aerial surveys in highway location - asprs

AERIAL SURVEYS IN HIGHWAY LOCATION

William T. Pryor, Highway EngineerDepartment of Design, Public Roads Administration

SHORTLY after the first World War highway engineers became interested inthe use of aerial photographs in highway focation. In 1924 aerial photo

graJ?hs and mosaics were u~ed in the location of parkways in the S~te of NewYork and during the same year photogrammetric methods of large scalemapping became available. Until World War II, however, general a~ceptanceofphotogrammetry as an aid in the highway engineering field has been relativelyslow.

We now have the benefit of experience by a number of State highway departments in the use of aerial phorographs and photogrammetric methods of mapping. It will be the purpose of this article to summarize the main points of whathas been learned regarding the various methods of aerial surveying and types ofphotogrammetric equipment that now appear to be best adapted fOl:,use in thesuccessive stages of highway location.

STAGES OF HIGHWAY LOCATION

In regions not adequately mapped small-scale maps must be prepared. Onsuch regional maps a system of highways can be planned, with each class of highway including National and State systems, secondary and land service roads represented. Terminal and major intermediate control points can be selected on thebasis of this classification. Once these controls have been selected, highway location may be carried out in the consecilt'ive stages shown in Figure 1. In referringto this figure particular attention is directed to the relationship between mapscales and width of coverage in each of the location stages. Though definite linesare shown dividing these stages, it will be understood that they consist of aflexible zone or band. .

1. Reconnaissance of Area. The purpose of reconnaiss~nceof area is to learnhow topography and land-use will control and affect selecfion of possible routes.In this first stage of highway location, the whole area of mapped topographybetween selected terminal points is studied. This area of land will contain allpossible routes for the proposed new highway. With the aid of maps or mosaicsand aerial photographs in reconnaissance of area, the engineer will choose anumber of possible routes for study and comparison.

2. Reconnaissance of Alternate Routes. The location engineer will now'compare the advantages and disadvantages of the possible highway routes. Withadequate topograpJ1ic ma,ps and aerial photographs, the topography of eachroute will be examined to determine which route is most direct and at the sametime interferes least with present and potential land-use. The examination willbe based on standards of alignment, grade, and cross section required to servethe traffic for which the road is to be built. Among t;J:1e factors which influencethe choice of a route may be:

Occurrence of badly drained ground.Character of soils.Drainage conditions.Character of bridge sites.N u,mber and cost of structures.Grades and curvature.Volume of excavation and embankment in relation to erosion, maintenance

cost, and possible hazards to traffic. .

429

430 PHOTOGRAMMETRIC ENGINEERING

Destruction of land and landscape values.Right of way costs and land damage claims.Directness of route to avoid unnecessary length between terminal points.

Upon completion of the reconnaissance of all possibl,e routes, one or morewill be selected for preliminary location survey.

3. Preliminary Location Surveys. Having selected a route or routes for preliminary location survey the engineer will obtain detailed topographic information on strip-areas. These strips may be from several hundred ya~ds to more thana mile or more in width, and must be wide enough to include the best of allpossible road locations on each alternate route.

Topographic ma.ps made by photogrammetric methods for this stage, mustbe of a scale which will furnish topographic and land-use detail to the accuracyrequired by existing conditions. Such conditions may vary between open undeveloped range lands with easy topography where relatively small-scale ma'ps

,may be adequate, to intensively developed urban areas or rugged mountaincountry where topographic maps 6f the largest scale with contour intervals assmall as five to two feet may be necessary.

SCALE - REPRESENTATIVE FRACTIONPublic Roads Administration Photo

FIG. 1. Stages in the location of a highway. Note the ease with which both map and verticalphotograph scales can be chosen for width of coverage required. For reconnaissance of area, useful scales will lie between 2,000 and 1,000 feet to one inch for widths of .10 to 50, and 3 to 10 milesrespectively; for reconnaissance of alternate routes 600 to 400 feet to one inch for 1 to 3 miles'and for preliminary location surveys 200 or 100 feet to one inch for topographic maps one quarte;to one mile wide. Location survey and contract construction plans are normally completed to 100or 50 feet to one'inch scales. Special and structure site plans are prepared to larger scales.

