SURVEYING

It is defined as the process of measuring horizontal distances, vertical distances and included angles to determine the location of points on. Above or below the earth surfaces. The term surveying is the representation of surface features in a horizontal

plane.

The process of determining the relative heights in the vertical plane is

referred as levelling.

Obiectives of SurveyingThe data obtained by surveying are used to prepare the plan or map

showing the ground features.When the area surveyed is small and the scale to which its result plotted is large, then it is known as Plan.

When the area surveyed is large and the scale to which its result plotted is small, then it is called as a MapSetting out of any engineering work like buildings. roads, railway tracks,

bridges and dams involves surveying.

Main divisions of surveying Plane surveyingGeodetic surveying

Concept:Since the shape of the earth is spheroidal, the line connecting any two

points on the earth surface is not a straight line, but a curve.

When the surveys extend over large areas or when the accuracy required is great, the curvature of earth has also to be taken into account.For small distances the difference and the subtended chord.

Plane Surveying

The surveying where the effect of curvature of earth is neglected and earth's surface is treated as plane. is called surveying.

The degree of accuracy in this type of surveying is comparatively low.

Generally when the surveying is conducted over the area less than gag Sg.Km. they are treated as plane surveying. Plane surveying is conducted for the purpose of engineering projects.

Geodetic Surveying The effect of curvature is taken into account. It is also known as Trigonometrical Surveying".

It is a special branch of surveying in which measurements are taken with high precision instruments.

Calculations are also made with help of spherical trigonometry.

It is generally adopted by the Great Trigonometrical Survey Department of India. (GTS). Classification of surveying Land Surveying

Marine or Navigation or Hydrographic Surveying

Astronomical Survey.

Land Surveying:

Land survey is a one. in which the relative points or objects on the earth's surface is determined.

Marine or Navigational or Hydrographic Survey:

Marine surveying is one in which in which the relative position of objects under water is determined.

Astronomical Surveying:

It is one in which observations are made to locate the heavenly bodies such as sun, moon and stars.

Classification of Land surveying

Topographical Survey:

It is used for determining the natural and artificial features of the country

such as rivers, lakes, hills and canals.

Cadastral Survey:

It is used to locate additional details such as boundaries of fields of fields, houses and other properties.

City Survey:

It is used for town planning schemes such as laying out plots, constructing streets, laying water supply and sewer lines. Engineering Survey :It is used to collect data for design and construction

of Engineering works such as roads, railways, bridges dams etc. Principles of SurveyingPrinciple 1:

A number of control points are fixed in the area concerned by adopting. Very accurate and precise methods.

The lines joining these control points will be control lines.

Other measurements are made to locate points inside these control lines. Thus. main triangles and traverses are formed first.

The main triangles and traverses are divided into smaller ones by using less rigorous methods.

By doing so. accumulation of errors is avoided and any local error can be easily identified.

If survey work is started from a part (smaller triangle or traverse) and proceeded to whole there are chances of errors getting multiplied at every stage.

Hence any survey work should be from whole to part and not from part to whole.

Principle 2:

New points should be fixed by atleast two independent measurements.

As per the Principle 2, the location of a new point involves one of the following.

(a) Measurement of two distances.

(b) Measurement of two angles

(c)Measurement one angle and one distance

Fig 1: It shows the method of locating R with reference to known length PQ byusing the known distances of PR (I1) and QR (I2)

Fig 2: It shows the method of locating R with reference to the length PQ by using the known angles QPR (61) and PQR (32)

Fig 3: it shows the method of locating R with reference to known length PQ by using the known distance of PR (I1) and known angle QPR (91)

CHAIN SURVEYING

This is the simplest and oldest form of land surveying of an area using lineal measurements only. It can be defined as the process of taking directmeasurement, although not necessarily with a chain.

Principle:

It is to form a network of triangles by using the distances measured.

Better accuracy will be obtained if the triangles thus formed are nearly

equilateral in shape.

Classification of surveying:

1. Chain Surveying

2. Compass Surveying

3. Theodolite surveying

4. Plane Surveying

5. Tachometric Surveying

EQUIPMENTS USED IN CHAIN SURVEYING:

These equipments can be divided into three namely :-(i) Those used for linear measurement. (Chain, steel band, linear tape)

(ii) Those used for slope angle measurement and for measuring right angle Eg. Abney level, clinometers, cross staff, optical squares)

(iii) Other items (Ranging rods or poles, arrows, pegs etc).

