Measurement of Vertical Heights from Single Oblique Aerial ... · Measurement at Vertical Heights...

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Measurement at Vertical Heights tram Single Oblique Aerial Photographs SYLVESTER P. GAY, JR., Utah Construction Company, San Francisco, California ABSTRACT: Nomographic methods for indirect measurement of vertical heights from images appearing on single-oblique aerial photographs are presented in this paper. The application of the methods is quite ele- mentary and may be learned by non-technical personnel in a matter of minutes. A measurement of a height may then be performed in two or three minutes with an accuracy of a very few per cent, depending prima- rily on the accuracy of photographic parameters and the skill of the opera- tor. (1) 1. INTRODUCTION I NA previous paper presented in this journal* the author described a method for performing indirect measurements of ground objects from single-oblique aerial photographs utilizing a set of nomographic charts. The measurements presented in that paper were limited to objects lying in a level plane---linear dimensions, areas, and angles. The same approach was subse- quently applied to measurement of ver- tical heights, and it was soon found that the method was even more suitable for this than for level measurement. Image param- eters are reduced to two, and the number of nomographs required is reduced to one. The method is nearly, though not quite, as simple as the corresponding technique used with vertical photographs in which the object height, t . h = d X altitude, where l is the image length of the height and d is the distance from the top of the image to the principal point of the photo- graph. Early in the development of the nomo- graphic method for measuring vertical heights, it became apparent that due to the character of different height images on a photograph, two solutions would be necessary-one for heights projecting short images on the photograph, and another for long images. Therefore, two different methods, equally simple, and each utiliz- ing a separate nomograph are presented in this paper. The difference between "long" and "short" images is determined by the practicality of accurate measure- ment of the image and accurate manipula- tion of the image parameters on the nomograph. The nomograph for long images may also be used to measure a component of the ground distance of a point from the nadir point of the aircraft, at the time of exposure of the photograph. Either height-measuring method may be taught in ten or fifteen minutes to non- technical personnel having little mathe- matical background. The principal skill required is ability to use a pair of dividers and to multiply two or three numbers to- gether. An operator proficient in the method should be able to measure a ground height from a photo image in about one or two minutes. Accuracy should be within a very few per cent, depending principally on accuracy of measurement of the image length. The foregoing is based, of course, on the premise that the operator is provided a photograph with accurately known param- eters of tilt, altitude, and focal-length, on which are marked accurate photo- graphic axes. Methods of determining these parameters are found, among other places, in the MANUAL OF PHOTOGRAM- METRY and in the aforementioned paper by the author, the latter containing several short-cuts, utilizing nomographs. * Gay, S. P., Jr., "Nomographic Solution to Oblique Photo Mensuration," PHOTOGRAM- METRIC ENGINEERING, Vol. XXII, no. 4, pp. 674-692. 900

Transcript of Measurement of Vertical Heights from Single Oblique Aerial ... · Measurement at Vertical Heights...

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Measurement at Vertical Heights tram SingleOblique Aerial Photographs

SYLVESTER P. GAY, JR.,

Utah Construction Company, San Francisco, California

ABSTRACT: Nomographic methods for indirect measurement of verticalheights from images appearing on single-oblique aerial photographs arepresented in this paper. The application of the methods is quite ele­mentary and may be learned by non-technical personnel in a matter ofminutes. A measurement of a height may then be performed in two orthree minutes with an accuracy of a very few per cent, depending prima­rily on the accuracy of photographic parameters and the skill of the opera­tor.

(1)

1. INTRODUCTION

I N A previous paper presented in thisjournal* the author described a method

for performing indirect measurements ofground objects from single-oblique aerialphotographs utilizing a set of nomographiccharts. The measurements presented inthat paper were limited to objects lying ina level plane---linear dimensions, areas, andangles. The same approach was subse­quently applied to measurement of ver­tical heights, and it was soon found thatthe method was even more suitable for thisthan for level measurement. Image param­eters are reduced to two, and the numberof nomographs required is reduced to one.The method is nearly, though not quite, assimple as the corresponding technique usedwith vertical photographs in which theobject height,

t .h = d X altitude,

where l is the image length of the heightand d is the distance from the top of theimage to the principal point of the photo­graph.

Early in the development of the nomo­graphic method for measuring verticalheights, it became apparent that due tothe character of different height images ona photograph, two solutions would benecessary-one for heights projecting shortimages on the photograph, and anotherfor long images. Therefore, two differentmethods, equally simple, and each utiliz-

ing a separate nomograph are presentedin this paper. The difference between"long" and "short" images is determinedby the practicality of accurate measure­ment of the image and accurate manipula­tion of the image parameters on thenomograph.