AERIAL SURVEYS IN HIGHWAY LOCATION 431

On such maps the highway engineer will make accurately measured preliminary road locations. He will locate the roadway, bridges, and drainage structures, and will classify preliminary contract items, and make calculations ofquantities and comparative estimates of cost on all feasible alternate roadlocations. Property lines will be laid down with determination of ownership,right of way, and land damage costs. The road location will be fitted in alignment, profife grade, cross section and structures into the topography. Havingcompleted this and the preceding highway location stages in consecutive order,the engineer will have obtained the essential" facts required to compare all preliminary locations and select the final road location. For the final location selected, alIgnment and profile grades will be computed and recorded on thepreliminary alignment plan and profile.

4. Location Surveys and Contract Plans. The engineer is now prepared tostake the road alignment, cross sections, structures and right of way lines on theground. He can then prepare his final plans and estimates for contract construction.

The object of highway location in all stages is the finding of that route andlocation which, beyond ail reasonable doubt, 'is the best available between theterminal points. Complete knowledge of topography, land-use, soil and drainage conditions, traffic volume and character and other factors in relation to required standards of high~ay alignment, profile grade, and cross section willmake such il location possible. Aerial survey methods adapted to each successive stage of location can provide this complete knowledge.

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FIG. 2. Selection of survey methods. This chart was designed for use in conjunction withthe chart showing stages in the location of a highway. A balance between methods and stagescould thus be rationalized more easily.


THE SELECTION OF SURVEY METHODS

Figure 2 shows the selection of survey methods in relation to ground coveragewidth and scale limits outlined for the stages in location of a highway shown onFigure 1.

Aerial Photographs and Mosaics

Pairs of aerial vertical photographs are not only the foundation of topographic map making by stereophotogrammetric methods, they are also a valuable auxiliary tool and should be used freely and consistently to supplement thetopographic maps as well as mosaics.

In the stages of area and route reconnaissance much of the informationneeded can be obtained from vertical photographs and mosaics a,lone, withoutmaps of any kind. (See Fig. 3.) Stereoscopic examination of succe.ssive pairs ofvertical photographs of adequate scale will, in fact, furnish information aboutthe topography and cultural features in both open country and urban areaswhich cannot be obtained from the best topographic maps of the largest possiblescale and most complete detail. Fundamental differences between photographsand maps should be kept in mind. For example, some are:

1. Photographs are perspective views, and in pairs under the stereoscopeshow the ground surface and things on it in three dimensions. A topographicmap shows only those details selected and placed on it in orthogonal projectionby the delineator.

2. Vertical photographs and mosaics are not maps but serve well as mapsubstitutes in the reconnaissance stages when accurate horizontal measurement is less important than complete knowledge of topography, soil conditions, drainage, existing structures and land-use incl uding all cultural features. The topographic maps ire essential in preliminary location surveysrequiring accurate horizontal measurements and precise determination ofdifferences in elevation.

3. A photograph is a complete record of the area covered from the cameraposition on the date it was taken. A topographic map may be prepared uponobtaining informa,tion from a number of partially complete sources by severalpersons at various times showing varyi,ng amounts of information to differentdegrees of accuracy. .

Aerial photographs and mosaics have been used on a number of highway survey projects. Such projects may be divided into three classifications:first, surveys in which both area and 'route reconnaissance stages have beenbased on stereoscopic study of successive pairs of photographs and on mosaicstied to ground control surveys by radial line methods; second, surveys whereform-lines have been traced by use of simple stereoscopic plotting equipment foruse on mosaics, (see Fig. 4), or by plltne table methods where topography is fromrolling to fairly smooth or nearly level; and third, surveys where large-scalephotographs, enlargements, and mpsaics of bridge sites and intensively developed areas have been used in reconnaissance studies in advance of preliminarylocation surveys, as shown in Figure 5.

In the use of aerial phot,-ographs and mosaics during the normal sequenceof highway location stages these points cannot be overemphasized: (1) eventhough maps may not be available photographs and mosaics have a definiteplace in the early stages up to and including reconnaissance of alternate routes;

Public Roads Administration Photo

FIG. 3. Portion of an index of photographs used as an uncontrolled mosaic for outlining a proposed highwayroute in unmapped country. This illustrates approximately one-fifth of a 125 mile selected route location which wasmade in an office by stereoscopic study of vertical photographs in reconnaissance of area between terminal points.

