It is constructed so that awire of small diameter, one end of which is anchored on the BOTTOM, passes around a calibrated sheave as the shipsteams along the route to be measured.

5313 taut wire mooring. A mooring arrangement in which a submerged FLOAT provides the upward force necessary tomaintain the system in a fixed position with reference to the sea bottom.

5314 tectonics. The study of origin and development of the broad structural features of the EARTH.

5315 telecommunication. Any transmission, emission, or reception of signs, SIGNALS, writing, IMAGES and SOUNDS orintelligence of any nature by wire, RADIO, visual or other electromagnetic systems.

5316 telegraph buoy. A BUOY used to mark the position of a submarine telegraph cable.

5317 telemeteorograph. General term for meteorological instruments whose indications are recorded at a distance.

243

5318 telemeter. An instrument for determining the distance from one point to another. Some such instruments employ aTELESCOPE and measure the angle subtended by a short base of known length.

The complete equipment for measuring any quantity, transmitting the results electrically to a distance point, andthere recording the values measured.

5319 telemeter: electronic. An electronic device that measures the phase difference or transit time between a transmittedelectromagnetic IMPULSE of known FREQUENCY and SPEED and its return.

5320 telemetry. The study and technique involved in measuring a quantity or quantities in place, transmitting this value to astation, and there interpreting, indicating, or recording the quantities; and transmitting commands from the station tocontrol the measurement device.

5321 telemotor. A hydraulic or electrical device controlling the action of power at a distance, specifically, a device whichcontrols the steering gear of a ship.

5322 telescope. An optical instrument used as an aid in viewing or photographing distant objects, particularly celestialobjects.

5323 telescope: meridian. A TELESCOPE whose motion along a VERTICAL CIRCLE is intended to lie in the plane of theMERIDIAN.

5324 telescope: photo zenithal. An instrument designed to take photos of stars culminating close to the zenith of a station inorder to determine the position of the station with high accuracy.

5325 telescope: reflecting. A TELESCOPE which collects light by means of a concave MIRROR.

5326 telescope: refracting. A TELESCOPE which collects light by means of a LENS or system of LENSES.

5327 telescope: zenith. An astronomical instrument for determining the LATITUDE by observing ZENITH DISTANCES oftwo STARS that culminate as close as possible in TIME, and at nearly the same ALTITUDE on opposite sides of theZENITH.

5328 telescopic alidade. See ALIDADE.

5329 telltale compass. See COMPASS.

5330 telluric current(s). See CURRENT: TELLURIC.

5331 temperature. The intensity or degree of heat.

5332 temperature correction (levelling). That CORRECTION which is applied to an observed difference of ELEVATIONto correct for the ERROR introduced when the temperature at which the LEVELLING RODS are used in the field isdifferent from the temperature at which they were standardized.

5333 temperature correction (tape). See TAPE: TEMPERATURE CORRECTION.

5334 temperature inversion. In OCEANOGRAPHY, a LAYER in which temperature increases with DEPTH. Also calledinversion layer.

5335 temperature lapse rate. Rate of decrease of temperature with increasing HEIGHT; usually an average rate over adistance, for example 100 METRES, is considered.

5336 temperature-salinity diagram. The plot of temperature versus SALINITY data of a water column. The result is aDIAGRAM which identifies the water masses within the column, the column's stability, and indicates the st value, andallows an estimate of the ACCURACY of the temperature and salinity measurements.

5337 template (or templet). A pattern or guide, usually constructed of paper, plastic, or metal, used to shape, delimit, orlocate an area.

A device used in RADIAL TRIANGULATION to represent the AERIAL PHOTOGRAPH; the template provides arecord of the directions of RADIALS taken from the PHOTOGRAPH.

5338 temporal (adj.). Pertaining to or limited by TIME.

5339 temporal variation. See MAGNETIC TEMPORAL VARIATION.

244

5340 temporary magnetism. See MAGNETISM.

5341 tension correction (tape). See TAPE: TENSION CORRECTION.

5342 tented ice. PRESSURE ICE in which two ICE FLOES have been pushed into the air, leaving an air space underneath.

5343 terminal. A number of berths grouped together, providing facilities for handling a particular form of cargo, e.g. oilterminal, container terminal.

5344 terrace. A relatively flat horizontal or gently inclined surface, sometimes long and narrow, bounded above and belowby rather abrupt SLOPES.

5345 terrain. Standing GROUND.A tract of country considered with regard to its natural features, configuration, etc.

5346 terrestrial camera. See CAMERA.

5347 terrestrial equator. See EQUATOR.

5348 terrestrial globe. See GLOBE.

5349 terrestrial latitude. See LATITUDE.

5350 terrestrial longitude. See LONGITUDE.

5351 terrestrial magnetic field. See GEOMAGNETIC FIELD.

5352 terrestrial magnetism. See GEOMAGNETISM.

5353 terrestrial meridian. See MERIDIAN: ASTRONOMICAL.

5354 terrestrial navigation. The kind of NAVIGATION which uses marks ashore for positioning.

5355 terrestrial photogrammetry. See PHOTOGRAMMETRY.

5356 terrestrial photograph. See PHOTOGRAPH.

5357 terrestrial pole. See POLE.

5358 terrigenous (adj.). Derived from the LAND.Applied to marine DEPOSITS, originating from the EROSION of the LAND as opposed to pelagic DEPOSITS.

5359 terrigenous sediments. DEPOSITS consisting of DEBRIS derived from the EROSION of land areas and usuallydeposited in the shallow parts of the SEA.

5360 territorial sea. A belt of water of a defined breadth but not exceeding 12 nautical miles measured seaward from theterritorial sea baseline.

5361 territorial waters. See TERRITORIAL SEA.

5362 tesla. The unit of magnetic flux density in the SI SYSTEM.

5363 test. The hard covering or supporting structure of many invertebrates; it may be enclosed within an outer layer of livingtissue; a SHELL.

5364 Texas tower. A fixed tower mounted on the CONTINENTAL SHELF or on a SHOAL, used to drill and operate gas orpetroleum wells, and to provide a platform for AIDS TO NAVIGATION and meteorological or oceanographic instru-mentation.

5365 thalweg. See TALWEG.

5366 thaw. Melting of SNOW and/or ICE, at the EARTH's surface, following a temperature rise above 0°C.

5367 thematic map. See MAP.

245

5368 theodolite. A precision surveying instrument consisting of an ALIDADE with a TELESCOPE. It is mounted on anaccurately graduated CIRCLE and is equipped with necessary LEVELS and reading devices. Sometimes, theALIDADE carries a graduated VERTICAL CIRCLE.

5369 theodolite: direction. A THEODOLITE in which the graduated HORIZONTAL CIRCLE remains fixed during a seriesof OBSERVATIONS, the TELESCOPE being pointed on a number of SIGNALS or objects in succession, and theDIRECTION of each read on the CIRCLE, usually by the use of MICROMETER MICROSCOPES.

5370 theodolite: face(s) of. See FACE(S) OF THEODOLITE.

5371 theodolite: repeating. A THEODOLITE so designed that successive measures of an angle may be accumulated on thegraduated CIRCLE, and a final reading of the CIRCLE made which represents the sum of the repetitions. The observedvalue of the angle is obtained by dividing the total arc passed through in making the series of OBSERVATIONS by thenumber of times the angle has been observed. Also called repeating instrument. See REPETITION METHOD OFOBSERVATION.

5372 theodolite: transit. See TRANSIT.

5373 theodolite axes. See AXIS: HORIZONTAL and AXIS: VERTICAL.

5374 theodolite drag. See DRAG (of THEODOLITE).

5375 theoretical error. See ERROR.

5376 thermal equator. See EQUATOR.

5377 thermal layer. See THERMOCLINE.

5378 thermocline. A vertical negative temperature gradient in some LAYER of a body of water, which is appreciably greaterthan the GRADIENTS above and below it; also a LAYER, in which such a GRADIENT occurs. The principalthermoclines in the OCEAN are either seasonal, due to heating of the surface water in summer, or permanent. Alsocalled discontinuity layer, or thermal layer.

5379 thermocline: transient. A small decrease in vertical temperature observed above the THERMOCLINE. It is a short-term PHENOMENON associated primarily with diurnal heating and wind mixing.

5380 thermogram. Record made by a THERMOGRAPH.

5381 thermograph. THERMOMETER used to give a graphical record of the time variations of temperature.

5382 thermohygrograph. See HYGROTHERMOGRAPH.

5383 thermometer. Instrument used in the measurement of temperature.

5384 thermometer: deep sea. A THERMOMETER for ascertaining the temperature of SEA WATER at any DEPTH. SeeTHERMOMETER: PROTECTED REVERSING, THERMOMETER: REVERSING, THERMOMETER:UNPROTECTED REVERSING.

5385 thermometer: dry-bulb. That of the two THERMOMETERS of a PSYCHROMETER whose bulb is bare and whichindicates the air temperature.

5386 thermometer: maximum. THERMOMETER used for measuring the highest temperature attained during a given inter-val of time, for example, a day.

5387 thermometer: minimum. THERMOMETER used for measuring the lowest temperature attained during a given inter-val of time, for example, a day.

5388 thermometer: protected reversing. A REVERSING THERMOMETER enclosed in a heavy glass case to protect itfrom HYDROSTATIC PRESSURE.

5389 thermometer: reversing. A DEEP-SEA THERMOMETER that records temperature at the desired DEPTH upon beinginverted and thereafter retains its reading until returned to the first position. Reversing thermometers are generally usedin pairs on WATER BOTTLES.

246

5390 thermometer: tape. A precision THERMOMETER fitted in a specially designed case to clip on and against a metalTAPE in order to determine TEMPERATURE CORRECTIONS for precision BASE or TRAVERSE tapemeasurements.

5391 thermometer: unprotected reversing. A REVERSING THERMOMETER which is not protected againstHYDROSTATIC PRESSURE. Used in conjunction with PROTECTED REVERSING THERMOMETERS todetermine THERMOMETRIC DEPTH.

5392 thermometer: wet-bulb. That of the two THERMOMETERS of a PSYCHROMETER whose bulb is wet and coveredby a film of pure water or ICE.

5393 thermometric depth. See DEPTH.

5394 thermometric levelling. See LEVELLING.

5395 thermoprobe. A TRANSDUCER used to measure temperature 'in situ' of ocean BOTTOM SEDIMENTS at DEPTHSbeneath the BOTTOM. Such measurements when combined with heat conductivity information provide a measurementof heat flow through the ocean bottom.

5396 third order triangulation. See TRIANGULATION CLASSIFICATION.

5397 three-arm protractor. See PROTRACTOR: THREE-ARM.

5398 three-point fix. See FIX.

5399 three-point fix method. One of the principal methods used on inshore HYDROGRAPHIC SURVEYS for establishingthe POSITION of the SURVEY VESSEL. It involves the measurement with SEXTANTS of two angles between threeknown STATIONS, the middle STATION being common to both angles, and plotting the vessel's POSITION gra-phically with a THREE-ARM PROTRACTOR.

5400 three-point problem. The determination of the horizontal POSITION of a point of observation from DATA comprisingtwo observed horizontal angles between three objects of known POSITION. The problem is solved either graphically bythe use of a THREE-ARM PROTRACTOR, or analytically by trigonometrical computation.

5401 threshold. In aeronautical terminology, the beginning of that portion of the RUNWAY usable for landing.

5402 threshold speed. The minimum SPEED of CURRENT at which a particular CURRENT METER will measure at itsrated reliability.

5403 thunder. Sharp or rumbling SOUND which accompanies LIGHTNING.

5404 thunderstorm. One or more sudden electrical discharges manifested by the flash of light (LIGHTNING) and a sharp orrumbling sound (THUNDER).

5405 tidal basin. See BASIN.

5406 tidal bore. See BORE.

5407 tidal constants. Tidal relations that remain essentially constant for any particular locality. Tidal constants are classed asharmonic and non-harmonic, the HARMONIC CONSTANTS consisting of the AMPLITUDES and EPOCHS, and theNON-HARMONIC CONSTANTS including those values determined directly from OBSERVATIONS, such as tidalranges and intervals.

5408 tidal constituent. See HARMONIC CONSTITUENT.

5409 tidal current. See CURRENT.

5410 tidal current chart. See CHART.

5411 tidal current signal. See SIGNAL.

5412 tidal (or tide) cycle. A complete set of tidal conditions, as those occurring during a TIDAL DAY, LUNAR MONTH orMETONIC CYCLE.

5413 tidal datum. See DATUM.

247

5414 tidal day. LUNAR DAY.The period of the daily CYCLE of the TIDES, differing slightly from the LUNAR DAY because of PRIMING and

LAGGING OF THE TIDE.

5415 tidal difference. Difference in TIME or HEIGHT of a HIGH or LOW WATER at a SUBORDINATE STATION andat a REFERENCE STATION for which predictions are given in the TIDE TABLES. The difference, when appliedaccording to sign to the prediction at the REFERENCE STATION, gives the corresponding TIME or HEIGHT for theSUBORDINATE STATION.

5416 tidal epoch. See PHASE LAG.

5417 tidal flat. A MARSH or sandy or muddy coastal flatland which is covered and uncovered by the RISE and fall of theTIDE.