The nomograph for long images mayalso be used to measure a component ofthe ground distance of a point from thenadir point of the aircraft, at the time ofexposure of the photograph.

Either height-measuring method maybe taught in ten or fifteen minutes to non­technical personnel having little mathe­matical background. The principal skillrequired is ability to use a pair of dividersand to multiply two or three numbers to­gether. An operator proficient in themethod should be able to measure aground height from a photo image inabout one or two minutes. Accuracy shouldbe within a very few per cent, dependingprincipally on accuracy of measurementof the image length.

The foregoing is based, of course, on thepremise that the operator is provided aphotograph with accurately known param­eters of tilt, altitude, and focal-length,on which are marked accurate photo­graphic axes. Methods of determiningthese parameters are found, among otherplaces, in the MANUAL OF PHOTOGRAM­METRY and in the aforementioned paperby the author, the latter containing severalshort-cuts, utilizing nomographs.

* Gay, S. P., Jr., "Nomographic Solution to Oblique Photo Mensuration," PHOTOGRAM­METRIC ENGINEERING, Vol. XXII, no. 4, pp. 674-692.

900

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MEASURING VERTICAL HEIGHTS FROM SINGLE OBLIQUE PHOTOS 901

One limitation of the nomographicmethods presented in this paper is that theheight to be measured must be a verticalheight, such as a flagpole, smoke-stack, orcorner of a building. I t will not measurerelative elevations of different objects aswill parallax methods utilizing stereoscopicpairs of photographs.

II. MEASURING HEIGHTS FROM SHORT

IMAGES

The height-factor chart (Figure 1) is thenomograph used in measuring verticalheights on an oblique photograph when theimage length is small, On this chart a num­ber, or height-factor, is determined usingimage and photographic parameters. Theheight-factor multiplied by the imagelength and the quotient of the altitude andfocal-length gives the correct height of theobject being measured:

altitucteHeight of object = ----­

focal lengthX image length X height factor (2)

To determine the proper height-factorfor a photographic image the photo inter­preter simply measures the perpendiculardistance from the center of the image tothe x-axis of the photograph, i.e., thevertical displacement of the image. Thisdistance is then located on the scale forthe proper focal-length at the left edgeof the height-factor chart, and a line isextended horizontally until it intersectsthe curve for the correct depression-angleof the photograph (images above x-axis)or tilt-angle (images below x-axis). At thisintersection, the height-factor is read be­tween vertical lines on the scales acrossthe top and bottom of the chart.

When the height-factor chart is repro­duced at the scale corresponding to thecorrect focal-length for a photograph, theoperator may simply take the vertical dis­placement of the image with dividers andapply it to the chart in one step. For theother focal-lengths shown, a pair of pro­portional dividers may be used in scalingoff the distance. This method may also beused for photographs of any focal length.The ratio is an inverse one. Thus, thechart coordinate for an image point on aphotograph of 7" focal-length would be12/7 of its value for an image point of thesame vertical displacement on a photo­graph of 12" focal-length.

The height-factor chart, then, is uni­versal in its use. It is not restricted to anyparticular depression-angle or focal-length.It requires no knowledge of mathematicsexcept as required by formula (2). Themanipulation of the chart is so elementarythat few advantages would accrue fromspecialization of the chart, such as adap­tion for one particular focal-length and de­pression-angle. The formula used with thechart, formula (2), may be recognized asthe same as that used for computinghorizontal distances on a map or on avertical photograph-multiplied by theheight-factor. This same formula is alsoused for computing horizontal distances onoblique photography with the authorsnomographic method, except that in thiscase, an "oblique" or level-line correction­factor is used instead of the height-factor.

The height-factor chart in its presentform is restricted, however, to photo­graphic images lying above, or beyond, thenadir point. On oblique photographs oflow tilt-angle, there is quite a sizeablephotographic area lying between the nadirparallel and the lower edge of the photo­graph, and it may occasionally be de­sirable to measure heights in this region.Large heights may be measured on thenomographic chart described in the follow­ing section, but it was not felt that theneed for measuring small images herejustified construction of additional curveson the height-factor chart. Furthermore,near the nadir-point where the cameraviewpoint is almost vertical, the image­length is necessarily very small and theheight-factor large, and the accuracy ofthe nomographic method rapidly decreases.Parallax methods using stereo pairs ofphotographs are much more applicable tothis situation. Such methods for obliquephotography are presently under develop­ment by the author.

