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. SCALE - FEET TO ONE INCH 8 ~ 0. 8 .- ~- 100 '" r50 , I00 50 , , l/) , \ IE: l/) 50 W W : I I­ ...J 20 V:=~=H~~~~~::C W , 30 ::: ::E II Plornle ME1IIlOS-20 IlID ~ FOOT I ! i COIITOUIIMTER"/AlS 20 ;l :I: 10 TlIMElIIOCOII lEIlll rHIITlIIllIIIS. I.5 :.:: l­e SlW.L SCALI YElTII:lL rHII_ I IlID ilEA REeOMIAIWIII:( MPS Io :I: 5 i I I e , b w ! I , ~ .~ 3 , , i I , j~llI/PIlClTlIIICI 4 IE: I Pllll£SS, STEIEOPlIlIIWPII 011 W I> ~ > iJ!i ~ P IUJIIUTOWIII-IO,5Il1D I 3 o .~ .", IQIT COII1OIIl IIlU'IlU <.> -, IE: """.,': 2 ffi e I 5 ~ z , ;:) 1.0 <.> I :" -: :;:::-::>::".' o .50 , e .~ IE: I - Cl I I COII1OIIl nlYll$ 6 I ~".,~ 5 .25 I 4 , , ~ ffi I I I I I I I I 2 .10 I 16 _18 -I~ 0 -I~ -0 -I~ -I~ ~ -I'" 18g -18g -I~ -I~ i'" SCALE - REPRESENTATIVE ,FRACTION Public Roads Administration Photo 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.