AERIAL SURVEYS IN HIGHWAY LOCATION William T. Pryor, Highway Engineer Department of Design, Public Roads Administration HORTLY after the first World War highway engineers became interested in S the 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 New York and during the same year photogrammetric methods of large scale mapping became available. Until World War II, however, general a~ceptanceof photogrammetry as an aid in the highway engineering field has been relatively slow. We now have the benefit of experience by a number of State highway depart­ ments in the use of aerial phorographs and photogrammetric methods of map­ ping. It will be the purpose of this article to summarize the main points of what has been learned regarding the various methods of aerial surveying and types of photogrammetric equipment that now appear to be best adapted fOl:,use in the successive stages of highway location. STAGES OF HIGHWAY LOCATION In regions not adequately mapped small-scale maps must be prepared. On such regional maps a system of highways can be planned, with each class of high­ way including National and State systems, secondary and land service roads rep­ resented. Terminal and major intermediate control points can be selected on the basis of this classification. Once these controls have been selected, highway loca­ tion may be carried out in the consecilt'ive stages shown in Figure 1. In referring to this figure particular attention is directed to the relationship between map scales and width of coverage in each of the location stages. Though definite lines are shown dividing these stages, it will be understood that they consist of a flexible zone or band. . 1. Reconnaissance of Area. The purpose of reconnaiss~nceof area is to learn how topography and land-use will control and affect selecfion of possible routes. In this first stage of highway location, the whole area of mapped topography between selected terminal points is studied. This area of land will contain all possible routes for the proposed new highway. With the aid of maps or mosaics and aerial photographs in reconnaissance of area, the engineer will choose a number of possible routes for study and comparison. 2. Reconnaissance of Alternate Routes. The location engineer will now'com­ pare the advantages and disadvantages of the possible highway routes. With adequate topograpJ1ic ma,ps and aerial photographs, the topography of each route will be examined to determine which route is most direct and at the same time interferes least with present and potential land-use. The examination will be based on standards of alignment, grade, and cross section required to serve the traffic for which the road is to be built. Among t;J:1e factors which influence the 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 more will be selected for preliminary location survey. 3. Preliminary Location Surveys. Having selected a route or routes for pre­ liminary location survey the engineer will obtain detailed topographic informa­ tion on strip-areas. These strips may be from several hundred ya~ds to more than a mile or more in width, and must be wide enough to include the best of all possible road locations on each alternate route. Topographic ma.ps made by photogrammetric methods for this stage, must be of a scale which will furnish topographic and land-use detail to the accuracy required by existing conditions. Such conditions may vary between open unde­ veloped range lands with easy topography where relatively small-scale ma'ps ,may be adequate, to intensively developed urban areas or rugged mountain country where topographic maps 6f the largest scale with contour intervals as small as five to two feet may be necessary.

SCALE - REPRESENTATIVE FRACTION Public Roads Administration Photo FIG. 1. Stages in the location of a highway. Note the ease with which both map and vertical photograph scales can be chosen for width of coverage required. For reconnaissance of area, use­ ful scales will lie between 2,000 and 1,000 feet to one inch for widths of .10 to 50, and 3 to 10 miles respectively; 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 normallycompleted to 100 or 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 pre­ liminary road locations. He will locate the roadway, bridges, and drainage struc­ tures, and will classify preliminary contract items, and make calculations of quantities and comparative estimates of cost on all feasible alternate road locations. Property lines will be laid down with determination of ownership, right of way, and land damage costs. The road location will be fitted in align­ ment, profife grade, cross section and structures into the topography. Having completed this and the preceding highway location stages in consecutive order, the engineer will have obtained the essential" facts required to compare all pre­ liminary locations and select the final road location. For the final location se­ lected, alIgnment and profile grades will be computed and recorded on the preliminary alignment plan and profile. 4. Location Surveys and Contract Plans. The engineer is now prepared to stake the road alignment, cross sections, structures and right of way lines on the ground. He can then prepare his final plans and estimates for contract construc­ tion. The object of highway location in all stages is the finding of that route and location which, beyond ail reasonable doubt, 'is the best available between the terminal points. Complete knowledge of topography, land-use, soil and drain­ age conditions, traffic volume and character and other factors in relation to re­ quired standards of high~ay alignment, profile grade, and cross section will make such il location possible. Aerial survey methods adapted to each suc­ cessive 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 with the chart showing stages in the location of a highway. A balance between methods and stages could thus be rationalized more easily. 432 PHOTOGRAMMETRIC ENGINEERING