5418 tidal harbour. See HARBOUR.

5419 tidal light(s). See LIGHT.

5420 tidal movement. The movement which includes both the vertical RISE and fall of the TIDE, and the horizontal FLOWof the TIDAL CURRENTS. This movement is associated with the astronomical TIDE-PRODUCING FORES of theMOON and SUN acting upon the rotating EARTH.

5421 tidal observations. See OBSERVATION.

5422 tidal prediction. See TIDE PREDICTION.

5423 tidal range. See RANGE OF TIDE.

5424 tidal rise. See RISE OF TIDE.

5425 tidal stream. See CURRENT: TIDAL.

5426 tidal theories. The different theories trying to describe the tidal phenomena.

5427 tidal water. Any water the level of which changes periodically due to tidal action. See TIDEWATER.

5428 tidal wave. See WAVE.

5429 tide. The periodic RISE and fall of the surface of OCEANS, BAYS, etc., due principally to the gravitational interactionsbetween the MOON, SUN and EARTH.

5430 tide: analysis. The mathematical processes by which the observed TIDE or TIDAL CURRENT are analyzed to obtainconstituents and statistics.

5431 tide(s): apogean. TIDES of decreased RANGE occurring when the MOON is near APOGEE.

5432 tide: astronomical. TIDE due to the attraction of the SUN and MOON, in contrast to a METEOROLOGICAL TIDE,caused by meterological conditions.

EQUILIBRIUM TIDE.

5433 tide: compound. A tide constituent with a SPEED equal to the sum or difference of the SPEEDS of two or more basicconstituents. Compound tides usually occur in SHALLOW WATER regions. See SHALLOW WATERCONSTITUENT.

5434 tide: diurnal. A TIDE in which the TIDAL CYCLE consists of one HIGH WATER and one LOW WATER eachTIDAL DAY. In British terminology also called single day tide.

5435 tide: double. A double-headed TIDE. Also called agger. See HIGH WATER: DOUBLE and LOW WATER:DOUBLE.

5436 tide: earth. Periodic movement of the EARTH's CRUST caused by the TIDE-PRODUCING FORCES of the MOONand SUN.

5437 tide: ebb. See TIDE: FALLING.

248

5438 tide(s): equatorial. TIDES that occur approximately every two weeks when the MOON is over the EQUATOR. Atthese times, the MOON produces minimum DIURNAL INEQUALITY in the TIDE.

5439 tide: equilibrium. A hypothetical TIDE based upon the assumption that the water responds, instantly to the TIDE-PRODUCING FORCES of the MOON and SUN, and forms a surface of EQUILIBRIUM under the action of theseforces, disregarding friction, inertia and the irregular distribution of the land masses.

5440 tide(s): equinoctial spring. Those TIDES occurring near the EQUINOXES when the FULL or NEW MOON and theSUN have little or low DECLINATION and SPRING TIDES of greater RANGE than the average occur, particularly ifthe MOON is also nearly in PERIGEE.

5441 tide: falling. The portion of the TIDE CYCLE between HIGH WATER and the following LOW WATER. Also calledebb tide. The opposite is RISING TIDE.

5442 tide: flood. See TIDE: RISING.

5443 tide: half. The condition or TIME of the TIDE when at the level midway between any given HIGH TIDE and thefollowing or preceding LOW TIDE.

5444 tide: high. See HIGH WATER.

5445 tide: leeward (or lee). A TIDAL CURRENT setting in the same direction as that in which the WIND is blowing. Alsocalled leeward tidal current.

5446 tide: low. See LOW WATER.

5447 tide: lunar. That part of the TIDE due solely to the TIDE-PRODUCING FORCES of the MOON, as distinguishedfrom that part caused by the forces of the SUN. See TIDE: SOLAR.

5448 tide: meteorological. A change in water level caused by local meteorological conditions, in contrast to anASTRONOMICAL TIDE caused by the attraction of the SUN and MOON.

5449 tide: mid-extreme. A level midway between the extreme HIGH WATER and extreme LOW WATER occurring at aplace. See HALF-TIDE LEVEL.

5450 tide: mixed. The TYPE OF TIDE in which a diurnal WAVE produces large INEQUALITIES in HEIGHTS and/ordurations of successive HIGH and/or LOW WATERS. This term applies to the TIDES intermediate to those predomi-nantly semidiurnal and those predominantly diurnal.

5451 tides: mixed diurnal. Diurnal tides which become semidiurnal with a considerable decrease of range when the moon'sdeclination is small.

5452 tides: mixed semidiurnal. Semidiurnal tides with noticeable inequality in corresponding extremes.

5453 tide(s): neap. The TIDES of decreased RANGE occurring near the TIMES of FIRST and LAST QUARTER.

5454 tide: oceanic. The periodic rise and fall of the earth's oceans resulting from gravitational interactions between the sun,moon, and earth.

5455 tide: partial. One of the harmonic components comprising the TIDE at any point. The periods of the partial tides arederived from various combinations of the ANGULAR VELOCITIES of EARTH, SUN, MOON, and STARS relative toeach other. See HARMONIC CONSTITUENT.

5456 tide(s): perigean. TIDES of increased RANGE occurring when the MOON is near PERIGEE.

5457 tide(s): predicted. The expected TIMES and HEIGHTS of the TIDE as given in the TIDE TABLES in advance of theiroccurrence.

5458 tide: primary. That part of a tidal UNDULATION that is the direct response to a TIDE-PRODUCING FORCE.

5459 tide(s): quarter-diurnal. The TIDE resulting from the distortion of the normal TIDE in SHALLOW WATER with fourHIGH WATERS and four LOW WATERS during one day. See also TIDE(S): SHALLOW WATER.

5460 tide: rising. The portion of the TIDE CYCLE between LOW WATER and the following HIGH WATER. Also calledflood tide. The opposite is FALLING TIDE.

249

5461 tide(s): secondary. Those additional TIDES HIGHER LOW WATERS and LOWER HIGH WATERS that occur twicea MONTH (when the MOON is over the EQUATOR) in a general pattern of DIURNAL TIDES.

5462 tide: semidiurnal. A TIDE in which the TIDAL CYCLE consists of two HIGH WATERS and two LOW WATERSeach TIDAL DAY, with comparatively little DIURNAL INEQUALITY.

5463 tide(s): shallow water. A distortional effect upon astronomically generated TIDE caused by SHALLOW WATERS.See also TIDE(S): QUARTER-DIURNAL.

5464 tide: single day. See TIDE: DIURNAL.

5465 tide: slack. See SLACK WATER.

5466 tide: solar. That part of the TIDE due solely to the TIDE-PRODUCING FORCES of the SUN, as distinguished fromthat part caused by the forces of the MOON.

5467 tide(s): solsticial. TIDES occurring near the TIMES of the SOLSTICES, when the TROPIC RANGE is especiallylarge.

5468 tide(s): spring. The TIDES of increased RANGE occurring near the TIMES of FULL MOON and NEW MOON.

5469 tide: storm. See STORM SURGE.

5470 tide: syzygy. The TIDE which takes place on the afternoon of the day the SUN and MOON are in SYZYGY. (Thisterm is now little used).

5471 tide: tropic. The TIDE that occurs twice monthly when the effect of the MOON's maximum DECLINATION north orsouth of the EQUATOR is greatest.

5472 tide: vanishing. The PHENOMENON occurring when a HIGH and LOW WATER 'melt' together into a period ofseveral hours with a nearly constant water level. The TIDE is in the diurnal category but is known as a 'vanishing tide'.

5473 tide: weather. See TIDE: WINDWARD.

5474 tide: windward. A TIDAL CURRENT setting to windward. Also called weather tide.

5475 tide-bound (adj.). Unable to proceed because of insufficient DEPTH of water due to tidal action.

5476 tide bulge. See TIDE WAVE.

5477 tide crack. The fissure at the line of junction between an immovable ICEFOOT or ICE WALL and FAST ICE, thelatter being subject to the RISE and fall of the TIDE.

5478 tide curve. Any graphic representation of the RISE and fall of the TIDE. TIME is generally represented by theabscissae and the HEIGHT of the TIDE by ordinates. For normal TIDES the curve so produced approximates a sinecurve. Also called marigram.

5479 tide cycle. See TIDAL CYCLE.

5480 tide gate. A restricted passage through which water runs with great SPEED due to tidal action.

5481 tide gauge. See GAUGE.

5482 tide-generating force. See TIDE-PRODUCING FORCE.

5483 tideland. LAND which is under water at HIGH TIDE and uncovered at LOW TIDE.

5484 tide lock. A LOCK situated between a BASIN or CANAL and TIDEWATER to maintain the water at a desired level asthe HEIGHT of the TIDE changes. Also called guard lock.

5485 tidemark. A HIGH WATER MARK left by TIDAL WATER.The highest point reached by a HIGH TIDE.A MARK placed to indicate the highest point reached by a HIGH TIDE, or, occasionally, any specified state of

TIDE.

250

5486 tide pole. See TIDE STAFF.

5487 tide-predicting machine. An instrument that computes, sometimes for years in advance, the TIMES and HEIGHTS ofHIGH and LOW WATERS at a REFERENCE STATION by mechanically summing the HARMONICCONSTITUENTS of which the TIDE is composed.

5488 tide (or tidal) prediction. The mathematical process by which the TIMES and HEIGHTS of the TIDE are determinedin advance from the HARMONIC CONSTITUENTS at a place. See HARMONIC ANALYSIS OF TIDE.

5489 tide-producing force(s). That part of the gravitational attraction of a CELESTIAL BODY which is effective inproducing the TIDES on EARTH. The SUN and MOON are the principal astronomic bodies that have a tide-producingeffect. The force varies approximately as the mass of the attracting body and inversely as the cube of its distance. Alsocalled tide-generating force.

5490 tide race. A strong TIDAL CURRENT or a CHANNEL in which such a CURRENT flows.

5491 tide range. See RANGE OF TIDE.

5492 tide reducer. The CORRECTION that must be applied to a recorded SOUNDING for the HEIGHT OF TIDE above orbelow the DATUM of reference at the TIME of SOUNDING.

5493 tide register. See GAUGE: TIDE.

5494 tide rip(s). Small WAVES formed on the surface of water by the meeting of opposing TIDAL CURRENTS or by aTIDAL CURRENT crossing an irregular BOTTOM. Vertical oscillation, rather than PROGRESSIVE WAVES, ischaracteristic of tide rips.

5495 tide signal. See SIGNAL.

5496 tide staff. A TIDE GAUGE consisting of a vertical graduated pole from which the HEIGHT OF TIDE at any TIME canbe read directly. Also called tide pole.

5497 tide staff: multiple. A succession of TIDE STAFFS on a sloping SHORE, so placed that the vertical GRADUATIONSon the several staffs form a continuous scale with reference to the same DATUM.

5498 tide station. See STATION.

5499 tide tables. TABLES listing daily predictions, usually a year in advance, of the TIMES and HEIGHTS of the TIDE.These predictions are usually supplemented by TIDAL DIFFERENCES and TIDAL CONSTANTS by means of whichadditional predictions can be obtained for numerous other places. See TIDE PREDICTION.

5500 tidewater. Water affected by TIDES or sometimes that part of it which covers the TIDELAND. The term is sometimesused broadly to designate the SEABOARD. See TIDAL WAVES.

5501 tide wave. A long-period wave associated with the TIDE-PRODUCING FORCES of the MOON and SUN; identifiedwith the rising and falling of the TIDE. Also called tide bulge. See TIDE, TIDAL MOVEMENT, STAND OF TIDE.

5502 tideway. A CHANNEL through which a TIDAL CURRENT runs.

5503 tie. A survey connection from a point of known POSITION to a point whose POSITION is desired. A tie is made todetermine the POSITION of a supplementary point whose POSITION is desired for mapping or reference purposes, orto close a SURVEY on a previously determined point. To tie in is to make such a connection. The point to which theconnection is made is termed a TIE POINT.

5504 tie in (v.i.). See TIE.

5505 tie net. See TRIANGULATION NET.

5506 tie point. Point of closure of a SURVEY either on itself or on another SURVEY.

5507 till. Unstratified glacial DRIFT consisting of CLAY, SAND, GRAVEL, and BOULDERS intermingled in any propor-tions. Also called boulder clay.

251

5508 tillite. ROCK formed of consolidated or lithified TILL and generally a record of a glacial EPOCH, older than that of thequarternary.

5509 tilt. The angle which anything makes with the horizontal.In PHOTOGRAMMETRY, the angle at the PERSPECTIVE CENTRE between the photograph perpendicular and

the PLUMB LINE, or other exterior REFERENCE DIRECTION; also, the dihedral angle between the plane of thePHOTOGRAPH and the horizontal plane.

5510 tilt: base. In PHOTOGRAMMETRY, the INCLINATION of the AIR BASE with respect to the horizontal.

5511 tilt: relative. The angular relationship between two overlapping VERTICAL PHOTOGRAPHS with no reference to anestablished DATUM.

5512 tilting level. See LEVELLING INSTRUMENT: TILTING LEVEL.

5513 time. The measurable aspect of duration.The HOUR of the DAY reckoned by the position of a celestial reference point relative to a reference CELESTIAL

MERIDIAN.An elapsed interval.