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FIG. 4. Using the spline line on a form-lined mosaic in reconnaissance of alternate routes in advanceof selecting a route for preliminary survey.


FIG. 5. Study of a proposed bridge location and approach connections by use of a photographicenlargement of a selected portion of a vertical photograph of the river and metropolitan area.


(2) with the beginning of .reconnaissance of alternate routes, photographs andmosaics should be supplememented by topographic maps to obtain best results-especially where existing topography is rolling or mountainous; (3) pairs ofvertical photographs examined under the stereoscope should be used in allstages of highway location.

Experience now ipdicates that certflin methods of aerial surveying and typesof aerial photogrammetric equipment appear to be particularly adapted toeach stage of highway location.

Trimetrogon Photography and Reconnaissance MappingIn unmapped regions li~e the one through which the Alaska Highway was

located and built, the highway locator can now take advantage of the trimetrogon method of aerial photography and rec.onnaissance mapping. This methodis most useful in regional reconnaissance and planning which p'recedes reconnaissance of area. It can be used in the reconnaissance of area stage in largeareas of undeveloped country.

The trimetrogon method was designed to cover extensive areas of unmappedcountry in a short time. Three aerial photographs are taken simultane~)Usly

with camera lenses having approximate focal lengths of 6 inches. Therefore atthe usual altitude of 20,000 feet the scale of the "central vertical photograph is1 :40,000 and the useful coverage extends 12 to 25 miles on each side of the aircraft flight line.

Map manuscripts are prepared to a scale of 1: 80,000 from the trimetrogonphotographs, and a,re published, mainly as aeronautical charts, at 1: 250,000or smaller scales. They show centers of population, major topographic featuresas mountmns and ranges of mountaIns, rivers, valleys, airfields, and some of themain systems of roads combined with boundaries of political subdivisions." Thebroad outlines of the topography are marked by contours or form-lines! at250- to 500- or 1,000-foot intervals. The maps should be supplemented in use byaerial photographs from which they were made.

Simple Stereoscopic Plotting EquipmentA number of simple portable stereoscopic plotters are available for mapping

to scales larger than 1 :31,680, as required in reconnaissance of area (see Figure2).

Stereocomparagraph and Contour Finder.-Stereoscopic plotters of thistype have been used within the zone of reconnaissance of area to make smallscale maps with topography delineated by form lines at intervals of twentyfeet. In the hands of a skilled operator, with adequate ground survey control,these instruments have been of real service to the highway engineer. Underfavor,able topographic conditions in fairly open' country, maps plotted by use ofthese instruments have been used in the smaller scale ranges for reconnaissanceof alternate routes. (See Figures 1 and 2.) The stereocomparagraph and contour finder are low in cost and will, no doubt, be used by many highway departments in the training of their staffs in the principles of stereoscopic plotting.

Other Stereoscopic Plotters.-The K.E.K. plotter (Fig. 12) has been usedin the preparation of topographic maps of value in highway location in reconnaissance of area and alternate routes. The essential advantages of this "instrument compared with the stereocomparagraph and the contour finder in map-ping for.highway purposes are: "

1 Where available vertical control is not adequate for contouring, form-lines spaced at regularintervals of elevation are drawn to outline the general shape and position of hills and valleys.


Correction for tilt in photographs can be made by instrument adjustmentbefore plotting begins.

True contours rather than topographic form-lines can be plotted.Adjustments can be made to correct" for v~riaitions in the sca,le of each

photogr~ph in the successive pairs to keep the map to a uniform scale.Other simple types of stereoscopic plotters which show promise in the

highway engineering field include:Mahap Plotter (Fig. 13)Cook PlotterLyon PlotterMultiscope (Fig. 14)Stereotop9graph

It is understood that these plotters are not yet available but when obtainable they should be very useful in the first two stages of highway location.

Advantages of this group of plotters include: Portability and low cost;training of engineers in use of stereoscopic plotting equipment.

Multiplex Aero-Projector (Fig. 15)The multiplex method of topographic mapping h~ an important place in

highway location.Excellent topographic maps may be compiled with multiplex equipment to

scales of 1: 6,000 with 10-foot contours; to 1: 20,000 with 20-foot contours (seeFigure 2). As indicated in Figure 1, maps to these scales meet the requirementsfor both reconnaissance stages of highway location.