Metric surveying chain A surveying chain is a device used to measure distance between two points on the ground.

Metric chains are available in lengths of 5 m, 1om, 2om and 30 m.

20m - 30 m chain is normally used for the field of surveying.

A surveying chain contains brass handles with brass eyebolt and collar, galvanized mild steel links and wire rings.

In the case of 20 m and 30 m chains, brass tallies are provided at every 5m length and indicating brass wire rings are attached at every meter length except where tallies are provided.

The distance between the outside faces of handles of a fully stretched out chain is the length of the chain.

The length of the chain, like 2om is engraved on the handles. While measuring the long distance, the chain will have to be used a

number of times.

Arrows are driven at the end of every chain length. For holding the arrows in position. grooves are cut in the outside face of the handles. The radius of the groove is the same as that the arrows. For convenient handling of the chain, the handle joint is made flexible so that it is possible to swivel to handle round the eye bolt.

Steel Bands: This may be 30m, 50m or 100m long and 13mm wide. It has handles similar to those on the chain and is wound on a steel cross. It is more accurate and but less robust than the chain. The operating tension and temperature for which it was graduated should be indicated on the band. Pegs

Wooden pegs of 15cm length and 3 cm square in section are used to

Establish the station points or the end points of a line on the ground.

They are tapered one end and are driven into the ground by using a

wooden hammer. About 4 cm is left projecting above the

ground.

Ranging Rod

Ranging rods should be straight and free from warps. The deviation in straightness should not exceed 5mm in a 2 m length.The ranging rod is painted in red and white in alternate band lengths of 200 mm each.The bottom end of the rod is fitted with apointed, hollow, cast iron shoe or steel shoe of 15 cm length.Plumb Bob

It consists of a solid conical piece and a string attached to it at its centre.When in use. the solid piece is at the bottom. It is used to test the verticality of the ranging rods and to transfer the points to the ground.

Plumb bob is used while doing chain surveying on sloping ground.

NECESSARY PRECAUTIONS IN USING CHAIN SURVEYING

INSTRUMENTS:

1. After use in wet weather, chains should be cleaned. and steel tapes should bedried and wiped with an oily rag.

2. A piece of colored cloth should be tied to arrow (or ribbon - attached) to

enable them to be seen clearly on the field.

3. Ranging rods should be erected as vertical as possible at the exact station point.

4. The operating tension and temperature for which steel bands/tapes are

graduated should be indicated.

5. Linen tapes should be frequently tested for length (standardized) and always after repairs.

6. Always keep tapes reeled up when not in use.

GENERAL PROCEDURE IN MAKING A CHAIN SURVEY:

1. Reconnaissance: Walk over the area to be surveyed and note the general layout, the position of features and the shape of the area.

2. Choice of Stations: Decide upon the framework to be used and drive in the station pegs to mark the stations selected.

3. Station Marking: Station marks should where possible be tied - in to a

permanent objects so that they may be easily replaced if moved or easily found during the survey. In soft ground wooden pegs may be used while rails may be used on roads or hard surfaces.

4. Witnessing: This consists of making a sketch of the immediate area around the station showing existing permanent features, the position of the stations and its description and designation. Measurements are then made from at least three surrounding features to the station point and recorded on the sketch. The aim of witnessing is to re-locate a station again at much later date even by others after a long interval.

5. Offsetting: Offsets are usually taken perpendicular to chain lines in order to dodge obstacles on the chain line.

6. Sketching: The layout on the last page of the chain book, together with the date and the name of the surveyor. the longest line of the survey is usually taken as the base line and is measured first.

CRITERIA FOR SELECTING A SURVEY LINES/OFFSETS:

During reconnaissance. the following points must be borne in mind as the criteria provide the best arrangement of survey lines,

a. Few survey lines: the number of survey lines should be kept to a minimum but must be sufficient for the survey to be plotted and checked.

b. Long base line: A long line should be positioned right across the site to

form a base on which to build the triangles.

c. Well conditioned triangle with angles greater than 30 and not

exceeding 150: It is preferable that the arcs used for plotting should intersect as close as 90 in order to provide sharp definition of the stations point.

d. Check lines: Every part of the survey should be provided with check lines that are positioned in such a way that they can be used for off- setting too. in order to save any unnecessary duplication of lines.

e. Obstacles such as steep slopes and rough ground should be avoided as far as possible.

f. Short offsets to survey lines (close feature preferably 2m)

should be selected: 80 that measuring operated by one person can be used instead of tape which needs two people.

g. Stations should be positioned on the extension of a check line or triangle.Such points can be plotted without the need for intersecting arcs.