THE SELECTION OF SURVEY METHODS Figure 2 shows the selection of survey methods in relation to ground coverage width and scale limits outlined for the stages in location of a highway shown on Figure 1.

Aerial Photographs and Mosaics Pairs of aerial vertical photographs are not only the foundation of topo­ graphic map making by stereophotogrammetric methods, they are also a valua­ ble auxiliary tool and should be used freely and consistently to supplement the topographic maps as well as mosaics. In the stages of area and route reconnaissance much of the information needed can be obtained from vertical photographs and mosaics a,lone, without maps of any kind. (See Fig. 3.) Stereoscopic examination of succe.ssive pairs of vertical photographs of adequate scale will, in fact, furnish information about the topography and cultural features in both open country and urban areas which cannot be obtained from the best topographic maps of the largest possible scale and most complete detail. Fundamental differences between photographs and maps should be kept in mind. For example, some are: 1. Photographs are perspective views, and in pairs under the stereoscope show the ground surface and things on it in three dimensions. A topographic map shows only those details selected and placed on it in orthogonal projection by the delineator. 2. Vertical photographs and mosaics are not maps but serve well as map substitutes in the reconnaissance stages when accurate horizontal measure­ ment is less important than complete knowledge of topography, soil condi­ tions, drainage, existing structures and land-use incl uding all cultural fea­ tures. The topographic maps ire essential in preliminary location surveys requiring accurate horizontal measurements and precise determination of differences in elevation. 3. A photograph is a complete record of the area covered from the camera position on the date it was taken. A topographic map may be prepared upon obtaining informa,tion from a number of partially complete sources by several persons at various times showing varyi,ng amounts of information to different degrees of accuracy. . Aerial photographs and mosaics have been used on a number of high­ way survey projects. Such projects may be divided into three classifications: first, surveys in which both area and 'route reconnaissance stages have been based on stereoscopic study of successive pairs of photographs and on mosaics tied to ground control surveys by radial line methods; second, surveys where form-lines have been traced by use of simple stereoscopic plotting equipment for use on mosaics, (see Fig. 4), or by plltne table methods where topography is from rolling to fairly smooth or nearly level; and third, surveys where large-scale photographs, enlargements, and mpsaics of bridge sites and intensively devel­ oped areas have been used in reconnaissance studies in advance of preliminary location surveys, as shown in Figure 5. In the use of aerial phot,-ographs and mosaics during the normal sequence of highway location stages these points cannot be overemphasized: (1) even though maps may not be available photographs and mosaics have a definite place in the early stages up to and including reconnaissance of alternate routes; :>­ tIl ....I:l:' :>­ t"' q<: ....­ ><: ot"' (") :>­ ....~ oz

Public Roads Administration Photo FIG. 3. Portion of an index of photographs used as an uncontrolled mosaic for outlining a proposed highway route in unmapped country. This illustrates approximately one-fifth of a 125 mile selected route location which was w made in an office by stereoscopic study of vertical photographs in reconnaissance of area between terminal points. w"'" 434 PHOTOGRAMMETRIC ENGINEERING

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

Public Roads Administration Photo FIG. 5. Study of a proposed bridge location and approach connections by use of a photographic enlargement of a selected portion of a vertical photograph of the river and metropolitan area. AERIAL SURVEYS IN HIGHWAY LOCATION 435

