STEREO COWPARAGRAPH~

B. B. Talley,Captain, Corps of Engineers

In desoribing the Stereo Comparagraph, it might be of interest to state how itoame into being, slnoe it was somewhat aooidental. In Deoember, 1935, while assem­bling material for a series of leotures to be presented betore the Instltute ot Geo­graphioal Exploration ot Harvard Unlversity, 1t w~s realized that there was no sIm­ple plotting instrument available 1n the United States whloh oould be readily trans­ported, set up, and adjusted ln a tew m1nu~es, and on whioh operators could betralned in a relatively shoru ·time. The stereoplanigraph, aerooartograph, and mUlti­plex are here, but ~hey are oomplioated in varying degrees and expensive beyond the

. means of most eduoational institutions. A simple plotting instrument was deslred,whioh would illustrate the prinolples of stereosoopl0 plott1ng, give resUlts ot atleast fair aco~racy, be inexpensive (to oost less than a first-class transit, level,or alldade), and be suffioiently simple so that a olass oould become entirely tamil­iar with its operation in the time allotted 'eaoh individual over a period ot threeweeks.

Commenoing With a hand-held lorgnette stereosoope, two strips of fi1m base onthe end ot whioh a black dot ot india ink had been spotted, the prinoiple of theinstrument was evolved.

When the sketohes of a pilot model were completed, Major A. W. Stevens, AirCorps, believing it to be of some ml1itary importance, authorized the fabrioation ofa pilot model in the shops at Wright Field•. Hoping it had some value, but not quiteoertain, I bought the lenses, et oetera, from personal tunds, and in about a monthtrom the time the initial design was oommenced, the p110t model was oompleted.

Still ln preparation tor the leotures at Harvard, I worked tor about a month atnight and on Sundays completing a series of tests on it. It was only when they we~eoompleted that it was realized that here was a simple instrument whioh was more thanthe illustration of a prinoiple, but one whioh should tind a plaoe among the variousstereosoopl0 plotting instruments.

The Fairohild Aerial Camera Corporation beoame interested 1n it about this timeand rights tor its oommeroial manufaoture were obtained by them, w1th shop rightsreserved to the United States by virtue of my employment. In the short time einoeFairohild have been interested, they have perfeoted the oommeroial mpdel shown inthe illustrations, and have 'tooled-Up' tor lts manufaoture 1n quantity. .

The instrument is so designed that it plots' true to the soal~t ot one of thephotographs ut11ize 1n ~ nneot10n wlth it. It. therefore, draws a true perspeotivetrom that photograph.

It possesses three outstanding·oharacterlstiost1. Simplioity. Without exoeption eaoh ot two or three hunare~ persons to Whom

I have ehown the instrument has within a tew min~tes grasped the principles ot itsoperation and has been able to see the 'floating mark' aotually 'tloat.' Two orthree days praotioe has been all that has normally been required to obtain skillin its operation. The plott~ng ll1ustrated hereWith Was done by a Corporal ln theCorps ot Engineers after three or four days praotioe. This is his ssoond -model.-

2. Acouraoy of Spot He1ght Determination. In the determination of relat1veelevations, most experienoed operators will return to the same miohrometer readingplus or minul about 0.05 mm, whioh from photographs taken from 10,000 teet oorres­ponds to about 7 feet.

3. Compaotness. The instrument is small, oompaot, and fits into a single,11ght hardwood oas8, oomplete with acoessories and oase. It weighs about 15 pounds.It oan be set-up in a matter of about tive minutes.

fo eXplain the prinolples of the instrument and to show what is involved 1n itsoperatIon. a more oomplete deecription and instruotions tor its operat1on follow.

Deeorlption and InstruotionsI~~ oeneral' !be .tereo oomparagraphis a 8Imple stereooomparator with drawing

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attaohment for the oompilation of topographio information direotly from vertioal aeri­al photographs, or trom oblique aerial photographs which have been reotified to thevertioal. It is tormed of the tollowing units:

a. Stiereosoopeb. Measuring Systemo. DraWing Attaohmentd. Alignment Meohanism (Not integral to instrument) .