5514 time: apparent. See TIME: APPARENT SOLAR.

5515 time: apparent solar. TIME measured by the apparent diurnal motion of the (true) SUN. Also called true solar time,and often apparent time.

5516 time: astronomic(al). SOLAR TIME in a DAY (ASTRONOMICAL DAY) that begins at NOON.

5517 time: civil. SOLAR TIME in a DAY (CIVIL DAY) that begins at MIDNIGHT.

5518 time: daylight saving. A variation of ZONE TIME, usually one HOUR ahead of that TIME. Sometimes called summertime.

5519 time: geologic(al). The division into a number of great ERAS, each represented by a group of ROCKS.

5520 time: Greenwich. TIME based upon the GREENWICH MERIDIAN as reference.

5521 time: Greenwich civil. See TIME: GREENWICH MEAN.

5522 time: Greenwich mean (GMT) (or Greenwich civil). MEAN SOLAR TIME at the GREENWICH MERIDIAN. SeeUNIVERSAL TIME (UT).

5523 time: local. TIME based upon the LOCAL MERIDIAN as reference, as contrasted with that based upon a ZONEMERIDIAN, or the MERIDIAN of Greenwich.

Any TIME kept locally.

5524 time: local apparent. The APPARENT SOLAR TIME for the MERIDIAN of the observer.

5525 time: local mean. The MEAN SOLAR TIME for the MERIDIAN of the observer.

5526 time: local sidereal. The SIDEREAL TIME for the MERIDIAN of the observer.

5527 time: mean. See TIME: MEAN SOLAR.

5528 time: mean solar. TIME measured by the diurnal motion of the MEAN SUN, which is supposed to move uniformly inthe CELESTIAL EQUATOR, completing the circuit in one TROPICAL YEAR. Often termed simply mean time.

5529 time: sidereal. TIME measured by the apparent diurnal rotation of the (true) VERNAL EQUINOX. It is counted from0 HOUR, when the VERNAL EQUINOX is on the MERIDIAN, through 24 HOURS. Naming the MERIDIAN ofreference is essential to its complete identification.

5530 time: solar. TIME based upon the ROTATION of the EARTH relative to the SUN. See TIME: APPARENT SOLAR,TIME: ASTRONOMICAL, TIME: CIVIL, TIME: MEAN SOLAR.

252

5531 time: standard. A variation of ZONE TIME adapted for use on or near LAND, with irregular but well-defined zonelimits.

5532 time: summer. See TIME: DAYLIGHT SAVING.

5533 time: true solar. See TIME: APPARENT SOLAR.

5534 Time: Universal. See UNIVERSAL TIME (UT).

5535 time: zone. The LOCAL MEAN TIME of a reference or zone meridian, whose TIME is kept throughout a designatedZONE. The ZONE MERIDIAN is usually the nearest MERIDIAN whose LONGITUDE is exactly divisible by 15°.

5536 time ball. A visual TIME SIGNAL in the form of a ball.

5537 timepiece. An instrument for measuring TIME. See CHRONOMETER, CLOCK, WATCH.

5538 time signal. See SIGNAL.

5539 time tag. Reference assigned to each measurement input from a sensor to identify the time (either CPU, GPS, or UTC)when a measurement was actually made by that particular sensor; time tags allow adjustment of data from differentsensors to a common time reference.

5540 time zone. See ZONE.

5541 tinted areas. Those areas on the NAUTICAL CHART that are symbolized by a flat colour to indicate the nature of thearea.

5542 title. In CARTOGRAPHY, an inscription, on the CHART or FAIR CHART, including any useful information and de-tails concerning the HYDROGRAPHIC SURVEY, the NATURAL SCALE, and geodetic, magnetic and tidal DATA.

5543 titration. A chemical method for determining the concentration of a substance in solution. In OCEANOGRAPHY, themost common titration is that for CHLORINITY.

5544 tolerance. The maximum allowable variation from a STANDARD or from specified conditions.

5545 ton. A unit of WEIGHT equal to 1,000 KILOGRAMS ('metric ton'), 2,240 POUNDS ('long ton'), or 2,000 POUNDS('short ton').

5546 tongue. A projection of the ICE EDGE up to several kilometres in length, caused by WIND and CURRENT.A relatively narrow strip of LAND.An INLET.A narrow, rapid CURRENT.A protrusion of water into a region of different temperature.

5547 tonnage. The carrying capacity of ships.

5548 topmark. A characteristic shape secured at the top of a BUOY or BEACON to aid in its identification.

5549 topmark buoy. A BUOY with a TOPMARK, or characteristic shape secured at the top to aid in its identification.

5550 topoangulator. An instrument used to measure vertical angles in the PRINCIPAL PLANE of an OBLIQUE PHOTO-GRAPH.

5551 topographic feature. A single feature of the surface of the EARTH such as a MOUNTAIN or VALLEY.

5552 topographic map. See MAP.

5553 topographic model. See RELIEF MODEL.

5554 topographic signal. See SIGNAL.

5555 topographic station. See STATION.

5556 topographic survey. See SURVEY.

253

5557 topography. The configuration of the surface of the EARTH, including its RELIEF, the position of its STREAMS,roads, cities, etc. The EARTH's natural and physical features collectively.

In OCEANOGRAPHY, the term is applied to a surface such as the sea bottom or a surface of given characteristicswithin the water mass.

5558 topography board. See FIELD BOARD.

5559 toponymy. The place names or the study of place names of a country or district.

5560 tornado. In North America, the name for an intense SPOUT of large diameter.

5561 torrent. Rushing STREAM of water; great downpour of RAIN.

5562 torrid zone. That part of the EARTH between the TROPIC OF CANCER and the TROPIC OF CAPRICORN. Alsocalled the tropics.

5563 torsion balance. A device for measuring very small forces of attraction resulting from GRAVITATION,MAGNETISM, etc., and consisting of a bar suspended horizontally by an elastic filament, one end of the bar beingsubjected to the influence of the attracting force to a greater degree than the other end. The attracting force is balancedand its comparative strength measured by the torsional reaction of the filament.

5564 total eclipse. See ECLIPSE.

5565 total magnetic intensity. The vector resultant of the intensity of the horizontal and vertical components of theEARTH's MAGNETIC FIELD at a specified point.

5566 towering. Apparent increase in the vertical dimension of an object near the HORIZON, due to large inequality ofATMOSPHERIC REFRACTION in the LINE OF SIGHT to the top and bottom of the object. The opposite is STOO-PING.

5567 tracer. Foreign substance introduced into the water by natural or artificial means which enables determination of watermovement through measurement of distribution and location of the substance at the same time.

5568 tracing cloth. A fine semitransparent linen or cotton cloth sized on one side and dull on the other.

5569 tracing paper. A thin tough semitransparent paper suitable for making tracings of drawings, or for miscellaneous use inhydrographic work, where permanence is not important or when pencil work is sufficient.

5570 track. The actual path or route of a craft over the GROUND or sea bottom, or its graphic representation. In AIRNAVIGATION also called track made good.

5571 track (v.t.). To follow the movements of an object as by RADAR or an optical system.To navigate by keeping a record, tabular or graphical of the past POSITIONS of a craft without regard for future

POSITIONS.

5572 track: dead reckoning. A line representing successive DEAD RECKONING POSITIONS of a craft.

5573 track: great-circle. The TRACK of a craft following a GREAT CIRCLE, or a GREAT CIRCLE which a craft intendsto follow approximately.

5574 track: intended. The anticipated path of a vessel relative to the EARTH.

5575 track: radar guided. See RADAR.

5576 track: recommended. A track, shown on a chart by either a dashed or a continuous line, recommended to all or onlycertain vessels.

5577 track chart. A chart showing recommended, required, or established tracks, and usually indicating turning points,courses, and distances.

5578 tracking. The process of observing the sequential changes in the position of a target to establish its motion.

5579 tracking camera. See CAMERA: BALLISTIC.

254

5580 track line of sounding. A continuous record of SOUNDINGS obtained by a ship on an extended voyage en routebetween its home port and the working ground. Also called cruise line of sounding.

5581 track made good. See TRACK.

5582 track plotter. A PLOTTER used to plot the track or course of a craft.

5583 trade winds. Persistent WINDS, mainly in the lower ATMOSPHERE, which blow over vast regions from a subtropicalANTICYCLONE toward the equatorial regions. The predominant directions of the trade winds are from NE in thenorthern hemisphere and from SE in the southern hemisphere.

5584 traffic lane. An area within defined limits in which one-way traffic is established. Natural obstacles, including thoseforming separation zones, may constitute a boundary.

5585 traffic separation scheme. A scheme which aims at reducing the risk of collision in congested and/or converging areasby separating traffic moving in opposite, or nearly opposite, directions.

5586 training wall. A wall, BANK, or JETTY, often submerged, built to direct or confine the FLOW of a RIVER or TIDALCURRENT.

5587 trajectory. The curved path of something hurtling through space.In METEOROLOGY, curve traced by the successive positions of a moving particle of air. Curve traced by the

successive positions of the centre of a selected synoptic system, such as a DEPRESSION.In OCEANOGRAPHY, the path followed by moving water particles.In electronics the path followed by electrons emanating from the cathode as in an electron tube.

5588 transceiver. A combination transmitter and receiver in a single housing, with some components being used by bothparts. See TRANSPONDER.

5589 transducer. Any device for converting energy from one form to another (electrical, mechanical, or acoustic).

5590 transducer: magnetostriction. A TRANSDUCER that depends for its operation on the interaction between the magne-tization and the deformation of a material having magnetostrictive properties.

5591 transformation. In PHOTOGRAMMETRY, the process of projecting a PHOTOGRAPH (mathematically, graphically,or photographically) from its plane onto another plane by translation, rotation, and/or scale change. The projection ismade onto a plane determined by the angular relations of the camera axes and not necessarily onto a horizontal plane.See also RECTIFICATION.

5592 transforming printer. A specially designed projection printer of fixed geometry used for transforming the obliquecomponents of a coupled camera installation, a MULTIPLE-LENS CAMERA, or a panoramic camera onto a planeperpendicular to the axis of the system.

5593 transhipment or lightening area. An area designated for transfer of cargo from one vessel to another to reduce thedraught of the larger vessel. Also called cargo transfer area.

5594 transient magnetism. See MAGNETISM: TEMPORARY.

5595 transient thermocline. See THERMOCLINE.

5596 transit. In ASTRONOMY, the apparent passage of a STAR or other CELESTIAL BODY across a defined line of theCELESTIAL SPHERE, as a MERIDIAN, PRIME VERTICAL, or ALMUCANTAR. When no line is specified, atransit across the MERIDIAN is usually intended. See MERIDIAN TRANSIT.

The apparent passage of a STAR or other CELESTIAL BODY across a line in the RETICLE of a TELESCOPE, orsome LINE OF SIGHT.

The apparent passage of a smaller CELESTIAL BODY across the disk of a larger CELESTIAL BODY.A surveying instrument composed of a HORIZONTAL CIRCLE graduated in circular measure and an ALIDADE

with a TELESCOPE which can be reversed in its supports without being lifted therefrom. Also, the act of making sucha reversal. A THEODOLITE having a TELESCOPE that can be transited in its supports is a transit, and is sometimestermed a transit theodolite. All modern THEODOLITES are transits.

An astronomical instrument having a TELESCOPE which can be so adjusted in position that the LINE OF SIGHTmay be made to define a VERTICAL CIRCLE. A transit used in astronomical work is usually termed either an astro-nomic(al) transit or a transit instrument.

In NAVIGATION, the position of two distant, fixed objects when they are in line to an observer; the line passingthrough them and the observer being a LINE OF POSITION. See also RANGE.

255

5597 transit: broken telescope. A precise astronomic TRANSIT or THEODOLITE in which the light passing through theOBJECTIVE LENS is reflected through a right angle by a prism placed in the HORIZONTAL AXIS. This hollow axisis continued beyond the supports, and the EYEPIECE is fitted at one end.

5598 transit: local. The apparent passage of a CELESTIAL BODY across the MERIDIAN of the observer. See alsoMERIDIAN TRANSIT.

5599 transit axis. See AXIS: HORIZONTAL.

5600 transit bearing. See BEARING.

5601 transit instrument. See TRANSIT.

5602 transitional water wave. See WAVE.

5603 transition layer. A water layer with a distinct change of the vertical gradient of a given parameter relative to adjacentlayers.

5604 transition zone. The water area between two opposing CURRENTS manifested by EDDIES, UPWELLING, RIPS, andsimilar turbulent conditions occurring either vertically or horizontally; or a zone between two water masses of differingphysical characteristics such as temperature and/or SALINITY.

5605 transit magnetometer. See MAGNETOMETER.

5606 transit micrometer. See MICROMETER.

5607 transit passage. In straits used for international navigation, all ships and aircraft enjoy the unimpeded right of transitpassage. This means the freedom of navigation and overflight solely for the purpose of continuous and expeditioustransit of the strait between one part of the high seas or an exclusive economic zone and another part of the high seas oran exclusive economic zone. See also INNOCENT PASSAGE.

5608 transit theodolite. See TRANSIT.