(2) with the beginning of .reconnaissance of alternate routes, photographs and mosaics should be supplememented by topographic maps to obtain best results -especially where existing topography is rolling or mountainous; (3) pairs of vertical photographs examined under the stereoscope should be used in all stages of highway location. Experience now ipdicates that certflin methods of aerial surveying and types of aerial photogrammetric equipment appear to be particularly adapted to each stage of highway location. Trimetrogon Photography and Reconnaissance Mapping In unmapped regions li~e the one through which the Alaska Highway was located and built, the highway locator can now take advantage of the trimetro­ gon method of aerial photography and rec.onnaissance mapping. This method is most useful in regional reconnaissance and planning which p'recedes recon­ naissance of area. It can be used in the reconnaissance of area stage in large areas of undeveloped country. The trimetrogon method was designed to cover extensive areas of unmapped country in a short time. Three aerial photographs are taken simultane~)Usly with camera lenses having approximate focal lengths of 6 inches. Therefore at the usual altitude of 20,000 feet the scale of the "central vertical photograph is 1 :40,000 and the useful coverage extends 12 to 25 miles on each side of the air­ craft flight line. Map manuscripts are prepared to a scale of 1:80,000 from the trimetrogon photographs, and a,re published, mainly as aeronautical charts, at 1:250,000 or smaller scales. They show centers of population, major topographic features as mountmns and ranges of mountaIns, rivers, valleys, airfields, and some of the main systems of roads combined with boundaries of political subdivisions." The broad outlines of the topography are marked by contours or form-lines! at 250- to 500- or 1,000-foot intervals. The maps should besupplemented in use by aerial photographs from which they were made. Simple Stereoscopic Plotting Equipment A number of simple portable stereoscopic plotters are available for mapping to scales larger than 1:31,680, as required in reconnaissance of area (see Figure 2). Stereocomparagraph and Contour Finder.-Stereoscopic plotters of this type have been used within the zone of reconnaissance of area to make small scale maps with topography delineated by form lines at intervals of twenty feet. In the hands of a skilled operator, with adequate ground survey control, these instruments have been of real service to the highway engineer. Under favor,able topographic conditions in fairly open'country, maps plotted by use of these instruments have been used in the smaller scale ranges for reconnaissance of alternate routes. (See Figures 1 and 2.) The stereocomparagraph and con­ tour finder are low in cost and will, no doubt, be used by many highway depart­ ments in the training of their staffs in the principles of stereoscopic plotting. Other Stereoscopic Plotters.-The K.E.K. plotter (Fig. 12) has been used in the preparation of topographic maps of value in highway location in recon­ naissance of area and alternate routes. The essential advantages of this"instru­ ment 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 regular intervals of elevation are drawn to outline the general shape and position of hills and valleys. 436 PHOTOGRAMMETRIC ENGINEERING

Correction for tilt in photographs can be made by instrument adjustment before 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 Plotter Lyon Plotter Multiscope (Fig. 14) Stereotop9graph It is understood that these plotters are not yet available but when ob­ tainable 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 (see Figure 2). As indicated in Figure 1, maps to these scales meet the requirements for both reconnaissance stages of highway location. Among the many advantages of- multiplex equipment from the standpoint of 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 the operator. 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 Autograph Figure 2 shows the range of large-scale topographic mapping which can be done by use of this stereophotogrammetric equipment. As shown on Figure 1 this range includes most map scales used in preliminary location. This stereo­ photogrammetric equipment is also effective in compiling maps to the smaller scales as used in the reconnaissance stages. One company equipped to do photo­ grammetric mapping by the Brock Process (Fig. 16) and another with the Ster­ eoplanigraph (Fig. 17) have each completed a number of aerial survey and topographic mapping projects for State highway departments. The Wild Autograph, manufactured in Switzerland, has not as yet been used in this country. It has been classed as precision photogrammetric equip­ ment by engineers w40 have had experience with it in Europe. The Wild Auto­ graph is now obtainable through the manufacturer's agent in this country. To bring out the many advantages of the accurate large scale topographic maps which can be compiled for use in highway location by the Brock and Stereoplanigraph methods, let us consider certain preliminary location survey projects where these methods were used. AERIAL SURVEYS IN HIGHWAY LOCATION 437