There is another important qualificationin the application of the nomographicmethod. It has been implied that a directmeasurement of the photographic image ismade. This is only true for those smallimages far from the nadir-point of the air­craft and near the principal line of thephotograph which are essentially parallelto the principal line. As images of verticallines on an aerial photograph always pointto the nadir, then many images on thesides of the photograph will be quite non­parallel with the principal line. For these

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Ii

F.I~ri.An,1e

"'l,~r.f ./tCI • 111101' L....~ .' H.ith. Ftclo,

'0oN

"d::r:o..,oC)::tl:>=::is:tIl..,::tl>-<(')

tIlZC)

ZtIltIl::tlZC)

FIG. 1. Height Factor Chart-for measuring heights from small photographic images.

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FIG. 2. Height Nomograph for measuring heights from large photographic images. Also used for determining componentof nadir point distance parallel to principal line.

Nadir Point Distance~rt..... ..g~sL""", "t'!!9!1!•.~2m2graoh - Lcirqe Images

,'SH .... .~ ,'14 .ft' 1.0" I tK ttl

MI''''' 'C.UI ("'- 'aloe".., O' At. 'fITWMI

i:!::t!1~[JJ

c:::eZC'l<:t!1:e>-l

n~t"'

:::t!1Ci:::>-l[JJ

"1:eoi:!::[JJ

ZC'lt"'t!1oIJ:lc:toc::t!1'0:::o>-lo[JJ

'0ov:.

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904 PHOTOGRAMMETRIC ENGINEERING

I-ZW~WU<l..JQ.(/)

0 Step 3..J L<lU

i=0::W>0

r- L ---j ,r>-/ Height of Object

I! ! I ! I I . I I Io .2H"H .6H .8H .9H

FIG. 3a. Basic technique for determining heightswith height nomograph-large images.

tom of the chart, where the height of theobject being measured is given as a fractionof the al titude.

I t can readily be appreciated that meas­urements are more accurate the longerthe image-length, but even for smallimages careful work should give quiteaccurate results. However, heights of lessthan about one-tenth the altitude areprobably more accurately measured onthe height-factor chart'. This generallylimits measurements from the height­nomograph-large images to lower altitudephotography. For the sake of accuracy itshould be mentioned that the aircraft alti­tude used is always the height above thebase of the object being measured.

An alternate technique for applying thevertical-displacements of the top andbottom of the image to the chart is shownin Figure 3b. This technique provides ameasure of what the author has beenstriving to do with all his nomographicmethods-that is, to make the measure­ment of objects from oblique photographsas simple and as similar to measurementfrom vertical photographs as is possible,(without requiring the use of an electroniccomputer), thereby speeding up the meas­urements and limiting the amount of newinformation which a photo interpretermust learn-new information in most casessimply being the manipulation of thenomographs.

In this technique for applying the ver­tical-displacemen ts, the displacemen t tothe top of the image is first taken withdividers, or proportional dividers, and the

images the component of the image parallelto the principal line must be measuredrather than the image itself. This is nogreat problem and may be performed quiteaccurately with a tube magnifier by plac­ing the scribed measuring lines parallel tothe x-axis.

As the height of the object being meas­ured approaches an appreciable fractionof the aircraft altitude, more and moreerror results from use of the height-factorchart. Another nomograph was developedto overcome the error and to take fulladvantage of the high degree of accuracypossible with longer images.

III. MEASURING HEIGHTS FROM LONG

IMAGES

\i\'hile the height method for shortimages resembles methods for measuringlevel-lying dimensions, the height methodfor Joil"ii'images is quite similar to the meas­urement of. pe'rpendicular heights fromvertical photographs. Derivation of thetwo is, in fact, almost identical.

The nomograph used in measurement ofheights from long photographic images isshown in Figure 2. After performing thenecessary operations on the chart, theoperator arrives at the height as a certainfraction of the aircraft altitude. The onlymathematics necessary, then, is the multi­plication of this fraction and the altitude.I n this respect the method is even simplerthan the corresponding technique forvertical photography, as a graphical stepreplaces a mathematical one.