2. Stereosoope: The stereosoope i' e1th~ of the refleoting type to whioh has beenadded a pair of matohed lenses for t e magnifioation of the detail of the photographsor a prismatio tiype oomposed of aa~ched prisms and lenses. Figure 1 shows the stereooomparagraph set up ready for operation.3. Keasurine System, The measuring system of the stereo oomparagraph consists of twomarks, m1 and 1IIg, in the oenters of two lenses whose separation may be varied, andsuoh variation measured by michrometer, II. The movement of the miohrometer sor'.serves both to affeot the movement of the measuring marks, and to indioate the extentof the ,movement of the floating mark formed by the fusion of the two index marks.4. Drawlng Attaohment: The drawing attachment oonsists of, a special pencil mountedat the end of a draWing arm rigidly oonneoted to the base or the instrument. Thestereo comparagraph i8 so designed that the penCil will follow the left index markover the lett photograph and draw true to the scale of that print The pencil isconstruoted to faoi11tate lifting off the paper to allow soanning of the photographswithout\.drawing. The stereo comparagraph is normally fitted with three pencils oon­taining leads of two degrees of hardness and a prioker for marking control pOints,From the ~aised position, points may be prioked by pressing down on the pencil. Aninner spring serv~s to return the pencil to the raised position. The draWing attao~

ment is equipped With arDIs of different lengtl1s to allow greater freedom 1"n plaoiDgthe draWing medium With respect to the photographs, and to permit the utilization oflarge prints.&. Alignment Meohanism: 'The stereo comparagraph is adapted to the attachment of a.tandard type drafting maohine, or any other suitable alignment mechanism to insurethe maintenance of the adjustment ot the instrument onoe 1t i8 established.6. Operation:

a. In operating the stereo comparagraph, differenoes in elevation are repre­sented by differenoes in parallax Whioh are measured by the m1ohrometer.Such measurement 1s obtained by bringing the fused image of the two in­dex marks into contact With the stereoscopio model, and noting the read­ing of the michrometer for each point so measured. The ditterence infeet oorresponding to the michrometer readings for two or more pointemay be obtained by examination of the photographs, trom parallax ts:bles1n a similar manner to obtaining logarithms ot numbers, (Parallax Tables,Supplement to Professional Pap~rsof the Air Survey Committee, No.8,British War Offioe) or through the manUfacturer, or in the absence ofparallax tables by oomputation by means ot the parallaX equation. Theparallax equation 18 the difference in elevation (in feet or meters] cor­responding to unit differenoe of parallax (usually 1 mm.].

b. The parallax equation may be expreseed asdh/dp = (H-h)2/fB

where:db equals the differenoe in elevation oorresponding to 1 mm ot parallaxdp equals unit parallax difference (1 111m).H equals the al t1tude from whioh the photographs were taken above sea

level (or o,ther datum plane). 'h equals the height of some known point in the photograph above ,the

seleoted datum.f equals the ,fooal lengt~ ot the taking oamera (in meters).B equals the length ot the stereosoopio base ot the photographs.

This equation is ,the derivative of the equation:p/f • B/(H-h) in Figure 2.

o. Ordinarily 1t i8 not neoessary to compute elevation differences tor parallaxdifferenoes of less than 1 mm., although it may be done if desired. Usuallyan interpolation along a straight l1ne plotted to a base ot 1 mm 1s suffi­cient over the corresponding range of elevations. For contouring, such aourve maY be tollowed to advantage sinoe by its use it is possible to read

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directly the michrometer setting corresponding to any desired contour level,or to determine any elevat10n d1reotly from any mlchrometer setting.

d. If vertical control be. abundant, the miohrometer settings for given eleva­tiona may be obtained direotly by stereosoopic adjustment of the floatingmar~s on the points of known elevation. Thus. the differenoe in the michro­meter setting oorresponding to the differenoe in the elevation of the two ormore points under consideration 1s a ready determinatton of the parallax forsl~1lar pointa~ It this method is utilized, it 1s preferable to take themean of several suoh read1ngs over different parts of the photographs, other­wise the effeot of tip or tilt may not be disclosed.