On these projects a number of alternate routes were selected for preliminary location survey: 1. Topographic strip maps one mile wide we~e prepared to a scale of 200 feet to one inch with contours at 5-foot intervals. 2. On these maps a number of preliminary road locations were laid out by use of the spline line (Fig. 6) in conjunction with the stereoscopic examina­ tion of the paired photographs. 3. In this way each preliminary location was carefully fitted to the topography in alignment (Fig. 7), profile grade, and cross section. 4. In making each trial location, such factors within the mile-wide zone of topography as land-use, number and length of bridges, right of way values and costs, drainage conditions, soils, together with location of points of scenic interest, and connections with other roads were carefully weighed. 5. For the preliminary road location lines, profiles were plotted, grade lines laid, cross sections plotted, structures located and excavation and embank­ ment yardage computed from topographic information provided by the map. 6. Estimates of cost of construction based on this preliminary survey information were prepared for each alternate location. 7. The best possible location on the chosen route could now be staked on the ground and contract construction plans prepared.

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

Ground surveys necessary for vertical and horizontal control for photogram­ metric mapping were carried out mainly along existing roads or across lands already in public ownership. One of the decided advantages of this type of aerial survey project is the fact that every practicable preliminary line within areas mapped can be located on the map and thoroughly anal)'~ed without trespass on private lands and disturbance or exciting of owners and the public. 438 PHOTOGRAMMETRIC ENGINEER1NG

Coordinating All Operations Reconnaissance and preliminary location surveys made on the ground are costly in terms of time and money. Effective use of aerial survey methods by highway engineers will go far to prevent a tendency to neglect or even omit either reconnaissance of area or that of alternate routes. The importance of re­ connaissance may be empha~ized ~ follows: On the project just described; had careful reconnaissance of area been omitted a number of desirable alternate routes might never have been consid­ ered. If the stage of reconnaissance of alternate routes had been neglected, or omitted, the zone of topography selected to be mapped for preliminary location survey might not have included the best of all possible locations between the terminal points. The best location within the mile-wide zone of topography mapped 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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'~';;"~"":;./ - Public Roads Administration Photo FIG. 7. Preliminary spline line location of a divided lane highway, with connections to other roads, 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 of highway location, available topographic maps having been too small in scale for preliminary location. Then ground methods were relied upon for preliminary location surveys. In difficult topography and areas of intensive land-use this meant loss of many of the advantages of aerial survey methods. Without detailed knowledge of topography furnished by a large-scale topographic map covering a wide strip-area on the best route selected in the re­ connaissance stages, a number of preliminary lines may be staked on the ground AERIAL SURVEYS IN HIGHWAY LOCATION 439 witho.ut finding the best location. Where adequate aerial photographs and topo­ graphic maps have been prepared for each of the successive stages of highway location only one road location need be staked on the ground, all preliminary locations having been made on paper. The best location afforded between the terminals of the highway is thus assured. GENERAL COMPARISON OF UNIT COSTS What are the relative unit costs of the several highway location survey methods discussed in the preceding pages? To answer this question let us con­ sider general unit costs in relation to scale of map and photograph and to methods of photogrammetric mapping. Figure 8 shows the unit cost range in dollars 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 with the items shown, Figure 8 will aid the reader in comparing unit costs of one method of obtaining highway location survey information with another. Con­ sideration of the unit costs should make it possible to choose the methpd adapted for use in each stage of highway location for a particular project. . The cost divisions fall into two separate groups. Areas one to four on the left indicate the purchase price of available topographic maps and aerial photo­ graphs. Areas five to eight on the right represent the general range in cost of aerial photography and topographic mapping. Maps and Photographs Available The cost per square mile of topographic maps purchased in quadrangle sheets from Government Departments'is indicated by area 1 on Figure 8. These maps are published to scales ranging (rom 1 :31,680 with 10-foot contour inter­ valp to 1: 125,000 with SO-foot contour intervals, and as discussed previously are useful in reconnaissance of area. . Aerial vertical photographs are also obtainable from Government Depart­ ments to scales usually ranging from 1: 10,000 to 1: 31 ,680. Area 4 indicates the general range in unit cost of vertical photographs having about 60 per cent endlap and 30 per cent sidelap for stereoscopic study. The Department of Agriculture, for example, has this type of photographs available to scale of 1:20,000 over whole farm land counties in many states. Photographic enlarge­ mentf;l of these photographs have a unit cost range shown by area 2. Both the vertical photographs and enlargements ape useful in reconnaissance, and are especially valuable for use in conjunction with the topographic maps. Photographic reproductions of multiplex manuscript maps to scales useful in both reconnaissance stages may be purchased when available. The unit cost range of these useful topographic maps is indicated by area 3. Mosaics of certain cities and urban areas have been prepared by Govern­ ment Departments. Copies of these mosaics when available to scales of about 1: 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 to compare with that obtained by aerial photography of a lflter date. Changes in city 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 aerial photographs that can be obtained at a nominal price of less than one dollar to less than one cent per square mile. The range in unit cost of each available map or photograph is governed largely by the number required for the location of a project. The highway locator will recognize the advantage of obtaining all .... o.... COST PER ACRE - DOLLARS 0.1.2 .5 LO 2 5 10 15 1 1100,000 ,200,000 50,000 I 600.000