Among the many advantages of- multiplex equipment from the standpointof the highway engineer are these:

The equipment is equally effective in both area and strip mapping.Precise adjustment for tilt, variation in scale of photographs, and orien

tation to datum plane controls can be quickly made under the eye of theoperator.

Multiplex equipment has been widely used by and for the armed forces,and trained operators should now be available.

Multiplex aero-projectors complete with all accessory eq1.!ipment is ob,tainable in this country.

Brock Process, Stereoplanigraph, and Wild AutographFigure 2 shows the range of large-scale topographic mapping which can be

done by use of this stereophotogrammetric equipment. As shown on Figure 1this range includes most map scales used in preliminary location. This stereophotogrammetric equipment is also effective in compiling maps to the smallerscales as used in the reconnaissance stages. One company equipped to do photogrammetric mapping by the Brock Process (Fig. 16) and another with the Stereoplanigraph (Fig. 17) have each completed a number of aerial survey andtopographic mapping projects for State highway departments.

The Wild Autograph, manufactured in Switzerland, has not as yet beenused in this country. It has been classed as precision photogrammetric equipment by engineers w40 have had experience with it in Europe. The Wild Autograph is now obtainable through the manufacturer's agent in this country.

To bring out the many advantages of the accurate large scale topographicmaps which can be compiled for use in highway location by the Brock andStereoplanigraph methods, let us consider certain preliminary location surveyprojects where these methods were used.


On these projects a number of alternate routes were selected for preliminarylocation survey:

1. Topographic strip maps one mile wide we~e prepared to a scale of 200feet to one inch with contours at 5-foot intervals.

2. On these maps a number of preliminary road locations were laid outby use of the spline line (Fig. 6) in conjunction with the stereoscopic examination of the paired photographs.

3. In this way each preliminary location was carefully fitted to thetopography in alignment (Fig. 7), profile grade, and cross section.

4. In making each trial location, such factors within the mile-wide zoneof topography as land-use, number and length of bridges, right of way valuesand costs, drainage conditions, soils, together with location of points of scenicinterest, and connections with other roads were carefully weighed.

5. For the preliminary road location lines, profiles were plotted, gradelines laid, cross sections plotted, structures located and excavation and embankment yardage computed from topographic information provided by the map.

6. Estimates of cost of construction based on this preliminary surveyinformation were prepared for each alternate location.

7. The best possible location on the chosen route could now be staked onthe ground and contract construction plans prepared.


FIG. 6. Using the spline line on a topographic map of large scale in the preliminary location ~f ahighway to fit the roadway to the controls of both topography and land-use.

Ground surveys necessary for vertical and horizontal control for photogrammetric mapping were carried out mainly along existing roads or across landsalready in public ownership. One of the decided advantages of this type ofaerial survey project is the fact that every practicable preliminary line withinareas mapped can be located on the map and thoroughly anal)'~ed withouttrespass on private lands and disturbance or exciting of owners and the public.

438 PHOTOGRAMMETRIC ENGINEER1NG

Coordinating All OperationsReconnaissance and preliminary location surveys made on the ground are

costly in terms of time and money. Effective use of aerial survey methods byhighway engineers will go far to prevent a tendency to neglect or even omiteither reconnaissance of area or that of alternate routes. The importance of reconnaissance may be empha~ized~ follows:

On the project just described; had careful reconnaissance of area beenomitted a number of desirable alternate routes might never have been considered. If the stage of reconnaissance of alternate routes had been neglected, oromitted, the zone of topography selected to be mapped for preliminary locationsurvey might not have included the best of all possible locations between theterminal points. The best location within the mile-wide zone of topographymapped to the scale of 200 feet to one inch might, under these circumstances,have been inferior to one entirely outside this mapped area.

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FIG. 7. Preliminary spline line location of a divided lane highway, with connections to otherroads, laid out on a large scale topographic map prepared by precise photogrammetric methods.Observe the coordinate grid and alignment notes.

In some cases aerial surveys have included only the reconnaissance stages ofhighway location, available topographic maps having been too small in scale forpreliminary location. Then ground methods were relied upon for preliminarylocation surveys. In difficult topography and areas of intensive land-use thismeant loss of many of the advantages of aerial survey methods.