5609 transit traverse. See TRAVERSE.

5610 translocation. A method to improve the accuracy of satellite positioning by simultaneously using several receivers. Onereceiver is deployed on a known position to obtain corrections required to match the known position and that derivedfrom satellite signals. These corrections are then transmitted to the other receivers.

5611 transmitter. One who or that which transmits or sends anything, particularly a RADIO TRANSMITTER.

5612 transparency. In OCEANOGRAPHY, the property of water to transmit light of different WAVE LENGTHS. Transpa-rency sometimes is measured in percent of RADIATION that penetrates a distance of one METRE. Sometimes it isexpressed as the average DEPTH at which a SECCHI DISC disappears and reappears.

In PHOTOGRAPHY, a photographic PRINT on a clear base, especially adaptable for viewing by transmitted light.Also, the light-transmitting capability of a material.

5613 transponder. A combined RECEIVER and TRANSMITTER whose function is to transmit SIGNALS automaticallywhen triggered by an INTERROGATOR. See TRANSCEIVER.

5614 transponder beacon. A beacon having a TRANSPONDER. Also called responder beacon.

5615 transport. The process by which a substance or quantity is carried past a fixed point, or across a fixed plane. InOCEANOGRAPHY and METEOROLOGY, such quantities are: Heat, MOMENTUM, mass, dissolved impurities,suspended particles, etc.

5616 transverse cylindrical orthomorphic projection. See PROJECTION: TRANSVERSE MERCATOR.

5617 transverse Mercator grid. The grid on a chart using the transverse Mercator projection.

5618 transverse Mercator projection. See PROJECTION.

5619 transverse wave. See WAVE.

5620 travelling wave. See WAVE: PROGRESSIVE.

256

5621 traverse. A method of surveying in which a sequence of lengths and DIRECTIONS of lines between points on theEARTH are obtained from field measurements, and used in determining POSITIONS of the points. Also called surveytraverse.

5622 traverse: astronomical. A TRAVERSE in which the GEOGRAPHIC(AL) POSITIONS of the STATIONS areobtained from ASTRONOMICAL OBSERVATIONS, and lengths and AZIMUTHS of lines are obtained bycomputation.

5623 traverse: closed. A TRAVERSE which starts and ends upon the same STATION, or upon STATIONS whose relativepositions have been determined by other SURVEYS.

5624 traverse: hanging. See TRAVERSE: OPEN.

5625 traverse: open (or open end). A TRAVERSE which starts from a STATION of known or adopted POSITION, butdoes not end upon such a STATION. Also called hanging traverse.

5626 traverse: plane table. A graphical TRAVERSE executed by PLANE TABLE methods.

5627 traverse: transit. A TRAVERSE in which the angles are measured with a TRANSIT or THEODOLITE and thelengths with a suitable measuring device.

5628 traverse net. See TRIANGULATION NET.

5629 traverse point. See TURNING POINT.

5630 traverse sailing. See SAILING.

5631 traverse station. See STATION.

5632 traverse table. A TABLE giving relative values of various parts of plane right triangles, for use in solving suchtriangles, particularly in connection with various SAILINGS.

5633 trench. A long, narrow and deep DEPRESSION of the SEA FLOOR with relatively steep sides.A long narrow VALLEY between two MOUNTAIN RANGES, especially a rift valley or a U-shaped valley.

5634 trend. The general direction of something, such as a coastline.

5635 triangle of error. See COCKED HAT.

5636 triangulate. The process of TRIANGULATION.

5637 triangulation. A method of surveying in which the STATIONS are points on the GROUND at the vertices of a chain orNETWORK of triangles. The angles of the triangles are measured instrumentally and the sides are derived bycomputation from selected triangle sides called BASES (or BASE LINES), the lengths of which are obtained fromdirect measurements on the GROUND.

5638 triangulation: arc of. A system of TRIANGULATION of limited width designed to progress in a single generaldirection. Also called chain of triangulation.

5639 triangulation: flare. A method of TRIANGULATION in which simultaneous OBSERVATIONS are made onparachute flares. This method is used for extending TRIANGULATION over lines too long to be observed by ordinarymethods.

5640 triangulation: floating. In hydrographic SURVEYING, a TRIANGULATION that may be carried out by means ofanchored BUOYS when surveying at such a distance from the LAND that the SIGNALS on the SHORE cannot be di-rectly used for locating the SOUNDINGS. Floating triangulation can be either connected to and extended from theSTATIONS of a shore triangulation or of a fully floating type in which case the POSITIONS of the BUOYS areobtained by means of OBSERVATIONS and measurements taken at sea. BUOYS are anchored in such a position as toprovide a framework for surveying the area.

5641 triangulation: order of. See TRIANGULATION CLASSIFICATION.

5642 triangulation: radial. See RADIAL TRIANGULATION.

257

5643 triangulation: ship-to-shore. A method of TRIANGULATION involving simultaneous OBSERVATIONS from threeshore STATIONS on a target carried by a ship offshore of the middle STATION. The middle STATION must be visiblefrom each of the two end STATIONS.

5644 triangulation classification. The classifying of TRIANGULATIONS into first-order, second-order, and third-ordertriangulation, according to the ERROR OF CLOSURE OF TRIANGLES. In first-order work, the average closure is notin excess of 1 SECOND; in second order work, it does not exceed 3 SECONDS; and in third-order work, it does notexceed 5 SECONDS. For the basic first-order work, the computed length through the NETWORK must agree with themeasured BASE within 1 part in 50 000, as a minimum, and average about 1 in 75 000, or better.

5645 triangulation net. ARCS OF TRIANGULATION, sometimes with lines of TRAVERSE, connected together to form asystem of loops or circuits extending over an area. Sometimes called tie net when used to tie small ISLANDS together.Also called traverse net or survey net.

5646 triangulation point. See STATION: TRIANGULATION.

5647 triangulation signal. See SIGNAL.

5648 triangulation station. See STATION.

5649 triangulation tower. A TRIANGULATION SIGNAL consisting of two separate structures, independent of one anotheran inner structure which supports the THEODOLITE and sometimes the TARGET or SIGNAL LAMP, and an outerstructure which supports the observer and his assistants and sometimes the TARGET or SIGNAL LAMP. Triangulationtowers are used to elevate the LINE OF SIGHT above trees, topographic features, and other obstacles that mightinterfere with the OBSERVATIONS. See BILBY STEEL TOWER.

5650 tribrach plate. A demountable, three-armed plate which is attached to an instrument stand or survey tower and onwhich the bottoms of the FOOTSCREWS of the instrument rest in milled grooves.

5651 tributary. A RIVER which joins a larger one.

5652 trigonometric levelling. See LEVELLING.

5653 trigonometric station. See STATION: TRIANGULATION.

5654 trilaterate. The process of TRILATERATION.

5655 trilateration. A method of surveying wherein the lengths of the triangle sides are measured, usually by electronicmethods, and the angles are computed from the measured lengths. Compare with TRIANGULATION.

5656 trim. The way in which a ship floats on the water, in relation to her fore-and-aft line, whether on an even keel or downby the bow or by the stern.

5657 tripod. A three-legged stand for THEODOLITE, CAMERA, SIGNALS, etc.

5658 tripod signal. See SIGNAL.

5659 tripping device. A device to pull equipment out of or to lower it into water.

5660 tripton. Collectively, all of the dead suspended particulate matter in aquatic habitats.

5661 trivet. A low support for a surveying instrument which is used where a TRIPOD cannot be used.

5662 trochoidal wave. See WAVE.

5663 tropical cyclone. CYCLONE of tropical origin of small diameter (some hundreds of kilometers). It is characterized byviolent WIND and torrential RAIN. Sometimes accompanied by a THUNDERSTORM.

5664 tropical month. See MONTH.

5665 tropical year. See YEAR.

5666 tropic higher high water. The HIGHER HIGH WATER of TROPIC TIDES.

5667 tropic higher low water. The HIGHER LOW WATER of TROPIC TIDES.

258

5668 tropic high water inequality. See DIURNAL INEQUALITY.

5669 tropic lower high water. The LOWER HIGH WATER of TROPIC TIDES.

5670 tropic lower low water. The LOWER LOW WATER of TROPIC TIDES.

5671 tropic low water inequality. See DIURNAL INEQUALITY.

5672 tropic of Cancer. The northern PARALLEL OF DECLINATION, approximately 23°27' from the CELESTIALEQUATOR, reached by the SUN at its maximum northerly DECLINATION, or the corresponding PARALLEL on theEARTH.

5673 tropic of Capricorn. The southern PARALLEL OF DECLINATION, approximately 23°27' from the CELESTIALEQUATOR, reached by the SUN at its maximum southerly DECLINATION, or the corresponding PARALLEL on theEARTH.

5674 tropic range. Short for GREAT TROPIC RANGE.

5675 tropics. See TORRID ZONE.

5676 tropic tide. See TIDE.

5677 tropopause. Upper limit of the TROPOSPHERE.

5678 troposphere. Lower part of the terrestrial ATMOSPHERE, extending from the surface up to a HEIGHT varying fromabout 9 km at the POLES to about 17 km at the EQUATOR, in which temperature decreases fairly regularly withHEIGHT.

From an analogy with the ATMOSPHERE, the term is sometimes applied in OCEANOGRAPHY to the upper oceanlayer of relatively high temperature that is found in middle and lower LATITUDES and within which strongCURRENTS are present. See STRATOSPHERE.

5679 tropospheric scattering. Diffusion of electromagnetic waves when traversing the troposphere. See also:SCATTERING.

5680 trough. A long DEPRESSION of the SEA FLOOR normally wider and shallower than a TRENCH.The lowest part of a WAVE between successive WAVE CRESTS. Also called wave trough.Region of the ATMOSPHERE in which the pressure is low relative to the surrounding regions at the same level. It is

represented on a WEATHER CHART by a system of nearly parallel ISOBARS or CONTOURS, of approximately V-shape, which are concave towards a DEPRESSION.

5681 trough compass. See DECLINATOIRE.

5682 true (adj.). Related to TRUE NORTH.Actual, as contrasted with fictitious, as true SUN.Related to a fixed point, either on the EARTH or in space, as TRUE WIND; in contrast with relative, which is

related to a moving point.Corrected, as TRUE ALTITUDE.

5683 true altitude. See ALTITUDE.

5684 true azimuth. See AZIMUTH.

5685 true bearing. See BEARING.

5686 true course. See COURSE.

5687 true horizon. See HORIZON.

5688 true meridian. See MERIDIAN.

5689 true north. See NORTH.

5690 true solar time. See TIME: APPARENT SOLAR.

5691 true sun. See SUN: APPARENT.

259

5692 true wind. WIND VECTOR in relation to the EARTH's surface. For a moving object it is the vector sum of theRELATIVE WIND and the VELOCITY of the object.

5693 trumpet buoy. See BUOY: SOUND.

5694 trunnion axis. See AXIS: HORIZONTAL.

5695 tsunami. A long-period sea wave produced by a submarine EARTHQUAKE or volcanic eruption. It may travelunnoticed across the OCEAN for thousands of MILES from its point of origin. It builds up to great HEIGHTS overshoal water. Also called tunami, TIDAL WAVE, seismic sea wave.

5696 tubular compass. See COMPASS.

5697 tufa. A chemical SEDIMENTARY ROCK composed of calcium carbonate or silica, precipitated from percolatingground water or from a SPRING.

5698 tuff. A ROCK formed of fragments of volcanic origin, DUST, ashes, etc., thrown out of a volcano during an eruption,frequently hardened to a ROCK. Hence tuff-cone, a volcanic cone built up of such material.

5699 tuff-cone. See TUFF.

5700 tunny net. A net built at sea for catching tunnies.

5701 turbidite. Turbidity current deposits, characterized by both vertically and horizontally graded bedding.

5702 turbidity. Reduced water clarity resulting from the presence of suspended matter. Water is considered turbid when itsload of suspended matter is visibly conspicuous, but all waters contain some suspended matter and therefore are turbid.

Reduced transparency of the ATMOSPHERE, caused by absorption and SCATTERING of RADIATION by solidor liquid particles, other than CLOUDS, held there in suspension.

5703 turbidity current. See CURRENT.

5704 turbulence. A state of fluid flow in which the instantaneous VELOCITIES exhibit irregular and apparently randomfluctuations, so that in practice only statistical properties can be recognized and submitted to analysis.

Superimposed on the mean motion of the air, an agitation composed of air motions which are uncoordinated and in astate of continuous change.

5705 turbulent flow. A FLOW characterized by TURBULENCE.

5706 turning basin. An enlargement of a channel for turning vessels. Also called manoeuvering basin.

5707 turning buoy. A BUOY marking a turn, as in a CHANNEL.

5708 turning point. In LEVELLING, a point on which both a minus sight (FORESIGHT) and a plus sight (BACKSIGHT)are taken on a line of DIRECT LEVELLING.

In TOPOGRAPHIC SURVEYS, any point on which the rod is held while the instrument (PLANE TABLE) ismoved to another STATION.

In a TRAVERSE, any point of junction of two LEGS. Also called traverse point.

5709 turn of the tide. See CHANGE OF TIDE.