METHOD OF MEASURING A CHAIN LINE

This is carried out by two assistants known as chain men, one acting as leading chairman and the other as follower. Ranging rods are inserted as close as possible to the station pegs in order that the position of the peg may be located from a distance.In the case of long lines. they are placed immediately between the stations and lined in by eye to enable a straight line to be measured.

To measure line AB, having previously positioned the ranging rods at both A and B. the chain men take one end each of the chain and check for defects. The leader equips himself with ten arrows and a ranging rod. the followers also takes a ranging rod.

The leader drags his end of the chain fon/vard to A1 and holds his ranging rod about one link short of the end.

The follower holds his end of the chain firmly against stations A and the surveyor lines in the leader's pole between A and B by closing one eye, sighting poles A and B and signaling the leader till he brings his pole into line AB. (The signaling usually adopted is to swing the left arm out to the left as an instruction to the leader to move his pole in that direction. Right arm is similarly used to indicate movement to the right. while both arms extended above the head then brought down indicate that the pole is in line)

The leader straightens the chain past the rod by sending gentle snake down the chain.

The follower indicates the chain is straight, and the leader put arrow at the end A1. The surveyor then walks along the chain, measuring any offsets required.

Upon the completion of these measurements. the leader drags his end to A2. taking nine arrows and his pole.

The follower moves to A1 and puts his pole behind the arrow and the surveyor lines in from A or A1. When the arrow has been inserted at A2. the surveyor removes the arrow at A1 and proceeds to take further offset measurements.

This procedure is repeated until the end of the line is reached or the chainman's arrows are exhausted. The collection of these arrows by the surveyor forms a check upon the number of chains measured.

METHOD OF SETTING OFFSET TO THE CHAIN LINE

Chain surveying principles have so far been applied to areas of land with straight boundaries. As most boundaries are irregular, the method of surveying their position is first to lay down a network of triangles which can be plotted and checked. From these survey lines, offsets are measured perpendicular from thechain line to points of detail.

Perpendicularity may be obtained in one of the following ways:

i. Judging with eye the right angle formed between the chain line and the tape(offset tape)

ii. By swinging the offset tape to obtain the shortest measurement.

iii. By setting out the right-angle with the optical square or cross -staff.

iv. By Pythagorass Theorem (3, 4. 5 method)

FIELD NOTES

Field notes for a chain survey are made in a note book usually with a double red line ruled up the middle of each page.

LEVELLING

PRINCIPLES AND DEFINITIONS:

Levelling is defined as the process of measuring the difference in height between points on the surface of the earth.

Level surface or Level Line:

This is a surface or line in which all points are at the same height and normal or at right angle to the full of gravity as shown by a plumb line.

A Horizontal surface or Horizontal line:

This is a plane flat surface or straight line which passes through a point at right angle to the pull of gravity at that point. It is therefore a tangent to the curve of a level surface.

A Datum Surface:

Datum surface is any level surface to which the elevations of all points may be referred. The mean sea level is usually adopted as datum.

A Reduced Level :The reduced level of a point is its height or elevation above the surface adoptedas a datum

Bench marks:

Bench marks are stable reference points the reduced levels of which are

Accurately determined by levelling.

Back sight:

This is the first reading taken with a levelling instrument in a levelling operation.

Foresight:

This is the last reading taken in a levelling operation.

Intermediate Sight:

This is the reading taken between the back sight and foresight in a leveling operation.

Turning Point or Change Point:

A change point or turning point is a staff station on which two staff readings

taken without changing the position of the instrument.PRINCIPLE OF DIFFERENTIAL LEVELLING

Differential levelling provides a means of accurately measuring height differences between points some tens of metres apart. A level is set up on a tripod and levelled so that the line of sight is horizontal.A graduated staff is held vertically over the first point and a reading made of the intersection of the cross-hair with the image of the staff (backsight - b). The same (or an identical) staff is then held vertically over the second point and a further reading made (foresight - f). The difference between the two readings is the difference in height between the two points: h=b-f

b is greater h is positive (i.e. there is a rise in elevation in moving from

than f then the first to the second point).