20,000I I PURCHASE: 6. AVAILABLE TOPO(;RAPHIC MAPS AND ~4o,OOO ..1 AERIAL PHOTOGRAPHY Ar.W TOPOGRAPHIC MAPPING .A RIAL VERTICAL PHOTOGRAPH 10,000 '" __'_Z --' 120.ooo Q J: '. .... "d U. 1 ::t: ~ 5,000 528 t; ...... 60,000 o ~::. « >-l 4J .... '" ..-. IE o ~ Z .. ,. r-~ Q I-c;;-.,:- - 2640 1 W 02,000 " ~ ~ ...... ,• 5 6 24,000 o 4' " ~ ~ .~, 1'. " , I- f,oOO . 2 ", '" " ~ ", " " i2,OOO~ trl I­ , >-l w I W ::

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

Public Roads Administration Photo FIG. 10. The rural highway located and built in a mountainous area. 442 PHOTOGRAMMETRIC ENGINEERING available maps and photographs in the initial stages of highway location before beginning additional aerial photography and topographic mapping. Aerial Photography and Topographic Mapping Wherever additional or new aerial photography is required for stereoscopic study or photogrammetric mapping, the highway locator may expect unit costs within the range of area 5. Unit costs of topographic mapping by the multi­ plex and other stereoscopic plotting methods lie within area 6. The unit costs of farge scale topographic mapping by the Brock Process and Stereoplanigraph range as shown in area 7. It is a~sumed that the Wild Auto­ graph, when available for use in this country, will do topographic mapping within similar unit cost limits. The unit costs of mapping shown in both areas 6 and 7 include necessary ground control surveys as well as vertical photographs taken 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