5710 twilight. The periods of incomplete darkness following SUNSET (evening twilight) or preceding SUNRISE (morningtwilight). Twilight is designated as civil, nautical, or astronomical, as the darker limit occurs when the centre of theSUN is 6°, 12°, or 18° below the CELESTIAL HORIZON, respectively.

5711 twilight stars. STARS suitably placed for ASTRONOMICAL OBSERVATION during morning or eveningTWILIGHT.

5712 two-way route. A route within defined limits inside which two-way traffic is established, aimed at providing safepassage of ships through waters where navigation is difficult or dangerous.

5713 tyfon. See TYPHON.

260

5714 type. In printing (TYPOGRAPHY) a metal block having a raised letter or figure which, when inked, is used to make animpression on paper or other material. Type can also be in the form of negative or positive stripping film.

5715 type face. A type of print, such as Roman, Egyptian, Caslon, etc.

5716 type of tide. The characteristic feature of the TIDE (TIDAL CURRENT) determined from the combination of itsdiurnal and semidiurnal components. TIDES are classified as SEMIDIURNAL, MIXED and DIURNAL, but there areno sharply defined limits separating the groups.

5717 typhon. A diaphragm HORN which operates under the influence of compressed air or steam. Called SIREN inCanadian terminology. Also called tyfon.

5718 typhoon. Name given to TROPICAL CYCLONES of the China Sea and, more generally, of the north-west Pacific.

261

U

5719 ultra high frequency (UHF). See FREQUENCY: RADIO.

5720 ultrashort wave. See WAVE.

5721 ultrasonic (adj.). Having a FREQUENCY above the audible RANGE.

5722 ultrasonic depth finder. See ECHO SOUNDER.

5723 ultrasonics. The science and technology relating to SOUND WAVES of FREQUENCIES above the audible RANGE.

5724 ultraviolet (adj.). Having a FREQUENCY immediately beyond the violet end of the visible SPECTRUM. Said ofRAYS of shorter WAVE LENGTH than visible light but longer than X-RAYS.

5725 uncontrolled mosaic. A MOSAIC composed of uncorrected PRINTS, the detail of which has been matched from printto print without GROUND CONTROL or other ORIENTATION.

5726 uncovered (adj.). Above water. The opposite is SUBMERGED.

5727 uncovers. See COVERS AND UNCOVERS.

5728 undercliff. A TERRACE or lower CLIFF formed by a LANDSLIDE. Also written as two words.

5729 undercurrent. A CURRENT below the surface, particularly one flowing in a direction or at a SPEED differing fromthe SURFACE CURRENT.

5730 underground mark. A STATION MARK set and plumbed below the centre of a SURFACE MARK and separatedtherefrom so as to preserve the STATION in case of accident to the SURFACE MARK. Also called subsurface mark.

5731 underkeel clearance. The distance between the lowest point of the ship's hull, normally some point on the keel, and thesea bottom.

5732 underkeel allowance. The estimated minimum UNDERKEEL CLEARANCE in a given channel accounting for theship's squat, movement due to swell, tide height etc.

5733 undertow. A seaward FLOW near the BOTTOM of a sloping BEACH.The subsurface return by gravity flow of the water carried up on SHORE by WAVES or BREAKERS. See also

BACKRUSH.

5734 underwater acoustics. A special field of technical acoustics dealing with the physics of sound propagation in thewater.

5735 underwater ice. See ANCHOR ICE.

5736 underwater navigation. See NAVIGATION.

5737 underwater photogrammetry. See PHOTOGRAMMETRY.

5738 underwater position fixing. A method of position fixing for surface vessels, submersibles or towed devices in relationto a network of underwater acoustic beacons on the seabed with known relative positions.

5739 underwater sound projector. A TRANSDUCER used to produce SOUND in water. The conversion from electricalenergy to SOUND is achieved through the use of either the piezoelectric or magnetostrictive effect. SeeMAGNETOSTRICTION and PIEZOELECTRICITY.

5740 underway (or under way) (adv.). Without MOORINGS; not secured in any way to the ground or a WHARF.In motion, particularly the start of such motion after a standstill.

5741 undulating (adj.). Having the form of WAVES or SWELLS, as undulating LAND.

5742 undulation. A continuously propagated motion to and from, in any fluid or elastic medium, with no permanenttranslation of the particles themselves.

262

5743 undulation of the geoid. The distance between the mathematical REFERENCE SPHEROID and the GEOID.

5744 unidirectional antenna. See ANTENNA.

5745 unifilar magnetometer. See MAGNETOMETER.

5746 uniform flow. Any CURRENT in which neither CONVERGENCE nor DIVERGENCE is present.

5747 unit magnetic pole. In magnetic theory, a fictitious entity analogous to a unit electric charge in electrostatic theory.

5748 United Nations (UN). The major international organization established to maintain international peace and security, todevelop friendly relations among nations, to achieve cooperation in solving international problems.

5749 United Nations Conference on Trade and Development (UNCTAD). Helps to restructure patterns of internationaltrade in order to enable developing countries to play their part in world commerce.

5750 United Nations Development Programme (UNDP). An UN agency which registers organizations and institutionslocated in developing countries which have the capacity for cooperative technical activities with organizations in otherdeveloping nations. The subject matter includes education and training courses, research and technological developmentactivities, consulting and expert services, and scientific and technical information and used primarily by institutions andgovernments in developing countries.

5751 United Nations Educational, Scientific, and Cultural Organization (UNESCO). Organized in 1945 to promotecollaboration among nations in education, science, and culture; UNESCO is the parent agency for theINTERGOVERNMENTAL OCEANOGRAPHIC COMMISSION.

5752 universal drafting machine. See PROTRACTOR: PARALLEL MOTION.

5753 universal level. See LEVEL: SPIRIT.

5754 Universal Time (UT). TIME as defined by the rotational motion of the EARTH and determined from the apparentdiurnal motions which reflect this ROTATION. Because of variations in the rate of rotation, Universal Time is notrigorously uniform. Also called Greenwich mean time.

5755 Universal Time 0 (UT0). The uncorrected TIME of the EARTH's ROTATION as measured by the TRANSIT ofSTARS across the observer's MERIDIAN. This ROTATION is referred to a fiducial mark on the ECLIPTIC whichapproximates the position of the MEAN SUN.

5756 Universal Time 1 (UT1). UT0 corrected for polar motion.

5757 Universal Time 2 (UT2). UT1 corrected for seasonal variations in the EARTH's ROTATION.

5758 Universal Time Coordinated (UTC). The time scale that is available from most broadcast time signals. It differs fromINTERNATIONAL ATOMIC TIME by an integral number of seconds. UTC is maintained within 1 second of UT1 bythe introduction of 1-second steps (leap seconds) when necessary, normally at the end of December. DUT1, anapproximation to the difference UT1 minus UTC, is transmitted in code on broadcast time signals.

5759 Universal Transverse Mercator (UTM) grid. See GRID.

5760 unprotected reversing thermometer. See THERMOMETER.

5761 unrecoverable station. See STATION.

5762 unsurveyed area. An area where survey data is non-existent or very poor.

5763 unsurveyed clearance depth. The estimated depth over an unsurveyed wreck or obstruction over which the exact depthis unknown but which is considered to have a safe clearance to the depth shown.

5764 upland. The higher GROUND of a region contrasted with the VALLEYS and PLAINS. Used in contrast toLOWLAND.

5765 uplifted reef. A CORAL REEF exposed above water level.

263

5766 upper branch. That half of a MERIDIAN or CELESTIAL MERIDIAN from POLE to POLE which passes through aplace or its ZENITH.

5767 upper culmination. Upper TRANSIT.

5768 upper limb. See LIMB.

5769 upper limit of navigability. The character of a river will, at some point along its length, change from navigable tonon-navigable. Very often that point will be at a major fall or rapids, or other place where there is a marked decrease inthe navigable capacity of the river. The upper limit will therefore often be the same point traditionally recognized as thehead of navigation, but may, under some of the tests described above, be at some point yet farther upstream.

5770 upper transit. See MERIDIAN TRANSIT.

5771 uprush. The rush of water up onto the BEACH following the breaking of a WAVE. See RUN-UP.

5772 upstream (adj. & adv.). Toward the SOURCE of a STREAM. The opposite is DOWNSTREAM.

5773 upwelling. The process by which water rises from a lower to a higher DEPTH, usually as a result of DIVERGENCEand offshore CURRENTS. See SINKING.

5774 upwind (adj. & adv.). In the direction from which the WIND is blowing. The opposite is DOWNWIND.

264

V

5775 vacuum lead. A CORER which makes use of HYDROSTATIC PRESSURE to force a tube into the BOTTOMSEDIMENT.

5776 valley. A long DEPRESSION or hollow in the LAND lying between HILLS or stretches of high GROUND and usuallyhaving a RIVER or STREAM flowing along the bottom.

5777 valley: median. See MEDIAN VALLEY.

5778 valley: submarine. A relatively shallow wide DEPRESSION with gentle SLOPES, the bottom of which grades conti-nuously downward. This term is used for features that do not have canyon-like characteristics in any significant part oftheir extent.

5779 valley line. See TALWEG.

5780 vane. A device to indicate the direction from which the WIND blows. Also called weather vane, wind vane. SeeANEMOMETER.

A SIGHT on an instrument used for observing BEARINGS, as on a PELORUS, BEARING CIRCLE, etc. Alsocalled sight (or sighting) vane.

A device to indicate the direction toward which the CURRENT flows.

5781 vanishing line. In PHOTOGRAMMETRY, the straight line on a PHOTOGRAPH upon which lie all the VANISHINGPOINTS of all systems of parallel lines parallel to one plane.

5782 vanishing point. In PHOTOGRAMMETRY, the IMAGE in the plane of a PHOTOGRAPH, of the point toward whicha system of parallel lines in the object space converges.

5783 vanishing tide. See TIDE.

5784 variate. A variate is a quantity that may take on any of the values of a specified set with a specified relative frequencyor probability often known as random variable. It is to be regarded as defined not merely by a set of permissible values,but by an associated frequency (probability) function expressing how often those values appear in a given situation.

5785 variation. The angle between the MAGNETIC and GEOGRAPHICAL MERIDIANS at any place, expressed inDEGREES east or west to indicate the direction of MAGNETIC NORTH from TRUE NORTH. Also called magneticvariation, or magnetic declination.

5786 variation: grid. Angular difference in direction between GRID NORTH and MAGNETIC NORTH. It is measured eastor west from GRID NORTH. Also called grivation, or grid magnetic angle.

5787 variation of the pole. A wandering of the POLES of the FIGURE OF THE EARTH with respect to its POLES ofROTATION.

5788 variometer. An instrument for comparing magnetic forces, especially of the EARTH's MAGNETIC FIELD.

5789 varsol. An oil used as the liquid in some modern MAGNETIC COMPASSES.

5790 varve. A sedimentary DEPOSIT, bed, or lamination deposited in one season. It is usually distinguished by colour orcomposition and used as an index to changes in the depositional environment.

5791 vectograph. A stereoscopic PHOTOGRAPH composed of two superimposed IMAGES which polarize light in planes90o apart. When these IMAGES are viewed through polaroid spectacles with the polarization axes at right angles, animpression of depth is obtained.

5792 vector. Direct connection between two points, either given as two sets of coordinates (points), or by direction anddistance from one given set of coordinates; or a point in a vector space defined by one set of coordinates relative to theorigin of a coordinate system.

5793 vector airborne magnetometer. See MAGNETOMETER.

5794 vector data presentation. Method of representing image features digitally by points, lines and polygons given throughtheir coordinates and appropriate code(s).

265

5795 vectorial angle. See COORDINATES: PLANE POLAR.

5796 veering wind. Clock-wise change of WIND direction, in either hemisphere.

5797 vein. A narrow LEAD or lane in PACK ICE.In mineralogy, a crack or fissure in the ROCKS of the EARTH's CRUST in which highly heated waters from below

have deposited from solution crystalline minerals (especially vein quartz) and, under certain circumstances, metallicminerals of economic importance.

5798 velocimeter. An instrument used for the 'in situ' measurement of the SPEED of SOUND in the SEA and other naturalwaters.

5799 velocity. A vector quantity equal to SPEED in a given direction.

5800 velocity of sound. See SOUND VELOCITY.

5801 veneer. A thin LAYER of SEDIMENT covering a rocky surface.

5802 vent. The orifice through which molten LAVA reaches the CRATER of a volcano. Eventually it may become choked asthe LAVA solidifies to form a plug or neck.

5803 Venturi tube. The trade name for a short tube of smaller diameter in the middle than at the ends. When a fluid flowsthrough such a tube, the pressure decreases as the diameter becomes smaller, the amount of the decrease beingproportional to the SPEED of FLOW and the amount of restriction.

5804 vernal equinox. See EQUINOX.

5805 vernier. A short, auxiliary SCALE situated alongside the graduated SCALE of an instrument, by means of whichfractional parts of the smallest division of the primary SCALE can be measured accurately.

5806 vertex (pl. vertices). The highest point; the vertices of a GREAT CIRCLE are the points nearest the POLE. See APEX.

5807 vertical (adj.). In the DIRECTION OF GRAVITY; perpendicular to the plane of the HORIZON.