This process can be repeated - the level can be moved to beyond the second point and the height difference between the second and a third point measured by the same process. Further repetitions will allow the height difference between widely separated points to be determined by accumulating the height differences between (temporary) intermediate points. The distance from level to staff is dictated by the steepness of the terrain and the clarity of the image viewed by the observer. Usually the maximum sight length is restricted to 50-6om.The sketch below shows a schematic illustration of a basic level:

level is mounted on a tripod, and has three levelling screws that (in

junction with a circular bubble) allow the level to be levelled. These screws

2 a limited range and the tripod head must be set approximately level

Jrehand by adjusting the tripod legs. is upper part of the level consists of a telescope tube with an objective lens and in eyepiece with a cross-hair. The line of sight (collimation axis) is defined by the line joining the centre of the cross-hairs with the focal point of the objective lens.The telescope is mounted on an axis that allows it to be rotated in the horizontal plane. The circular bubble is not very sensitive and is not the sole means of levelling the level. Older levels will have tubular bubbles attached to the side of the telescope.and the footscrews are used to level this bubble, which then provides a horizontal line of sight in the direction of the collimation axis.

Automatic Compensator:

Modern levels will all use some form of automatic compensator, which allows the user to level the instrument with the circular bubble only. Any small departures are compensated by the compensator. The figure below shows a schematic illustration of one type of compensator.

In this device the image of the object is deflected by a fixed mirror to pass through a prism, after which it is deflected by another mirror to the eyepiece. The prism is suspended by wires and its orientation changes as the telescope tube is tilted. The geometry of the device is designed so that any tilt of the telescope tube is Compensated by a tilt of the prism and the collimation axis remains horizontal.The compensator has a limited range (a few minutes of arc) and the level must be levelled reasonably well using the circular bubble before the compensator will work correctly.

Types of Level:

Broadly speaking, there are three classes of level:

a)Builders/Engineers Level: As implied by the name. these are used by

builders and engineers. Their design is basically as described earlier, and they use graduated staffs in which the smallest graduation is 1cm. millimetres must be estimated, and the accuracy of a single reading will be about 2-3mm.

b) Digital Level: This type of level uses a special bar-coded staff. The image of the staff passes through the objective lens and then via a beam splitter to a photo detector array. where it is digitized. The microprocessor compares this image to a copy of the bar code and calculates the staff reading, which is displayed and/or stored. The sensitivity of the device is such that single reading accuracies of 0.2mm to 0.3mm can be achieved, and sight lengths can be extended up to 1oom.

c)Precise Level: This is a modification of the conventional level in which a

parallel plate micrometer is placed in front of the objective lens. This allows the image of the staff graduation to be moved up or down by very small

measurable amounts. For sight lengths of under 5om, single reading

accuracies of 0.02mm to 0.03mm can be achieved. As precision improves, so prices increase. It is tempting to use a builder's level for reasons of economy, and many tidal institutions have done so. However, if

measured small changes in mean sea level are to be meaningful, the stability of the TGBM must be unquestioned. and accuracies of 1mm or better are desirable for the levelling connection. Precise levels have been used and will continue to be used, but if a new level is to be acquired, the best option would be a digital level.

There are three basic types of level in common use, namely.

(a) Dumpy level

(b) Tilting level

(c) Automatic Level

Other types of level include:

Hand levels, bricklayer's level, Cowley level, spirit level, digital level

etc.

Brick layers level:

This is the simplest form of level. It consists of glass tube filled with liquid which contains an air bubble.This tube is set in a wooden block in such a

way that when the instrument is placed upon a horizontal surface. the bubbles float centrally in the tube.

Cowley level:

This is a modern builder's instrument. It has no telescope nor levelling bubble but by mean of reflecting mirrors one attached to a pendulum contained in a metal case about 130mm square by 50mm thick. The

instrument shows a horizontal line of sightclaimed to be accurate within 6mm per 30m.

Spirit Level:

This consists of a glass tube in shape and filled with spirit. A small air bubble isenclosed in it. This tube is inserted in a wooden container and a metal strip is fixed at the top to protect the glass cylinder.