To use the height-nomograph-largeimages, the vertical displacements to thetop and bottom of the image are deter­mined on the photograph and are appliedto the chart in the same manner as pre­viously explained for the height-factorchart, com men ts concerning other focal­lengths being equally applicable. However,on the height-nomograph-Iarge images,both positive and negative vertical-dis­placement is presented, and the photo­graphic depression angle is always used­never the complementary tilt-angle. Hori­zontal Jines are extended from the twovertical-displacement values until theyboth intersect the correct depression-anglecurve for the photograph (see Figure 3a).The horizontal distance, L, between thesetwo intersections is then measured, or pre­ferably taken with dividers, and is appliedto the left side of the scale at the very bot-

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MEASURING VERTICAL HEIGHTS FROM SINGLE OBLIQUE PHOTOS 905

~ ·Ste I 2zW

TSlep 2:EwL)

I<f 1.J11.

Ien 1-1--Sle 3-----

0 f-L--J.J Yz<fL)

Y, ........ -e-~

a: Iw>0

FIG. 3b. Alternate technique for determiningdistance, L, on height nomograph-largeimages.

proper depression-angle intersection islocated on the chart and marked. Then,as with vertical photography, the image­length, or in this case, the difference invertical-displacements, is taken with thedividers and is laid off vertically down­ward from the marked point. Dividers arenext rotated about the bottom point, andthe distance, L, to the depression-anglecurve is taken for application to the heightscale.

On the height-nomograph-large imagesdepression-angle curves have been plottedfor every degree from 0 to 90. I t can beseen that application to the chart of de­pression-angle values measured to withina fraction of a degree is less simple and notas accurate as for whole degrees. Therefore,it is suggested that the depression-anglevalue be rounded off to the nearest wholedegree; or if necessary, two measurementsmay be made for the curve either side ofthe correct depression angle, and the finalanswer weighted accordingly.

Although not specifically developed fordetermining nadir-point distances, theheight-nomograph-large images servesan important auxiliary function for thisuse. The nadir-point distance is the dis­tance of a point on the ground from a pointdirectly underneath the camera at thetime of exposure. To fully define theposition of the ground point, it is neces­sary to know both the location of the nadirand the azimuth of the point from the na­dir with respect to some known azimuth.The height-nomograph merely gives thecomponent of the nadir point distanceparallel to the camera axis, or on thephotograph, parallel to the principal line.

I t is determi ned on the chart at the properintersection of (1) vertical-displacementof the image point and (2) photographicdepression-angle by the scales at top andbottom of the chart in fractions and multi­ples of the altitude. There is an additionalconversion scale at the bottom for deter­mining this component in nautical milesfor an aircraft at 10,000 feet altitude,which may be quickly converted for otheraltitudes by multiplying the ratio of actualaltitude to 10,000 feet.

The component of nadir-point distanceperpendicular to the principal line maybe determined nomographically by pre­viously mentioned methods developed bythe writer. The ground point may then belocated, provided the position of the nadirand the azimuth of the photographic axisis known.

The ove:-rid!ng family of curves in thelower left of the height-nomograph-largeimages is used for measuring negativenadir-point distances and for determiningheights from images lying below the nadiron the photograph. The method used isexactly the same as for points and imageslying above the nadir.

IV. DERIVATION OF MATHEMATICS AND

CONSTRUCTION OF NOMOGRAPHS

Derivation of the mathematical formu­las required for construction of the nomo­graphic-charts used in measuring verticalheights from oblique photographs isnearly as simple as the use of the chartsthemselves. The derivation of the formulaon which is based the height-nomograph­large images will be explained first. Allmathematics occurs on the principalplane-the plane which includes the prin­cipal ray, or camera axis, and the nadir­point (refer to Figure 4.).

The five independent parameters neces­sary to measure a vertical height, h, froma large image are as follows:

Photographic parameters

H-aircraft, or camera, altitude abovebase of object.

j-focal-length of camerat-tilt-angle of camera, the complec

ment of depression-angle, O.

Image parameters

Yl-vertical-displacement to bottom ofimage.

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906 PHOTOGRAMMETRIC ENGINEERING

FIG. 4. Side view of oblique photo geometryfor measuring vertical heights.

where k is any point. Therefore, combin­ing (3) and (4) and using fl's to top andbottom of image

This, very simply, is the formula utilizedin the measurement of vertical heightsfrom large images.

The height-factor chart for measuringheights from small images is based on aformula equivalent to (5) but derived by anindependent method utilizing a helpingplane through the base of the image per­pendicular to the camera axis. The for­mula is:

Y2-vertical-displacement to top of im­age.