5808 vertical (n.). The direction in which the force of GRAVITY acts. A vertical line, plane, etc.

5809 vertical axis. See AXIS.

5810 vertical circle. See CIRCLE.

5811 vertical clearance. The minimum vertical space available for passage.

5812 vertical collimator. See COLLIMATOR.

5813 vertical control. See CONTROL.

5814 vertical control datum. See DATUM.

5815 vertical deflection. See DEFLECTION OF THE VERTICAL.

5816 vertical dilution of precision. See DILUTION OF PRECISION.

5817 vertical intensity of the earth's magnetic field. The strength of the vertical component of the EARTH's MAGNETICFIELD.

5818 vertical light(s). See LIGHT.

5819 vertical parallax. See PARALLAX: Y-.

5820 vertical photograph. See PHOTOGRAPH.

5821 vertical plane. See PLANE.

266

5822 very close pack ice. SEA ICE the CONCENTRATION of which is practically 10/10ths with little if any water present.

5823 very high frequency (VHF). See FREQUENCY: RADIO.

5824 very low frequency (VLF). See FREQUENCY: RADIO.

5825 very open pack ice. SEA ICE the CONCENTRATION of which ranges between 1/10ths 3/10ths.

5826 vessel traffic service (VTS). Any service implemented by a relevant authority primarily designed to improve safety andefficiency of traffic flow and the protection of the environment. It may range from simple information messages, toextensive organization of the traffic involving national or regional schemes.

5827 viaduct. A structure consisting of a series of arches or towers supporting a roadway, waterway, etc., across adepression, etc.

5828 vibrating needle. A MAGNETIC NEEDLE used in compass adjustment to find the relative intensity of the horizontalcomponents of the EARTH'S MAGNETIC FIELD and the MAGNETIC FIELD at the compass location. Also calledhorizontal force instrument. See COMPENSATION OF MAGNETIC COMPASS.

5829 vibration. Periodic motion of an elastic body or medium in alternately opposite directions from EQUILIBRIUM;OSCILLATION.

The motion of a vibrating body during one complete CYCLE; two OSCILLATIONS.

5830 vigia. A PINNACLE, ROCK, or SHOAL the existence or POSITION of which is doubtful, or a warning note to thiseffect on a CHART.

5831 vignetting. In PHOTOGRAPHY, a gradual reduction in DENSITY of parts of a photographic IMAGE due to thestopping of some of the RAYS entering the LENS.

5832 violent storm. WIND with a speed between 56 and 63 KNOTS (Beaufort scale wind force 11).

5833 virtual image. See IMAGE.

5834 visibility. The extreme horizontal distance at which prominent objects can be seen and identified by the unaided eye.

5835 visibility: flight. The VISIBILITY forward from the cockpit of an aircraft in flight.

5836 visibility: meteorological. The greatest distance at which a black object of suitable dimensions can be seen andrecognized against the horizon sky, or, in the case of night observations, could be seen and recognized if the generalillumination were raised to the normal daylight level.

5837 visibility: radius of. The radius of a circle limiting the area in which an object can be seen under specified conditions.

5838 visible horizon. See HORIZON: APPARENT.

5839 visual display unit (VDU). See DISPLAY.

5840 visual line of position. A LINE OF POSITION determined by visual OBSERVATION of a LANDMARK, AID TONAVIGATION, etc.

5841 voe. An INLET, BAY, or CREEK.

5842 volcanic ash. Uncemented pyroclastic material consisting of fragments mostly under 4 millimetres in diameter. Coarseash is 1/4 to 4 millimetres in grain size; fine ash is below 1/4 millimetre. A constituent of some marine SEDIMENTS.

5843 volt. The unit of potential difference or electromotive force in the SI SYSTEM.

5844 vulgar (or common) establishment. See ESTABLISHMENT.

267

W

5845 wadi. A watercourse that is permanently dry or dry except for the rainy season.

5846 wake. The path of disturbed water or air behind a craft in motion.

5847 waning moon. See PHASES OF THE MOON.

5848 want of correspondence. See PARALLAX: Y-.

5849 warm front. Any non-occluded FRONT which moves in such a way that warmer air replaces colder air.

5850 warm pool. A body of warm water entirely surrounded by cold water.

5851 warning: gale. Meteorological message intended to warn those concerned of the occurrence or expected occurrence ofa WIND of Beaufort force 8 or 9 over a specified area.

5852 warning: hurricane. Meteorological message intended to warn those concerned of the occurrence or expectedoccurrence of a WIND of BEAUFORT force 12 over a specified area.

Tropical cyclone warning relating to a HURRICANE.

5853 warning: navigation. See NAVIGATIONAL WARNING.

5854 warning: storm. Meteorological message intended to warn those concerned of the occurrence or expected occurrenceof a WIND of BEAUFORT force 10 or 11 over a specified area.

More generally, forecast of severe weather conditions.

5855 warping buoy. A BUOY so located that lines to it can be used for the movement of ships.

5856 wash. The visible or audible motion of agitated water; especially that caused by the passage of a vessel.

5857 washovers. Small DELTAS formed on the lagoon side of a BAR separating the LAGOON from the OPEN SEA.STORM WAVES breaking over the BAR deposit SEDIMENT on the lagoon side in the form of DELTAS. Also calledwave deltas.

5858 watch. A small TIMEPIECE of a size convenient to be carried on the person.

5859 watch: comparing. A HACK WATCH, particularly one having its ERROR determined by comparison with aCHRONOMETER.

5860 watch: hack. A WATCH used for timing OBSERVATIONS of CELESTIAL BODIES regulating ship's CLOCKS, etc.

5861 watch: stop. A WATCH that can be started, stopped, and reset at will, to indicate elapsed time.

5862 watch buoy. See STATION BUOY.

5863 water bottle. A device for collecting WATER SAMPLES from varying DEPTHS. They are lowered in an 'open' stateto the required DEPTH and can be closed before being hauled up by means of a MESSENGER. Water bottles can beoperated in series that is with more than one bottle on the wire rope so that WATER SAMPLES can be taken at anumber of DEPTHS on the same CAST. Also called water specimen cup.

5864 water bottle: insulated (or insulating). A WATER BOTTLE specially designed for measuring temperatures andcollecting WATER SAMPLES at moderate DEPTHS. This type of WATER BOTTLE is insulated to maintain thetemperature constant while being brought to the surface.

5865 water bottle: reversing. A WATER BOTTLE specially designed for use in deep sea waters. The bottle is lowered bywire and can be closed watertight at any desired DEPTH, by means of a MESSENGER allowed to slide down the wireand strike the reversing mechanism making the bottle turn upside down. Thus the enclosed WATER SAMPLE is notcontaminated by water at higher levels or lost by leakage after the bottle is brought on board. REVERSINGTHERMOMETERS are attached to reversing water bottles. See EKMAN BOTTLE and NANSEN BOTTLE.

5866 water colour. See SEA WATER: COLOUR OF.

268

5867 water column. A vertical continuum of water from sea surface to sea-bed.

5868 water content. A ratio; 100 multiplied by the weight of water in a bottom sediment sample divided by the weight of thedried sample, expressed as a percentage.

5869 watercourse. A STREAM of water, a RIVER or a BROOK; also an artificial CHANNEL for the conveyance of water.

5870 water front. LAND at the end of a STREAM, HARBOUR, etc. The part of a city or town on such LAND; WHARF ordock area.

5871 water levelling. See LEVELLING.

5872 waterline. The line marking the junction of LAND and water.The line along which the surface of the water touches a vessel's hull.Also written as two words.

5873 water-parting. A summit or boundary line separating the drainage districts of two STREAMS or COASTS; a divide orWATERSHED.

5874 water sample. A portion of water brought up from a certain DEPTH to determine its composition, or physicalproperties viz: temperature, SALINITY and DENSITY.

5875 water sampler. Any device for obtaining a WATER SAMPLE. Also called water sampling device. See WATERBOTTLE.

5876 watershed. WATER-PARTING.The whole region or area contributing to the supply of a RIVER or LAKE; drainage area; catchment area or BASIN.

5877 water signal. See SIGNAL.

5878 water sky. Typical dark patches and strips on low CLOUDS over a water area enclosed in ICE or behind its edge.

5879 water specimen cup. See WATER BOTTLE.

5880 water vapor. Water in the gaseous phase.

5881 waterway. A line of water (RIVER, CHANNEL, etc.) which can be utilized for communication or transport.

5882 watt. The unit of power in the SI system.

5883 wave. A disturbance which moves through or over the surface of the medium with SPEED dependent upon theproperties of the medium.

A ridge deformation, or UNDULATION of the surface of a liquid.

5884 wave: capillary. A WAVE whose VELOCITY of propagation is controlled primarily by the surface tension of theliquid in which the WAVE is travelling. Water waves of length less than one INCH are considered to be capillarywaves.

5885 wave: carrier. A RADIO WAVE used as a vehicle for conveying intelligence, generally by MODULATION. Alsocalled carrier.

5886 wave: cellular. System of SURFACE WAVES in which the OSCILLATION of water is limited as by solidBOUNDARIES. The PERIOD is the same in adjacent cells.

5887 wave: compressional. A WAVE in an elastic medium which causes an element of the medium to change its volumewithout undergoing rotation.

5888 wave: continuous. A series of WAVES of like AMPLITUDE and FREQUENCY. See WAVE: MODULATED,PULSE.

5889 wave: cycloidal. A very steep, symmetrical WAVE whose CREST forms an angle of 120 DEGREES. The wave formis that of a cycloid. A TROCHOIDAL WAVE of maximum steepness.

269

5890 wave(s): damped. WAVE(S) the AMPLITUDE of which becomes progressively smaller.

5891 wave: direct. A subdivision of the space wave component of the electromagnetic GROUND WAVE. This WAVE ispropagated directly through space, i.e., it follows the shortest path between TRANSMITTER and RECEIVER.

5892 wave: edge. An ocean wave travelling parallel to a COAST, with CRESTS normal to the COASTLINE. Such a WAVEhas a HEIGHT that diminishes rapidly seaward and is negligible at a distance of one WAVE LENGTH offshore.

5893 wave: electromagnetic. WAVE(S) of associated electric and magnetic fields characterized by variations of the fields.The electric and magnetic fields are at right angles to each other and to the direction of propagation.

An electromagnetic wave is coherent if the phase-time relationship is constant throughout the propagation path.

5894 wave: forced. A WAVE generated and maintained by a continuous force, in contrast to a FREE WAVE that continuesto exist after the generating force has ceased to act.

5895 wave: free. A WAVE that continues to exist after the generating force has ceased to act, in contrast with a FORCEDWAVE that is generated and maintained by a continuous force.

A free wave on a water surface is one created by a sudden impulse, thereafter influenced only by friction, thedimensions of the BASIN, and the dispersive character of the water medium it moves in. Most ocean surface wavesexcept TIDAL WAVES are free waves. See OSCILLATION.

5896 wave: gravity. A WAVE whose VELOCITY of propagation is controlled primarily by GRAVITY. Water waves oflength greater than 2 INCHES are considered gravity waves.

5897 wave: ground. That portion of a RADIO WAVE in proximity to and affected by the GROUND, being somewhatrefracted by the lower ATMOSPHERE, diffracted by the surface of the EARTH and retarded by the resistivity of theEARTH. Such a WAVE travels more or less parallel to the surface of the EARTH. See WAVE: SKY.

5898 wave: indirect. Any WAVE which arrives by an indirect path having undergone an abrupt change of direction byREFRACTION or REFLECTION. See WAVE: SKY.

5899 wave: internal. A WAVE that occurs within a fluid whose DENSITY changes with DEPTH, either abruptly at a sharpsurface of discontinuity (an INTERFACE) or gradually. Also called boundary wave.

5900 wave: longitudinal. A WAVE in which the direction of displacement of each particle in the medium is perpendicular tothe WAVE FRONT.

5901 wave: Love (or Q-wave). A TRANSVERSE WAVE propagated along the BOUNDARY of two elastic media whichboth have rigidity, i.e. both media must be capable of propagating TRANSVERSE WAVES.

A surface seismic wave in which the particles of an elastic medium vibrate transverse to the direction of theWAVE's travel, with no vertical component.

5902 wave: modulated. A WAVE which varies in some characteristic in accordance with the variations of aMODULATING WAVE. See WAVE: CONTINUOUS.

5903 wave: modulating. A WAVE which modulates a CARRIER WAVE.

5904 wave: oscillatory. A WAVE in which each individual particle oscillates about a point with little or no permanentchange in position. The term is commonly applied to progressive oscillatory waves in which only the form advances, theindividual particles moving in closed ORBITS, as ocean waves in deep waters. Distinguished from a WAVE OFTRANSLATION.

5905 wave(s): periodic. WAVES which are repeated at regular intervals.

5906 wave: pressure. See PRESSURE WAVE.

5907 wave: progressive. A WAVE which is manifested by the progressive movement of the wave form. The opposite is aSTANDING WAVE. Also called travelling wave.

5908 wave: propagation. The travel of waves through or along a medium.

5909 wave(s): radio. ELECTROMAGNETIC WAVES of FREQUENCIES generally higher than those of audible SOUNDWAVES but lower than those of heat and light waves. Also called hertzian waves.