Without detailed knowledge of topography furnished by a large-scaletopographic map covering a wide strip-area on the best route selected in the reconnaissance stages, a number of preliminary lines may be staked on the ground


witho.ut finding the best location. Where adequate aerial photographs and topographic maps have been prepared for each of the successive stages of highwaylocation only one road location need be staked on the ground, all preliminarylocations having been made on paper. The best location afforded between theterminals of the highway is thus assured.

GENERAL COMPARISON OF UNIT COSTS

What are the relative unit costs of the several highway location surveymethods discussed in the preceding pages? To answer this question let us consider general unit costs in relation to scale of map and photograph and tomethods of photogrammetric mapping. Figure 8 shows the unit cost range indollars per square mile and per acre based on the best information available.Although the experience of others may lead to unit costs in slight variance withthe items shown, Figure 8 will aid the reader in comparing unit costs of onemethod of obtaining highway location survey information with another. Consideration of the unit costs should make it possible to choose the methpd adaptedfor use in each stage of highway location for a particular project. .

The cost divisions fall into two separate groups. Areas one to four on theleft indicate the purchase price of available topographic maps and aerial photographs. Areas five to eight on the right represent the general range in cost ofaerial photography and topographic mapping.

Maps and Photographs AvailableThe cost per square mile of topographic maps purchased in quadrangle

sheets from Government Departments'is indicated by area 1 on Figure 8. Thesemaps are published to scales ranging (rom 1 :31,680 with 10-foot contour intervalp to 1: 125,000 with SO-foot contour intervals, and as discussed previouslyare useful in reconnaissance of area. .

Aerial vertical photographs are also obtainable from Government Departments to scales usually ranging from 1: 10,000 to 1: 31 ,680. Area 4 indicates thegeneral range in unit cost of vertical photographs having about 60 per centendlap and 30 per cent sidelap for stereoscopic study. The Department ofAgriculture, for example, has this type of photographs available to scale of1: 20,000 over whole farm land counties in many states. Photographic enlargementf;l of these photographs have a unit cost range shown by area 2. Both thevertical photographs and enlargements ape useful in reconnaissance, and areespecially valuable for use in conjunction with the topographic maps.

Photographic reproductions of multiplex manuscript maps to scales usefulin both reconnaissance stages may be purchased when available. The unit costrange of these useful topographic maps is indicated by area 3.

Mosaics of certain cities and urban areas have been prepared by Government Departments. Copies of these mosaics when available to scales of about1: 20,000 and 1: 10,000 can be obtained at unit costs lying within areas 2 and 3.Such mosaics are valuable in furnishing land-use information of one date tocompare with that obtained by aerial photography of a lflter date. Changes incity growth can be determined and evaluated in relation to future trends affect-ing a proposed highway location. .

Thus far we have considered general area topographic maps and aerialphotographs that can be obtained at a nominal price of less than one dollar toless than one cent per square mile. The range in unit cost of each available mapor photograph is governed largely by the number required for the location ofa project. The highway locator will recognize the advantage of obtaining all

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of the unit costs in relation to Highway Location Surveys and the scale of photography and mapping required.

AERIAL SURVEYS IN HIGHWAY LOCATION


Fig. 9. A highway located and built to fit the topography and rural land-use.

441


FIG. 10. The rural highway located and built in a mountainous area.

Public Roods Administration Photo

FIG. 11. The modern highway in an urban area complete with grade separationsand rotary intersections.

available maps and photographs in the initial stages of highway location beforebeginning additional aerial photography and topographic mapping.

Aerial Photography and Topographic MappingWherever additional or new aerial photography is required for stereoscopic

study or photogrammetric mapping, the highway locator may expect unitcosts within the range of area 5. Unit costs of topographic mapping by the multiplex and other stereoscopic plotting methods lie within area 6.

The unit costs of farge scale topographic mapping by the Brock Process andStereoplanigraph range as shown in area 7. It is a~sumed that the Wild Autograph, when available for use in this country, will do topographic mappingwithin similar unit cost limits. The unit costs of mapping shown in both areas6 and 7 include necessary ground control surveys as well as vertical photographstaken according to specifications for the requirements of the contour interval,scale of the topographic map, and photogrammetric method of mapping.