Dependent parameters used in the der­ivation are D 1 and D 2 , the nadir-pointdistances (on principal plane) for image­points representing the bottom and topof the object, and fll and fl2, the interiorcamera-angles for these two points. Thenadir-point distances aid in the derivationin the same manner as with the corre­sponding derivation for vertical photog­raphy.

From the diagram (Figure 4) it is seenby similar triangles that

11 D2 -D1

H=iJ-;-'

---0.

and therefore,

11= H (1- Dl).D2

Also it is seen that

Dk = H tan (t + (3k),

[tan (t +fJ1)]

1I=H 1 .tan (t + fJ2)

I 0.-0'=1

(3)

(4)

(5)

H cos fJl cos fJ211 = - I. ' (6)

j cos (t + fJ,) sin (t + (32)

where tv = Y2 - Yl, the component of theimage parallel to the principal line of thephotograph. Rather than present here thederivation for (6) it is simpler to merelyshow its equivalence to formula (5).

From (5), substituting sine and cosinevalues for the tangent, and simplifying:

r cos (t + (3,) sin (t + 82) lL' - sin (t + fJ,) cos (t + (32) J

1I=H .cos (t + (3,) sin (t + (32)

Using sum of angles reductions for thenumerator, multiplying, and gatheringterms:

. [COS #1 sin fJ2 - co, (J, si" ,'"1 , 11I=H ----------- .

cos (t + (3,) sin (t +#,)

Again changing the form of the numerator:

(tan (32 - tan fJ,) cos {3, cos tJ211 = H ------------ .

cos (t + (3,) sin (t + /3,)

From Figure 4 it may be seen that,

Y2 - y, I.tan {32 - tan {3, = -7- = 7 .

Therefore, substituting:

I. cos fJ1 cos fJ211 = H - ---------- .

j cos (t + fJ,) (sin t + fJ2)

This formula is identical to (6), thus prov­ing the equivalence of (5) and (6).

The difference in the methods for meas­uring heights from long and sh~rt imagesis provided by a simplifying assumptionmade with formula (6) which cannot bemade with (5). This assumption is that asthe image becomes smaller and approacheszero length, fll and f3 2 approach a commonvalue flo. Therefore, for images of zerolength the common value f30 may be sub­stituted in (6) to give

H cos2fJo11=-1. . (7)

j cos (t + flo) sin (t + fJo)

Substitution of the common value f30 in(5) gives the result, h = 0, but in formula(7), h = 0, only when tv = 0. Therefore, (7)may be used when tv ~ 0, by assuming thevalue of flo to occupy a point midwaybetween f31 and fl" i.e., the image mid­point. Detailed studies have not been per­formed as to the error involved in thisassumption, but preliminary calculationsshow that the error is very small whenmeasuring heights of less than about one

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MEASURING VERTICAL HEIGHTS FROM SINGLE OBLIQUE PHOTOS 907

(9)

(13)

are plotted for whole number values of tfrom 0° to 90° corresponding on the chartto depression-angle value of 90° to 0°.The yd! values are plotted directly; mkvalues, logarithmically.

The mk values plotted on the logarithmicscale act similarly to a slide rule. A givendistance along the scale represents thedifference in logarithms of the numbers atthe ends of the given distance. The num­ber, or antilogarithm, corresponding to thegiven distance is the quotient of the twonumbers. Thus, the distance, L, in Figure3 actually represents the quotient:

1It1 tan (t + (31)

1It2 tan (t + (3,)

This distance applied to the proper scalewould give the value of (13) which, whensubtracted from unity (10), is the properaltitude fraction. However, by going onestep further, the values on the scale at thebottom of the height nomograph-largeimages have already been subtracted fromunity. Application of the distance L to thescale thus solves (10) completely, givingthe height-measurement directly as afraction of the altitude.

The mathematics for the component ofthe nadir-point distance lying on theprincipal-plane has already been derived­formula (4). And by comparing (4) and(11), the latter being the equation for thecurves on the height-nomograph-largeimages, it is seen that these curves givethe direct solution for nadir-point distance

tan (t+{3I)altitude fraction = 1 - . (10)

tan (t + (3,)

The formula for the family of curves onthe nomograph is simply

tenth the aircraft altitude, increasing,however, with increasing nadir-point dis­tance of the height. Image parameters formeasuring heights from small images are,therefore, ly and Yo, rather than YI and Y2.