270

5910 wave: reflected. A WAVE that has had its direction of motion changed by REFLECTION.

5911 wave: refracted. A WAVE that has had its direction of motion changed by REFRACTION.

5912 wave: short. A RADIO WAVE shorter than those of the standard broadcasting band.In OCEANOGRAPHY, WAVES under conditions where the relative DEPTH (water depth/wave length) is greater

than 0.5 and where the PHASE VELOCITY is independent of water depth, but dependent upon WAVE LENGTH.

5913 wave: sky. An indirect RADIO WAVE which travels from the transmitting ANTENNA into the sky, where theIONOSPHERE bends it back toward the EARTH. Also called ionospheric wave. See WAVE: GROUND.

5914 wave: solitary. A WAVE OF TRANSLATION consisting of a single CREST rising above the undisturbed liquid level,without any accompanying TROUGH, in contrast with a WAVE TRAIN.

5915 wave: sound. A mechanical disturbance advancing with finite VELOCITY through an elastic medium and consisting oflongitudinal displacements of the ultimate particles of the medium, that is, consisting of compressional and rare-factional displacements parallel to the direction of advance of the disturbance; a LONGITUDINAL WAVE. Also calledsonic wave, or acoustic wave.

5916 wave: spherical. A WAVE whose wave front surfaces are concentric spheres. Such WAVES propagate from a pointsource.

5917 wave: standing. A type of WAVE in which the surface of the water oscillates vertically between fixed points, calledNODES, without progression. The points of maximum vertical RISE and fall are called antinodes or LOOPS. At theNODES, the underlying water particles exhibit no vertical motion but maximum horizontal motion. At the antinodes theunderlying water particles have no horizontal motion and maximum vertical motion. They may be the result of twoequal progressive WAVE TRAINS travelling through each other in opposite directions. Also called stationary wave.

5918 wave: storm. A wind-generated sea surface wave of great HEIGHT. See STORM SURGE.

5919 wave: surface. A progressive GRAVITY WAVE in which the disturbance (that is, the particle movement in the fluidmass as well as the surface movement) is confined to the upper limits of a body of water. Strictly speaking this termapplies to those progressive GRAVITY WAVES whose celerity depends only upon the WAVE LENGTH.

5920 wave: tidal. The wave motion of the TIDES.In popular usage, any unusually high (and therefore destructive) water level along a SHORE. It usually refers to

either a STORM SURGE or TSUNAMI.

5921 wave: tide. See TIDE.

5922 wave: transitional water. A progressive GRAVITY WAVE in water whose DEPTH is less than 1/2 but more than1/25 the WAVE LENGTH. Often called shallow water wave.

5923 wave: transverse. A WAVE in which the direction of propagation of the WAVE is normal to the displacements of themedium, for example, a vibrating string. The GRAVITY WAVE in which fluid parcels move in circular ORBITS is anexample of a mixed transverse-longitudinal wave.

5924 wave: trochoidal. A progressive OSCILLATORY WAVE whose form is that of a prolate cycloid or trochoid. It isapproximated by WAVES of small AMPLITUDES.

5925 wave: ultrashort. A RADIO WAVE shorter than 10 METRES.

5926 wave: wind. A WAVE resulting from the action of WIND on a water surface. See FETCH.

5927 wave age. The state of development of a wind-generated sea surface wave, conveniently expressed by the ratio of wavespeed to wind speed. WIND SPEED is usually measured at about 8 METRES above STILL WATER LEVEL.

5928 wave base. The greatest DEPTH at which SEDIMENT on the SEA FLOOR can just be stirred by the oscillating water.

5929 wave crest. The highest part of a WAVE.

271

5930 wave deltas. See WASHOVERS.

5931 wave direction. The direction from which WAVES are moving.

5932 wave forecasting. The theoretical determination of future wave characteristics; usually from observed or predictedmeteorological or oceanographic PHENOMENA.

5933 waveform. The graphical representation of a WAVE, showing variation of AMPLITUDE with time. Also written astwo words.

5934 wave front. The leading side of a WAVE.

5935 wave generation. The creation of WAVES by natural or mechanical means.In WAVE FORECASTING the growth of WAVES caused by a WIND blowing over a water surface for a certain

period of time. The area involved is called the generating area or FETCH.

5936 wave group. A series of WAVES in which the WAVE DIRECTION, WAVE LENGTH, and WAVE HEIGHT varyonly slightly.

5937 wave height. The vertical distance between a WAVE CREST and the preceding WAVE TROUGH.

5938 wave interference. The PHENOMENON which results when WAVES of the same or nearly the same FREQUENCYare superposed. It is characterized by a spatial or temporal distribution of AMPLITUDE of some specified characteristicdiffering from that of the individual superposed WAVES.

5939 wave length. The distance between corresponding points of two successive PERIODIC WAVES in the direction ofpropagation, for which the OSCILLATION has the same PHASE. Unit of measurement in METRES. Also written asone word.

5940 wavemeter. An instrument for measuring WAVES. One used for ocean waves usually measures HEIGHT andPERIOD; one used for ELECTROMAGNETIC or SOUND WAVES usually measures length. Also written as twowords.

5941 wave of translation. A WAVE in which the individual particles of the medium are shifted in the direction of wavetravel, as ocean waves in shoal waters. Distinguished from an OSCILLATORY WAVE.

5942 wave period. The time, in SECONDS, required for a WAVE CREST to traverse a distance equal to one WAVELENGTH.

5943 wave refraction. See REFRACTION OF WATER WAVES.

5944 wave spectrum. In ocean wave studies, a graph showing the distribution of WAVE HEIGHTS with respect to FRE-QUENCY in a wave record.

5945 wave train. A series of WAVES moving in the same direction. See WAVE: SOLITARY.

5946 wave trough. See TROUGH.

5947 wave velocity. The SPEED at which the individual wave form advances.A vector quantity that specifies the SPEED and direction with which a SOUND WAVE travels through a medium.

5948 waxing moon. See PHASES OF THE MOON.

5949 way. The motion of a vessel through the water.

5950 weather (adj.). Of or pertaining to the direction from which the WIND is blowing, as opposed to LEE.

5951 weather. State of the ATMOSPHERE at a given time, as defined by the various meteorological elements.

5952 weather analysis. Operation of studying the general state of the ATMOSPHERE over a region, with respect toSYNOPTIC CHARTS. Also called synoptic analysis.

5953 weather buoy. An Ocean Data Acquisition System (ODAS) used for collecting oceanographic or meteorologicalinformation. ODAS buoys are up to 40 feet in diameter.

272

5954 weather forecast. Statement of the expected meteorological conditions for a specified period, and for specified area orportion of air space.

5955 weather chart (or map). See CHART.

5956 weathered ice. Hummocked POLAR ICE subjected to weathering which has given the HUMMOCKS and PRESSURERIDGES a rounded form. If the weathering continues, the surface may become more and more even.

5957 weather ship. See OCEAN STATION VESSEL.

5958 weather shore. SHORE that is to windward of a vessel. See LEE SHORE.

5959 weather side. The side of a vessel toward which, or the side of a CHANNEL from which the WIND is blowing.

5960 weather signal. See SIGNAL.

5961 weather tide. See TIDE: WINDWARD.

5962 weather vane. See VANE.

5963 weber. The unit of magnetic flux in the SI SYSTEM.

5964 wedge. In optics, a refracting prism of very small deviation such as those used in the EYEPIECE of someSTEREOSCOPES.

In METEOROLOGY, term synonymous with RIDGE, though usually applied to a fast-moving RIDGE between twoDEPRESSIONS or TROUGHS.

5965 weight. The force with which a body is attracted by GRAVITY.Any block or mass of material used for its heaviness.The relative value of an OBSERVATION, source, or quantity when compared with other OBSERVATIONS, sour-

ces, or quantities of the same or related quantities. The value determined by the most reliable method is assigned thegreatest weight.

5966 weighted mean. A value obtained by multiplying each of a series of values by its assigned WEIGHT and dividing thesum of those products by the sum of the WEIGHTS.

5967 weir. A DAM erected across a RIVER to raise the level of the water.A fence of stakes set in a RIVER or along the SHORE as a trap for fish.The word is now restricted to smaller works, the larger are called DAMS.

5968 well: appraisal. A borehole drilled to evaluate an oil field or gas field.

5969 well: development. A borehole drilled, usually from a production platform, after an oil field or gas field has beenjudged capable of economic production.

5970 well: deviated. A borehole drilled at an oblique angle to evaluate or remove oil or natural gas reserves not directlybelow the drilling structure.

5971 well: offshore. A borehole that produces or is capable of producing oil or natural gas.

5972 well: plugged. A borehole which has been sealed with a plug and permanently abandoned.

5973 well: production. A borehole, producing oil or natural gas, which is covered by a seabed installation of valves andpipelines for the controlled removal of the product. See also SUB-SEA COMPLETION.

5974 well: suspended. A borehole that has been temporarily abandoned pending future development. An obstruction up to 5metres high may remain on the sea-floor.

5975 wellhead: offshore. See WELL: OFFSHORE.

5976 wellhead: submerged. A submarine structure projecting some distance above the seabed and capping a temporarilyabandoned or suspended oil or gas well.

273

5977 westerlies. Zone, lying between the approximate LATITUDES 35o and 65o in each hemisphere, in which the air motionis mainly from west to east, especially in the high TROPOSPHERE and low STRATOSPHERE. Near the EARTH'ssurface, the zone is particularly well marked in the southern hemisphere.

5978 westing. The distance a craft makes good to the west. The opposite is EASTING.

5979 wet-bulb thermometer. See THERMOMETER.

5980 wet compass. See COMPASS: LIQUID.

5981 wet density. The ratio of the WEIGHT of the solid particles of a BOTTOM SEDIMENT and the contained moisture toits total volume.

5982 wet dock. See DOCK.

5983 wet fog. See FOG.

5984 wetlands. The term "wetlands" means those areas that are inundated or saturated by surface or ground water at afrequency and duration sufficient to support, and that under normal circumstances do support, a prevalence ofvegetation typically adapted for life in saturated soil conditions. Wetlands generally include SWAMPS, MARSHES,BOGS and similar areas.

5985 wharf. A structure serving as a berthing place for vessels.

5986 whirlpool. Water in rapid rotary motion. See EDDY.

5987 whirlwind. General term for a small-scale rotating column of air.

5988 whitecap. A CREST of a WAVE which becomes unstable in deep water, toppling over or breaking. See BREAKINGOF WAVES.

5989 wind. Air motion relative to the EARTH'S surface. Unless it is otherwise specified, only the horizontal component isconsidered.

5990 wind circulation. The flow of air through a given area.

5991 wind direction. See DIRECTION OF WIND.

5992 wind driven current. See CURRENT.

5993 wind force. Number on a progressive scale (BEAUFORT WIND SCALE) corresponding to the effects produced byWINDS within a range of speeds.

Force exerted by the wind on a construction, object, etc.

5994 wind rose. For a given STATION and period of time, a star-shaped DIAGRAM indicating the relative frequencies ofdifferent DIRECTIONS OF WIND, sometimes also the frequencies of groups of WIND SPEEDS in different directions.

5995 wind set-up. The vertical rise in the STILL WATER LEVEL on the leeward side of a body of water caused by windstresses on the surface of the water.

The difference in STILL WATER LEVELS on the windward and the leeward sides of a body of water caused bywind stresses on the surface of water.

Synonymous with wind tide. Wind tide is usually reserved for use on the OCEAN and large bodies of water. Windset-up is usually reserved for use on RESERVOIRS and smaller bodies of water. See TIDE: METEOROLOGICAL.

5996 wind speed. Ratio of the distance covered by the air to the time taken to cover it. The 'instantaneous speed' or, morebriefly, the 'speed', corresponds to the case of an infinitely small time interval. The 'mean speed' corresponds to the caseof a finite time interval.

5997 wind tide. See WIND SET-UP.

5998 wind vane. See VANE.

5999 wind velocity. See WIND VECTOR.

274

6000 wind vector (or wind velocity). Vector drawn in the sense and direction of the WIND and of length proportional toWIND SPEED.

6001 window. In computer applications, a rectangular subdivision of the screen showing information without affecting otherparts of the screen; a method allowing to view different items of information (e.g. different data sets or graphics) at thesame time. On graphical displays a window may be selected with the cursor and subsequently be enlarged (ZOOM).

6002 windward (adj. & adv.). In the general direction from which the WIND blows; in the WIND; on the WEATHER SIDE.The opposite is LEEWARD.

6003 windward tide. See TIDE.

6004 wind wave. See WAVE.

6005 winter ice. More or less unbroken, level SEA ICE of not more than one winter's growth originating from YOUNG ICE.Thickness from 15 cm to 2 m.

6006 winter solstice. See SOLSTICE.

6007 wire: base. See BASE TAPE (or WIRE).

6008 wire: invar. A wire made of INVAR metal used in measurement of geodetic BASES.

6009 wire: sounding. See SOUNDING.

6010 wire and bar sweep. See BAR SWEEPING and WIRE DRAG SWEEP.

6011 wire angle. The angle measured between the SOUNDING WIRE, or the oceanographic wire, and the VERTICAL.