CRITERIA FOR SELECTING LEVELLING DATUM

For all surveys a level line is chosen to which the elevation of all point is related to as datum or datum surface.This can be any surface but the most commonly used datum is mean sea level measured as ordinance datum. All points referred to ordinance datum are said tohave their height above ordinance datum (AOD).On many construction and Civil Engineering sites. mean sea level is not often used as a datum for levelling. Instead, a permanent feature of some sort is chosen on which to base all works and this is given an arbitrary height (referred to as datum) to suite the site conditions.

CONSTRUCTION AND USE OF BENCH MARKS

Bench marks are permanent reference marks or points whose reduced levels are accurately determined by levelling. They are classified into two namely:

1. Permanent or ordinance bench marks (OBM), and

2. Temporary or transferred bench marks (TBM)

Permanent or ordinance bench marks (OBM)

Ordinance bench marks are those which have been established by the

ordinance survey and are based on the ordinance datum. The most common types are permanently marked on buildings and walls by a

cut in vertical brickwork or masonry or indicated by an arrow or crows foot mark. On horizontal surface, OBM consist of a rivet or bolt with the position of the reduced level shown for both types.

All ordinance survey bench marks (OBM) have been in place for some time and may be affected by physical disturbance or local subsidence. To safeguard against this. it is always advisable to include at least two OBM in leveling schemes where ordinance datum is being used.

Temporary or transferred bench marks (TBM)

These are marks set up on stable points near construction sites to which all

levelling operations on that particular site will be referred.These are often used when there is no ordinance bench mark (OBM) close to the site.The height of TBM may be assumed at some convenient value (usually

100.00m) or may be accurately established by levelling from the nearest OBM.The position of TBM should be fixed during the initial site reconnaissance. Permanent existing features should be used where possible. In practice, 20mm diameter steel bolts 1oomm long are driven into existing door steps, foot path. low wall etc. Any TBM set up on site must be leveled with reference to main bench mark (OBM) or some other agreed datum.

Methods of Levelling

Method 1: It is done with only one setting of the instrument.Method 2: When the two station points are wide apart and the instrument

is set up at more than one point and the levelling is carried out.Method 1: With only one setting of the instrument The instrument is set up at a point between P and Q and the temporary

adjustments carried out. The levelling staff is held at P, the elevation of which is known already. A back sight is taken on the staff held at P. The staff is then held at Q and he foresight is taken.

Height of the instrument= Known elevation of P + the staff reading at P

=100.00+ 2.10 = 102.10 m

Elevation of Q =Height of the instrument - the staff reading at Q

=102.10 -1.80 = 100.30 mMethod ll: When the station points are wide apart, the instrument is setup for at more than one Q point and levelling is done (Height of Collimation Method)

A change point (C.P) is established in between P and Q.

A back sight is taken at P and a fore sight is taken at the change point.

The instrument is shifted to another point between the change point and I

A back sight is taken at the change point and a fore sight is taken at Q.

Any numbers of change points are established as required.

This method is known as Height of Collimation method.

The elevation of change point = Elevation of P + Back sight at P Fore sight at change point (C.P)

=100.00+1.60-1.10 = 100.50 m

The second height of the instrument

= the elevation of change point + Back Sight at change point

=100.50 + 1.25 = 101.75 mThe elevation of Q = the second height of instrument foresight at Q

101.75 -1.81 = 99.94m

Rise and Fall Method of calculating the level

The staff readings of the points observed from the same setting of the instrument are compared.

It is found whether a point is above or below the preceding point.

If the point is above. the staff reading will be less than the preceding point.The difference between the staff readings is called rise.

If the point is below the preceding point, the staff reading will be greater than that at the preceding point. The difference between the staff readings is termed fall.

The difference between the staff readings at P and Q = 2.10 - 1.80 = 0.30 (rise)Hence, level of Q = Elevation of P + Rise 100.00+O.30 = 100.30 m

Fly Levelling

Any number of change points are established as required during levelling. This method is known as fly levelling. it is adopted to find the difference in level between two points. when

(i) The two points are too far away

(ii) the difference in level between two points is large

(iii) there are no obstructions in between the two points concerned.

Calculation of Areas

One of the purposes of surveying is to determine the area. to be surveyed.