Formula (7), it may be seen, is equiva­lent to formula (2) where:

cos2 {3oheight factor = --,-- (8)

cos (t + (3o) sin (t + (3o)

From Figure 4,

{3o = tan- 1 ~f

The above two formulas, (8) and (9), arethe equations for the family of curves onthe height-factor chart. The vertical axisof the chart is the Yo/! axis; the horizontalaxis is the height-factor axis. The lattervalues are plotted logarithmically; theformer, directly for direct application ofvertical-displacement values with rule ordividers. Curves are plotted for values oft from 0° to 90°.

After construction of the chart was be­gun, it was discovered that the curves weremirror images about the height-factor axis,and that curves for values of depression­angle above the axis corresponded to thosefor tilt-angle below the axis. Therefore, thebottom half of the nomograph was elimi­nated, the top half sufficing for both.

Explanation of the construction of theheight-nomograph-large-images has beendeferred to the last to show the readertricks which are possible with nomographs,and which although at first appearingcomplicated, are actually quite elemen­tary. The following solution, it may beadded, was not arrived at initially by de­duction, but through trial and error.

The formula on which the height­nomograph-large images is based is (5)where,

1/tk = tan (±{3k), (11)

where

(12)

k being any point. The vertical axis, aswith the height-factor chart, is the Yk/!axis; the horizontal, the mk axis. Curves

FIG. 5. Vertical displacements, Yl and Yz, arethe same for all boards in fence, proving valid­ity of methods for performing measurements ofvertical heights from images located anywhereon the photograph.

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908 PHOTOGRAMMETRIC ENGINEERING

in one operation. Thus, it was only neces­sary to add the proper scales for nadir­point distance along the horizontal axis.

So far in this discussion, the mathe­matics has not departed from the prin­cipal plane, and explanations for the nomo­graphs have blandly implied that themethod is valid for any point on theoblique photograph. A non-rigorous, butlucid, proof of the validity of the mathe­matics for any height'image on the photo­graph is here presented.

I n Figure 5 is shown a schematic dia­gram of an oblique photograph containinga board fence parallel to the x-axis of thephotograph. All the boards in the fence arevertical and of equal height, and on thephotograph they all project identical ver­tical displacements. Therefore, the fore-

SPECIAL CAMERAS ARE BEING TESTED

Special automatic cameras which make a rec­ord of a radar screen are being experimentallyput into operation by the Civil Aeronautics Ad­ministration to evaluate the Air Traffic ControlRadar Beacon System.

Use of the special cameras manufactured byGordon Enterprises, North Hollywood (Calif.)at C.A.A. radar sites in New York, Chicago,Washington and Norfolk has been announcedby D. M. Stuart, Director of the C.A.A. Tech­nical Development Center, Indianapolis.

The move may presage use of such camerasto record almost all aircraft landings and take­offs. Extension to marine use might in the fu­ture provide definite records of responsibilityfor such disasters as the Andrea Doria-Stock-

going mathematics, which was derivedonly for the center board, i.e., the boardlying on the principal line, will give anidentical height for all the boards, pro­vided the vertical displacements, or com­ponent of the image parallel to the y-axis,is applied to the formulas or nomographs.

V. ACKNOWLEDGMENTS

The writer would like to acknowledgethe aid and encouragement given by MajorByron L. Schatzley, USAF, who was re­sponsible for the establishment and super­vision of this project at Wright Air De­velopment Center, Wright Patterson AirForce Base, Ohio, and would also like tothank personnel of the Computer Labora­tory, WADC, for computation of points forthe nomographs.

holm collision which occurred off Nantucket onJuly 25.

The special cameras, made to C.A.A. specifi­cations, will record information on plan-positionradar indicators during special flight tests orduring periods in which it is desired to recordactual movement of air traffic along airways orin traffic patterns near airports, Stuart said.

"\iVhen the information is recorded on film, itis possible to make detailed studies after thetest period has been completed to be sure thatno significant data has been overlooked,"Stuart said.

The Gordon Enterprises cameras, using35 mm film in 100 foot magazines, are operatedautomatically by the sweep of the radar an­tenna. I n addition to photographing the radarscreen, the camera simultaneously photo­graphs a clock face, so that the exact time isincluded as part of each picture. A data card focpertinent information is also photographed.

One feature of the cameras is that they canconstantly photograph the radar screen unat­tended by an operator. Except for changing offilm and winding of the timepiece, the cameraswill perform in excess of 500 hours without re­pair or adjustment.