6012 wire angle indicator. A device used to measure the angle of the SOUNDING WIRE, or the oceanographic wire, fromthe VERTICAL. It consists of a PROTRACTOR with a weighted plumb arm. The indicator is suspended from the wireat two points and the plumb arm points to the WIRE ANGLE. Also called inclinometer.

6013 wire drag. An apparatus for surveying rocky areas where normal sounding methods are insufficient to insure thediscovery of all existing OBSTRUCTIONS, PINNACLES, ROCKS, etc., above a given depth or for determining theleast DEPTH of an area. It consists essentially of a buoyed wire towed at the desired DEPTH by two LAUNCHES.Often shortened to drag. See DRAG (v.t.).

6014 wire drag survey. See SURVEY.

6015 wire drag sweep. See SURVEY: WIRE DRAG.

6016 wireless. See RADIO.

6017 wire sounding. See SOUNDING.

6018 witness mark. A MARK placed at a known distance and direction from a property corner, BASE TERMINAL, orsurvey STATION, to aid in its recovery and identification.

6019 Wollaston current meter. A CURRENT METER similar but much larger and heavier than the EKMAN CURRENTMETER. It is equipped with recording mechanisms and is suitable for measuring CURRENTS up to six KNOTS. TheWollaston current meter is designed for use from ships at anchor.

6020 word. An addressable subdivision of storage memory of a computer system. Usually an integer number of BYTES (e.g.4 bytes = 32 bit) indicating the standard length of number representation in storage.

6021 working chart. See CHART.

6022 working pendulum. See PENDULUM.

6023 World Geodetic System. A global geodesic reference system developed by the USA for satellite position fixing andrecommended by IHO for hydrographic and cartographic use.

6024 World Meteorological Organization (WMO). Promotes international cooperation in the field of meteorology.

275

6025 World Wide Navigational Warning Service (WWNWS). A service established for the purpose of coordinating thetransmissions of radionavigational warnings in geographical areas using coastal and satellite communication services.

6026 wrack. See ROCKWEED.

6027 wreck. The ruined remains of a stranded or sunken vessel which has been rendered useless. See also DANGEROUSWRECK, and DERELICT.

6028 wreckage. Goods or parts of a wrecked vessel washed ashore or afloat; remains of a WRECK.

6029 wreck buoy. A BUOY marking the location of a WRECK.

6030 w/t station. See RADIO STATION.

276

X

6031 x-axis. A horizontal axis in a system of RECTANGULAR COORDINATES; that line on which distances to the right orleft (east or west) of the reference line are marked, especially on a MAP, CHART, or graph.

6032 x-coordinates. See COORDINATES: PLANE RECTANGULAR.

6033 xerography. A process of electrostatic printing which employs a special selenium-coated PLATE, useable many timesover.

6034 x-parallax. See PARALLAX: ABSOLUTE STEREOSCOPIC.

6035 x-rays. Electromagnetic RADIATION of the same nature as visible radiation but of an extremely short WAVELENGTH less than 10-2 micrometers. X-rays are used in radar technology.

277

Y

6036 yard. A unit of length equal to 3 FEET, 36 INCHES, or 0.9144 METRE.

6037 yaw (or yawing). The ship's oscillation about a vertical axis approximately through the center of gravity of the vessel.

6038 y-axis. A vertical axis in a system of RECTANGULAR COORDINATES; that line on which distances above or below(north or south of) a reference line are marked, especially on a MAP, CHART, or graph.

The line which is perpendicular to the X-AXIS and passes through the ORIGIN.

6039 y-coordinates. See COORDINATES: PLANE RECTANGULAR.

6040 year. A period of one REVOLUTION of the EARTH around the SUN.

6041 year: anomalistic. The period of one REVOLUTION of the EARTH around the SUN, from PERIHELION toPERIHELION, averaging 365 DAYS, 6 HOURS, 13 MINUTES, 53.0 SECONDS in 1900, and increasing at the rate of0.26 second per century.

6042 year: civil. A YEAR of the Gregorian calendar of 365 DAYS in common YEARS, or 366 DAYS in leap (or bissextile)years.

6043 year: sidereal. The period of one apparent REVOLUTION of the EARTH around the SUN, with respect to a fixedpoint, or a distant STAR devoid of proper motion, being 365 DAYS, 6 HOURS, 9 MINUTES, and 9.5 SECONDS in1900 and increasing at the rate of 0.0001 second annually.

6044 year: tropical. The period of one REVOLUTION of the EARTH around the SUN, with respect to the VERNALEQUINOX. Because of PRECESSION OF THE EQUINOXES, this is not 360o with respect to the STARS, but 50"3less. A tropical year is about 20 minutes shorter than a SIDEREAL YEAR, averaging 365 DAYS, 5 HOURS, 48MINUTES, and 46 SECONDS in 1900 and is decreasing at the rate of 0.00530 second annually. Also calledastronomical, equinoctial, natural, or solar year.

6045 Y-level. See LEVELLING INSTRUMENT: Y LEVEL.

6046 young ice. Newly formed LEVEL ICE in the transition stage of development from ICE RIND or PANCAKE ICE toWINTER ICE. Thickness from 5 to 15 cm.

6047 y-parallax. See PARALLAX.

278

Z

6048 zenith. The point where the direction of the PLUMB LINE produced above the HORIZON meets the CELESTIALSPHERE.

6049 zenithal projection. See PROJECTION: AZIMUTHAL.

6050 zenith camera. A special CAMERA so designed that its OPTICAL AXIS may be pointed accurately toward theZENITH. It is used for the determination of ASTRONOMICAL POSITIONS by photographing the position of theSTARS.

6051 zenith distance. The vertical angle between the ZENITH and the object which is observed or defined. Zenith distance isthe complement of the ALTITUDE. See ZENITH DISTANCE: DOUBLE.

6052 zenith distance: double. A value of twice the ZENITH DISTANCE of an object, obtained by OBSERVATION and notby mathematical process.

6053 zenith telescope. See TELESCOPE.

6054 zero sounding(s). See SOUNDING.

6055 zodiac. The band of the sky extending 8.5° either side of the ECLIPTIC. The SUN, MOON, and navigationalPLANETS are always within this band, with the occasional exception of Venus. The zodiac is divided into 12 equalparts, called 'signs', each part being named for the principal CONSTELLATION originally within it.

6056 zodiacal light. White or yellowish light which spreads out, in the night sky, more or less along the ZODIAC from theHORIZON on the side on which the SUN is hidden. It is observed when the sky is sufficiently dark and theATMOSPHERE sufficiently clear.

6057 zone. An area in all parts of which the same TIME is kept. In general, each zone is 15° of LONGITUDE in width, withthe GREENWICH MERIDIAN (0° LONGITUDE) designated as the central MERIDIAN of zone 0 and the remainingzones centered on a MERIDIAN whose LONGITUDE is exactly divisible by 15°. Zones are used in connection withZONE, STANDARD, and DAYLIGHT SAVING TIMES. Also called time zone.

6058 zone: equiphase. That region in space within which the difference in PHASE of two radio signals is indistinguishable.

6059 zone: equisignal. That region in space within which the difference in AMPLITUDE of two radio signals (usuallyemitted by a single STATION) is indistinguishable.

6060 zone meridian. The CENTRAL MERIDIAN of each zone of a grid system.

6061 zone of silence. See BLIND ZONE.

6062 zone time. See TIME.

6063 zoom. A method of enlarging graphics on a graphical display, usually a function provided by the hardware of thescreen. Either a selected WINDOW may be enlarged to cover the entire screen or by repeatedly pressing a key, astepwise or continuous enlargement of the screen contents can be invoked,keeping the graphics centered at the screen'scentre.

6064 Zulu time. See UNIVERSAL TIME.

279

ANNEX A

AcronymsASCII American Standard Code for Information InterchangeBSHC Baltic Sea Hydrographic CommissionCCIR Comité Consultatif International des RadiocommunicationsCGS Centimetre-Gram-Second SystemCIRM Comité International Radio-Maritime

(International Radio-Maritime Committee)DOP Dilution of PrecisionDORIS Determination d'Orbite et Radiopositionnement intégrés par SatelliteDSL Deep Scattering LayerDTM Digital Terrain ModelEAHC East Asia Hydrographic CommissionEAtHC Eastern Atlantic Hydrographic CommissionEBCDIC Extended Binary Coded Decimal Interchange CodeECDB Electronic Chart Data BaseECDIS Electronic Chart Display and Information SystemED European DatumED Existence DoubtfulEEZ Exclusive Economic ZoneEGC Enhanced Group Call (INMARSAT)EP Estimated PositionFAO Food and Agricultural OrganisationFIG Fédération Internationale des Geomètres

(International Federation of Surveyors)GDOP Geometric Dilution of PrecisionGEBCO General Bathymetric Chart of the OceansGLOSS Global Sea-Level Observing SystemGMDSS Global Maritime Distress and Safety SystemHDOP Horizontal Dilution of PrecisionHO Hydrographic OfficeIAC International Advisory Committee on Research in the Natural SciencesIACOMS International Advisory Committee on Marine SciencesIAG International Association of GeodesyIAGA International Association of Geomagnetism and AeronomyIAHR International Association for Hydraulic ResearchIAL International Association of theoretical and applied LimnologyIALA International Association of Lighthouse AuthoritiesIAMAP International Association of Meteorology and Atmospheric PhysicsIAPSO International Association for the Physical Sciences of the OceanIASPEI International Association of Seismology and Physics of the Earth's InteriorIAT International Atomic TimeIAVCEI International Association of Volcanology and Chemistry of the Earth's Interior (IUGG)IBCCA International Bathymetric Chart of the Caribbean Sea and Gulf of MexicoIBCEA International Bathymetric Chart of the Eastern AtlanticIBCM International Bathymetric Chart of the MediterraneanIBCRSGA International Bathymetric Chart of the Red Sea and Gulf of AdenIBCWIO International Bathymetric Chart of the Western Indian OceanIBWM International Bureau of Weights and MeasuresICA International Cartographic AssociationICAO International Civil Aviation OrganizationICES International Council for the Exploration of the SeaICOS International Committee of Onomastic SciencesICSEM International Commission for the Scientific Exploration of the Mediterranean SeaICSU International Council of Scientific UnionsIERS International Earth Rotation ServiceIGC International Geological CongressesIGU International Geographical UnionIHB International Hydrographic BureauIHC International Hydrographic ConferenceIHO International Hydrographic OrganizationIIP International Ice PatrolIMO International Maritime OrganizationINMARSAT International Maritime Satellite Organization

280

INT Chart International ChartIOC Intergovernmental Oceanographic Commission of UNESCOISBA International Sea Bed AuthorityISLW Indian Spring Low WaterISPRS International Society of Photogrammetry and Remote SensingISRU International Scientific Radio UnionITU International Telecommunications UnionIUGG International Union of Geodesy and GeophysicsIUGS International Union of Geological SciencesIUPAP International Union of Pure and Applied PhysicsLOP Line of PositionLOS Law of the SeaMBSHC Mediterranean and Black Seas Hydrographic CommissionMKS system Meter Kilogram Second systemMSC Maritime Safety Committee (IMO)M.S.L. Mean Sea LevelM.T.L. Mean Tide LevelMWARC Maritime World Administrative Radio ConferenceNAD North American DatumNavarea Navigational Warning AreaNAVTEX Automatic Narrow Band Printing SystemNHG Northern Hydrographic GroupNSHC North Sea Hydrographic CommissionNtM Notice to MarinersOECD Organization for Economic Cooperation and DevelopmentPA Position ApproximatePAIGH Pan American Institute of Geography and HistoryPD Position DoubtfulPDOP Positioning Dilution of PrecisionPIANC Permanent International Association of Navigation CongressesRHC Regional Hydrographic CommissionRMS Root Mean Square ErrorROV Remote Operated VehicleSAD South American DatumSAR Synthetic Aperture Radar, Search and RescueSCAR Scientific Committee on Antarctic ResearchesSLAR Side-looking Airborne RadarSCOR Scientific Committee on Oceanographic ResearchSI System InternationalSON Sub-Committee on Safety of Navigation (IMO)SRC Sub-Committee on Radio Communications (IMO)SSS Side Scan SonarSWPHC South-West Pacific Hydrographic CommissionTDOP Time Dilution of PrecisionTSS Traffic Separation SchemeUNCLOS United Nations Convention on the Law of the Seaulcc ultra large crude carrier (< 300 000 t)UN United NationsUNCTAD United Nations Conference on Trade and DevelopmentUNDP United Nations Development ProgrammeUNEP United Nations Environmental ProgramUNESCO United Nations Educational, Scientific and Cultural OrganizationUSCHC U.S/Canada Hydrographic CommissionUT0 Universal Time 0UT1 Universal Time 1UT2 Universal Time 2UTC Universal Time CoordinatedUTM Universal Transverse Mercator (Grid)VDOP Vertical Dilution of Precisionvlcc very large crude carrier (> 200 000 t)VHO Voluntaring Hydrographic OfficeVLBI Very Long Baseline InterferometryVTS Vessel Traffic ServiceWGS World Geodetic SystemWMO World Meteorological OrganizationWWNWS World Wide Navigational Warning Service