g. It ls easy to determine differences 1n elevation between any two points 1na given area by any of the above methods, thereby accomp11sh1ng one of themost valuable funotlons of the 1nstrument. Such elevation differences maybe rapidly obtained from suitable photographs and used to oheck, or to sup­plement the infor.nation obtained from ordinary topographio maps, or for theoompilation ot maps ln unknown terrltoryc The application of this method indet.er.uln1ng the best route between two points in rough terrain for roadbuilding, or right of way reoonna1ssance, and so forth, is valuable.

t. Llnes of constant elevation are traced rapldly.by clamping the miohrometerat the read1ng corresponding to the desired elevation and viewing the float­lng mark as it is held in contact With the stereosoopio model While movingthe instrument over the photographs, causing the pencll to traoe a oontinuousline. Planimetrl0 detail, such as roads, rivers, et cetera, 1s compiled1n a similar manner, except that the m1chrometer setting ls varied to main­ta1n tbe floating mark in oontaot With the stereoscop10 model dur1ng plot­ting.

,. ~netruot1on8:

a. For convenience of instruction ln the use of the stereo oomparagraph, astereosoopio pair ot photographs of Fort Ano1ent, near Lebanon, Ohio (repu­t,ably the largest. rema1ning earth work 01: the Mound BUilders in NorthAmer1oa), made With a 12-i:l.Ch, K-3B, aerial camera at an altitude of about.10.000 feet above the ground have been mounted in correct stereoscop1c ad­Justment and recopied to scale. This enables a st~d8nt operator to becometaml1iar w1th the 1nstrument, and to obtain praotioe in stereoscopic plot..t,lng before he undertakes the adjustment of a similar pair.

b. On the r1ght photograph of the Fort Ancient stereoscop1c pa1r have beenplotted three positions known in elevation. To sket.ch the 20-toot for~ linesot this mounted pair, it is only neoessary to set the 1nstrument up With 1tsstereosoopic base parallel to the stereosoopic base ot the photographs (al­rea4Y adJusted), and fuse the float1ng mark over the marked points and ob­tain the m1ohrometer reading cor~esponding to the1r respeot1veEaevatlons andfrom these data obtain the mlchrometer settings correspond1ng to the oontour1nterval, and proceed as prevlo~sly outlined. This will be faoilitated Ifthe:michrometer readings corresponding to the known eleVations are plottedas absoissae and the elevations ae ordinates, and elevatlons 1n 1norementsot 90 feet obtained therefroo.

o. With the lnstrument for whioh these photographs were adjusted, the m1ohro­meter read1ngs corresponding to the known elevat10ns were as tollows:

Elevat10n Michrometer Sett1ng(feet) (mm. )659 9.35680 9.18940 7.12

(Tbe JI'.1ohrometer setting depends upon the spac1ng of the photographs and\he 1n1t1al settlng of the index marks, and w11l probably differ aocordingto the prefereno e of dl1:ferent operators, and has no bearing on the finalNault. Hovever t the d1tferenoe in parallax corresponding to equal differ­enoes 1n elevation should be the same tor all operators when the 1nstrument

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1s adJusted to the same, or to similar photographso) Thus, in the Case athand, the parallax difference between 659 feet and S80 teet 1s 0017 ma.,and between 659 feet and. 940 feet is 2~23 8m. After plotting the curve aspreViously outlined, it 18 determined that the mlohrometer settings for theinterven1ng elevations in inorements of 20 teet ares (Read from the ourve)

Elevat10n Michrometer Setting(teet) (m.)660 9.~4

6aO 9.18700 9.02720 8.~6

it oetera.d. Figure 3 shows the results Ot drawing the torm l1nes on a oopY of the lett

photograph Ot this pair (Photograph FA-ll). Figure 4 1s a more oonventionalplott1ng on draWing paper.

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