Unit costs of all aerial photography and topographic mapping by these

PHOTOGRAMMETRIC ENGINEERING442


separate methods will be affected by such factors as: topography, land-use, andsize of area; ground cover, season of year, and weather; location of project anddetails to be shown on the map together with scale of photographs and maps aswell as contour interval required.

Ground Surveys

Finally, ground survey unit costs will largely lie within or to the right of area8. It will be understood that this range of unit costs include:

1. Preliminary location surveys by ground methods where they have notbeen done by aerial methods.

2. Special large-scale topographic mapping at site locations of bridges,grade separations, rotary intersections, and so on. .

3. Location Surveys-the staking of the located highway including alignment (centerline), cross sections, structures, and right of way.

In areas 5 to 8 of Figure 8, we have considered methods of surveying forhighway location that cost from two dollars to ten thousand dollars per squaremile. With this in mind, it would be well to reflect, before proceeding to theconclusions, that for each of the successive stages of highway location thelocating engineer will choose the aerial survey method best adapted for use onhis particular project, according to: (a) its size and location, whether in mappedor unmapped country; (b) the character of the two kinds of major controls between terminal points-those arising from the needs of man and those set bythe forces of nature; (c) types of aerial surveying and photogrammetric equipment immediately available.

The highway locator can obtain full value from his use of aerial surveymethods if he keeps in mind that there are ~wo main uses for aerial photographsin all stages of his work: (1) interpretation, and (2) measurement.

1. Interpretation of topographic features in relation to uses of land bynature and man is accomplished most effectively by application of the stereoscopic principle. By examination of pairs of vertical photographs under thestereoscope, pictured ground conditions, trees and other cover, and culturalfeatures may be s~en in three dimensions and interpreted as though goingover every square yard on foot.

2. Data for measurement of horizontal distances and ,determination' ofdifferences in elevation are provided by topographic maps. These maps canbe prepared by use of aerial photographs and photogrammetric equipmentwith requisite ground control surve~. Map accuracy increases progressivelywith increase in map Elcale and decrease in contour interval until the highwayis located and staked for construction.

CONCLUSIONS

The possibilities in the use of aerial survey methods are not yet fully appreciated by highway engineers. Enough work has been done in this field, however, to bring out these fundamental principles relating to the use of variousaerial surveying methods and types of photogrammetric equipment in highwaylocation:

1. To be fully effective aerial surveys should be carried out in a series ofcoordinated stages, each of which provides the ground work for the succeedingstage. Omission of a single link in the chain, which begins after selection ofterminal points in regional reconnaissance and planning and ends with location

FIG. 12. The K. E. K. Plotter.

PHOTOGRAMMETRIC ENGINEERING

surveys and contract plans, may result in loss of many of the advantages ofaerial survey methods.

2. Each type of aerial survey has a place in highway location. Aerial photographs and mosaics are valuable in the reconnaissance stages. Small-scale topographic maps are tools most useful in the reconnaissance of area stage. Largescale topographic maps are useful tools eliminating arduous arid costly groundwork in the preliminary location survey stages.

3. Each type of photogrammetric equipment has a place in aerial surveys forhighways (see Figure 2). Selection of the best types of pllOtogrammetric equipment will be based on the contour interval required on the topographic maptogether with the mapping scale and range in contour interval inherent in thedesign of the equipment.

Finally, it will be remembered that complete information about topographyand land-use can be obtained easily by stereoscopic study and interpretation ofaerial photographs of adequate scale; that aerial photographs are basic to application of the 6tereoscopic principle in photogrammetric mapping; and thatthe most-effective use of these methods will help the highway locator in attainingthe best highway lQcation between given terminal points. The cooperativeefforts and skills of photogrammetric engineers and the designers and manufacturers of aerial surveying and photogrammetric equipment will bring aboutimproved and more extensive use of these methods.

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AERIAL SURVEYS IN HIGHWAY LOCATION 445)

FIG. 13. The Mahan Plotter.

FIG. 14. The Multiscope.

FIG. 15. The Bausch and Lomb Multiplex Aero Projector in use.

FIG. 16. The Brock Stereometer.

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FIG. 17. The Stereoplanigraph in operation.

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aerial surveys in highway location - asprs

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