The area of the land obtained by surveying actually refers to the area as projected on a horizontal plane. There are different methods of computing the area of land using the data obtained by surveying.

Calculation of area by Trapezoidal Rule

ln trapezoidal. a convenient base line is established.

Perpendicular distances from the base line to the boundary of the land concerned are measured at regular (equal) intervals along the baseline.These perpendicular distances are called ordinates.

ERRORS IN LEVELLING

There are a large number of potential sources of error in levelling. Many of these are only significant for precise levelling over long distances. For the short segments of levelling that will occur in connecting a TGBM to nearby benchmarks there are only three worth mentioning:

Collimation Error

Error due to Earth Curvature

Error due to Refraction

Collimation Error:

Collimation error occurs when the collimation axis is not truly horizontal when the instrument is level. The effect is illustrated in the sketch below. where the collimation axis is tilted with respect to the horizontal by an angleIn this particular example, the effect is to read too high on the staff. For a typical collimation error of 20", over a sight length of 5om the effect is 5mm. If the sight lengths for backsight and foresight are equal, the linear effect is the same for both readings. When the height difference is calculated, this effect cancels:

8h = (b+s.at) - (f+s.at) = b - f

That is, the effect of the collimation error is eliminated if sight lengths are kept equal.Earth Curvature:

Due to the curvature of the Earth. the line of sight at the instrument will deviate from a horizontal line as one moves away from the level:

Ideally one would like the line of sight to be a curved line which is everywhere perpendicular to the direction of gravity. The error in staff reading due to Earth curvature is given by: Where s is the sight length and R is the radius of curvature of the Earth. For a sight length of 1oom the effect is only 1mm. As with collimation error. the effect is eliminated by using equal sight lengths forfore- and backsights.

Refraction:

The variable density of the Earth's atmosphere causes a bending of the ray from the staff to the level. The effect is illustrated in the sketch below:

The light ray is bent in a path which has a curvature less than that of the Earth's surface, and the combined effect is smaller than that due to Earth curvature alone:

The effect of refraction is almost totally eliminated by using equal fore- and

backsights (because atmospheric conditions along the fore- and backsights will not be completely identical, there will be a small residual error). Level staff

A level staff, also called levelling rod, is a graduated wooden or aluminum FOLthe use of which permits the determination of differences in elevation.

Two sides of a modern surveyor's levelling rod. Metric graduations on the left. imperial on the right. Levelling rods can be one piece, but many are sectional and can be shortened for storage and transport or lengthened for use. Aluminum rods may adjust length by telescoping sections inside each other, while wooden rod sections are attached to each other with sliding connections or slip joints.

There are many types of rods, with names that identify the form of the graduations and other characteristics. Markings can be in imperial or metric units. Some rods are graduated on only one side while others are marked on both sides. lf marked on both sides, the markings can be identical or. in some cases, can have imperial units on one side and metric on the other.

Reading a rod

In the photograph on the right, both a metric (left) and imperial (right) leveling rods are seen. This is a two-sided aluminum rod. coated white with markings in contrasting colours. The imperial side has a bright yellow background.

The metric rod has major numbered graduations in meters and tenths of meters(e.g. 18 is 1.8 m - there is a tiny decimal point between the numbers). Between the major marks are either a pattern of squares and spaces in different colours or an E shape (or its mirror image) with horizontal components and spaces between of equal size. In both parts of the pattern. the squares. lines or spaces are precisely one centimeter high. When viewed through an instrument's telescope, the observer can easily visually interpolate a 1 cm mark to a quarter of its height, yielding a reading with accuracy of 2.5 mm. On this side of the rod. the colours of the markings alternate between red and black with each meter of length.

The imperial graduations are in feet (large red numbers), tenths of a foot (small black numbers) and hundredths of a foot (unnumbered marks or spaces between the marks). The tenths of a foot point is indicated by the top of the long mark with the upward sloped end. The point halfway between tenths of a foot marks is indicated by the bottom of a medium length black mark with a downward slopedend. Each mark or space is approximately 3mm, yielding roughly the same accuracy as the metric rod.

