ns, 
  
35 
to make the needed revisions and/or corrections by field surveys. In this case, 
complete remapping by photogrammetric methods, utilizing correct control, is 
first necessary. 
Repeated use is made of either one or both of the preceding methods, after 
the maps are initially "corrected and/or revised and completed," and subsequent 
accuracy and completeness tests prove the maps are not yet sufficiently accu- 
rate and/or complete. 
Engineering use of photographs and topographic maps 
  
To accomplish the essential and detailed engineering design when direct 
procedure is employed in the preliminary survey, the aerial photographs and 
maps are used simultaneously. The photographs are examined stereoscopically, 
interpreted, and analyzed to obtain the qualitative information needed, which 
is not, or cannot be, shown on the maps. The maps provide all the dimensional 
data, including shape, size, and position. The service standards established 
by the class of highway to be located and designed, as determined according to 
the traffic to be served, govern in detail all engineering work subsequently 
performed. The design of the highway is accomplished in all of its physical, 
geometric, and esthetic aspects by comparison procedures of fitting the highway 
to the topography and land use, horizontally, using a spline line or appropri- 
ate templates on the topographic maps, and cross sectionally, using the cross 
section of highway on representative cross sections of the ground. 
Qualitatively the engineering design is done by photographically inter- 
preting and analyzing the aerial photographs. Quantitatively the engineering 
design is done by using the contours on the maps and grading contours of the 
highway imposed thereon, or by using cross sections of ground surface measured 
from contours on maps, or measured on stereoscopic models while the maps are 
compiled or later in relation to initially positioned centerline and cross sec- 
tions of highway, including excavation, side ditches, embankment, and cross 
drainage; or by using a digital model of the ground and cross sections of high- 
way. Electronic methods of computation are employed, as advantageous. The 
initially designed location is examined and adjustments made therein, as desir- 
able and feasible, to attain the best possible location for the highway on the 
surveyed route on the basis of qualitative information, physical features, 
quantitative data, rights-of-way, traffic services, and the like. 
The detailed qualitative information needed and utilized in this phase of 
the work is that which specifically affects highway engineering decisions. As 
obtained from the aerial photographs, the individual factors which may associ- 
ate in any combination to govern such decisions in regard to topography and 
land use are composition, prevalence, relationships, influence, interference, 
adaptability, feasibility, scope, and value. Such pertinent factors regarding 
topography, which is the composite configuration of the ground, include its 
form and the natural features on it, its character and condition; type and 
intensity of ground cover; character and area of drainage; type, condition, and 
probable thickness of soils; class and condition of ground structure; and water 
and wind erosion. Also such pertinent factors regarding land use, in relation- 
ship to its intensity and condition, include the rural--undeveloped (pristine), 
farming, grazing, and recreational; the urban--residential, recreational, and 
commercial; industrial--manufacturing, mining, storage, exchange, and other 
services; special site uses--cemeteries, churches, public buildings, historical, 
archeological, and defense; and transportation facilities--highways, railroads, 
navigable rivers, streams, and lakes, and airports.