Public Roods Administration Photo FIG. 11. The modern highway in an urban area complete with grade separations and rotary intersections. AERIAL SURVEYS IN HIGHWAY LOCATION 443

separate methods will be affected by such factors as: topography, land-use, and size of area; ground cover, season of year, and weather; location of project and details to be shown on the map together with scale of photographs and maps as well as contour interval required. Ground Surveys Finally, ground survey unit costs will largely lie within or to the right of area 8. It will be understood that this range of unit costs include: 1. Preliminary location surveys by ground methods where they have not been 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 align­ ment (centerline), cross sections, structures, and right of way. In areas 5 to 8 of Figure 8, we have considered methods of surveying for highway location that cost from two dollars to ten thousand dollars per square mile. With this in mind, it would be well to reflect, before proceeding to the conclusions, that for each of the successive stages of highway location the locating engineer will choose the aerial survey method best adapted for use on his particular project, according to: (a) its size and location, whether in mapped or unmapped country; (b) the character of the two kinds of major controls be­ tween terminal points-those arising from the needs of man and those set by the forces of nature; (c) types of aerial surveying and photogrammetric equip­ ment immediately available. The highway locator can obtain full value from his use of aerial survey methods if he keeps in mind that there are ~wo main uses for aerial photographs in all stages of his work: (1) interpretation, and (2) measurement. 1. Interpretation of topographic features in relation to uses of land by nature and man is accomplished most effectively by application of the stereo­ scopic principle. By examination of pairs of vertical photographs under the stereoscope, pictured ground conditions, trees and other cover, and cultural features may be s~en in three dimensions and interpreted as though going over every square yard on foot. 2. Data for measurement of horizontal distances and ,determination' of differences in elevation are provided by topographic maps. These maps can be prepared by use of aerial photographs and photogrammetric equipment with requisite ground control surve~. Map accuracy increases progressively with increase in map Elcale and decrease in contour interval until the highway is located and staked for construction.

CONCLUSIONS The possibilities in the use of aerial survey methods are not yet fully ap­ preciated by highway engineers. Enough work has been done in this field, how­ ever, to bring out these fundamental principles relating to the use of various aerial surveying methods and types of photogrammetric equipment in highway location: 1. To be fully effective aerial surveys should be carried out in a series of coordinated stages, each of which provides the ground work for the succeeding stage. Omission of a single link in the chain, which begins after selection of terminal points in regional reconnaissance and planning and ends with location 444 PHOTOGRAMMETRIC ENGINEERING surveys and contract plans, may result in loss of many of the advantages of aerial survey methods. 2. Each type of aerial survey has a place in highway location. Aerial photo­ graphs and mosaics are valuable in the reconnaissance stages. Small-scale topo­ graphic maps are tools most useful in the reconnaissance of area stage. Large­ scale topographic maps are useful tools eliminating arduous arid costly ground work in the preliminary location survey stages. 3. Each type of photogrammetric equipment has a place in aerial surveys for highways (see Figure 2). Selection of the best types of pllOtogrammetric equip­ ment will be based on the contour interval required on the topographic map together with the mapping scale and range in contour interval inherent in the design of the equipment. Finally, it will be remembered that complete information about topography and land-use can be obtained easily by stereoscopic study and interpretation of aerial photographs of adequate scale; that aerial photographs are basic to ap­ plication of the 6tereoscopic principle in photogrammetric mapping; and that the most-effective use of these methods will help the highway locator in attaining the best highway lQcation between given terminal points. The cooperative efforts and skills of photogrammetric engineers and the designers and manu­ facturers of aerial surveying and photogrammetric equipment will bring about improved and more extensive use of these methods.

FIG. 12. The K. E. K. Plotter. 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. ., '

AERIAL SURVEYS IN HIGHWAY LOCATION 447

FIG. 17. The Stereoplanigraph in operation.

ERRATA Vol. XII, No.3, Sept. 1946, p. 299. In the line reading "From the similar triangles VP1p and VI1p" delete the sub 1 after the 1. The formula immediately below that line should read: Vl vp --=-. VP1 Vp The seventh line from the bottom of the page: change (n and N) to (i and I). Page 300, Fig. 3. The angle a is established by the intersection of the Negative Plane and Lens Plane. The angle ~ by the Lens and easel Planes. Page 301. In the line following equation (1) change vlw to vlu.