Classes of rods

Surveyor's view of the levelling rod with the crosshair. This indicates a reading of

1.422 m

Rods come in two classes:

1. Self-reading rods (sometimes called speaking rods).

2. Target rods.

Self-reading rods are rods that are read by the person viewing the rod through the telescope of the instrument. The gradations are sufficiently clear to read with good accuracy. Target rods, on the other hand, are equipped with a target. The target is a round or oval plate marked in quarters in contrasting colours such as red and white in opposite quarters. A hole in the centre allows the instrument user to see the rod's scale. The target is adjusted by the Rodman according to the instructions from the instrument man. When the target is set to align with the crosshairs of the

instrument, the Rodman records the level value. The target may have a vernier to allow fractional increments of the graduation to be read.

Topographer's rods

Topographer's rods are special purpose rods used to ease conducting

topographical surveys. The rod has the zero mark at mid-height and the

graduations increase in both directions away from the mid-height.

In use, the rod is adjusted so that the zero point is level with the instrument (or the surveyor's eye if he is using a hand level for low-resolution work). When placed at any point where the level is to be read, the value seen is the height above or below the viewer's position. An alternative topographer's rod has the graduations numbered upwards from the base.

Stadia rods

A normal levelling rod can be used for stadia measures at shorter distances (up to about 125 m). For longer distances, special stadia rods are better suited. In order to provide good visibility at long distances, stadia rods are typically wider than levelling rods with larger markings. Since very fine gradations are not necessary for long sights. they may be left off the dedicated stadia rod.

USES OF LEVELLING

In the context of tidal measurements, levelling is used for the following purposes:

Referencing of Tide Gauges: To determine and check the vertical stability of the tide gauge bench mark (TGBM) with respect to reference points(benchmarks) in its immediate vicinity. In order to isolate any local movements,there should be at least three such benchmarks. and the levelling should be repeated on an annual or semi-annual basis. Connection to GPS Reference Points: To determine its regional stability and to separate sea level rise from vertical crustal motion, the TGBM should be connected via GPS to reference stations fixed in a global co-ordinate system.Generally speaking, the GPS antenna cannot be directly placed on the TGBM and a GPS reference point must be established a short distance away. This must be connected to the TGBM by levelling.

Connection to National Levelling Network: Mean sea level is used to definevertical datums for national surveying and mapping - hence the TGBM must be connected to the national levelling network. Connection to the network will also allow all tide gauges to be connected to each other. providing information on

spatial variations in mean sea level.

Contouring

A contour is defined as an imaginary line joining points of the same height or elevation above or below a datum. These are shown so that the relief or

topography of an area can be interpreted (a factor greatly used in civil

engineering)

The difference in height between successive contours is known as the vertical interval (VI) and this interval dictates the accuracy to which the ground is represented. The value of (VI) chosen for any application depends on;

(a) Scale of the plan

(b) Intended use of the plan

(c) The costs involved

(cl) The nature of the terrain

Generally, a small vertical interval of up to 1m is required for engineering projects. large scale survey plans and surveys on fairly even sites. A wider vertical interval is used in hilly or broken terrain.

Electronic instruments such as total stations are normally used to collect data for contouring and contours are plotted by using computer software and hardware.

If drawn manually. contours can be obtained other directly or indirectly using mathematical interpolation or graphical interpolation techniques.

CONTOUR CHARACTERISTICS

(i) Contour lines close upon themselves some where each to its own elevation. If not within the limit of the map.

(ii) Contour lines cannot intersect one another whether they are of the same elevation or not.

(iii) Contour lines on the tops of ridges and in the bottom of valleys either close or run in pairs within the limits of the map. no single line can ever run between two of higher or lower elevation.

(iv) Contour lines indicate uniform slopes when they are equally spaced, convex slopes when they are farther apart with increasing elevations and concave slopes when becoming closer together with increasing elevations

USES OF CONTOUR MAPS

1. Location of possible routes for roads. dams etc.

2. Laying out building sites:

The position of hill tops, basins, steep slopes, etc can be seen from contour plans

to avoid sitting buildings on exposed hill top and risking possible soil creep. or in

basins which may form natural drainage area.

3. Calculation of volumes.

4. Determination of inter visibility between stations.

INTERPRETATION OF MAPS, LAYOUT AND

ENGINEERING SURVEY PLAN

Maps and Plans:

The presentation of the measurements made in land surveying is in the form of maps, plans or diagram with the information recorded in a suitable manner on the drawing by the scale representation. use of conversional signs, tabulation of notes, etc. The most suitable method of representing the information depends on the nature of the survey work.

On maps. the scale is too small to allow every feature to be properly presented to scale. Thus conversional symbols are used to represent feature which would otherwise be too small to be recognized.

Plans on the other hand show all features on the ground correctly to scale.

Symbols or methods are used to represent ground features on a plan. Different organizations have slightly different conversions for representing detail on plans, but the best are those forms which are simple. clear, and cannot be misinterpreted.

TYPES OF MAPS:

There are different types of maps namely:

1. Geographical maps

2. Atlas

3. Topographical maps

4. Engineering mapsGENERAL REQUIREMENT OF A MAP OR PLAN

(a) The title

(b) The scale

(c) The North Point (north direction)

(d) Border lines

(e) An explanation or legend, as to the symbols employed.

(f) A terrier. showing the acreages held by various owners

(g) The surveyor's sign.

Special requirements include:

(h) Contours or spot levels.

(i) Constructional lines and symbols for building and civil engineering works.

(j) Location of the survey

(k) Details of the control grid used 1. ERRORS IN EQUIPMENT

a) Collimation Errors

This can be a serious source of error in levelling if sight length from

instrument position is not equal since the collimation error is proportional to tt. difference in sight length. Hence in all types of levelling. sights should be kept equal particularly by back sight and the foresights.

(b) Parallax:

This effect must be eliminated before any readings are taken.

(c) Defect of the Staff

This is possible that staff graduations may be incorrect and new repaired staff should be checked against the steel tape. Particular attention should be paid to the base of the staff to see if it has become badly worn.

This does not affect height difference if the same staff is used for all the levelling, but introduces some errors if two staffs are being used for the same sources of levels.

When using a three section staff it is important to ensure that the staff is properly extended by examining the graduations on either staff of each joint. If the joints became loose. the staff should be returned for repair.

(d) The Pod Defect:

The stability of tripods. should be checked before any field work commence by testing to see the tripod head is screwed and the shoes at the base of each are not loose.

2. ERRORS IN HANDING EQUIPMENT

(a) Staff Not vertical

Since the staff is used to measure a vertical difference between ground and line of collimation, failure to hold the staff vertical will result in incorrect readings.

(b) Instrument not Level

For automatic level this source of error is unusual, but for a tilting level in which the tilting screws has to be adjusted for each reading.

This is a common mistake. the best procedure here to ensure that the main

bubble is centralized before and after a reading is taken.

3. ERRORS IN READING AND BOOKING

Extra care must be taken when reading the staff since an inverted images result in faulty reading being recorded by inexperienced observer. although the image usually diminishes with practice

Another source of reading error is sighting the staff over too long distance when it becomes impossible to take accurate reading. It is therefore recommended that sighting distance should be limited to 6om. but where absolutely unavoidable this may be increased to a maximum of 1oom.

Many mistakes are made during the booking of the readings and the general rule is that staff sightings must be carefully entered into the levelling table immediately after reading.

4. Errors Due to Displacement of Equipment

If the instrument is setup on soft or marshy ground, it may settle and alter theheight of collimation.Change points must be chosen so that when turning the staff round or when replacing it after removal no alteration of height takes place. Always choose stable change points on hand ground and mark the staff position with chalk.

5. Errors clue to Natural Causes

The wind causes vibration of the level, tripod and the staff particularly when it is fully extended thereby making accurate sighting impossible. Always shelter the staff and keep short sights on windy weather.The sun can cause an apparent vibration of the staff owing to irregular refraction.It also affects the bubble by causing unequal expansion of the level and tripod. In

hot weather length of sight are reduced to at least 0.5m above the ground

through- out the length. The ray-shade in front of the instrument should be

extended or shade the instrument with umbrella.Rain makes accurate work difficult and unpleasant; rain dropping on theobjective glass and condensation on the eye piece make sighting impossible. For

precise work it is advisable to wait for better weather condition or ray shade canbe used or protecting the instrument with umbrella also be kept short.

6. SOURCES OF ERROR IN LEVELLING

There are five sources of errors in levelling and their importance must be

appreciated and precaution taken to reduce their effects. These sources includes.

1. Instrumental error in equipment.

2. Error in handling the equipments

3. Error due to displacement of equipment 4. Error in reading and booking

5. Error due to natural causes.