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Some of the direct manners in which aerial photographs are used are in 
the obtainment and utilization of information and in delineation of proposed 
route locations in stereoscopic correspondence with topography where they are 
intended to be positioned on the ground. All this is possible because of the 
three-dimensional concepts achieved by stereoscopic examination of the photo- 
graphs. Other direct ways in which aerial photographs are used are in illus- 
trating the relationship of one controlling factor to another in their detail, 
number, size, proximity, and scope; in demonstrating problems such as those 
related to traffic, topography, drainage, soils, landscaping, snow, sun expo- 
sure, separation of. roads, existing and needed road access and where it should 
be restricted or eliminated, bridges and other structures, land use, rights- 
of-way, and property severance; and in demonstrating proposed designs and/or 
solutions to problems. Moreover, aerial photographs are used directly for 
right-of-way determination and designation, and in right-of-way dealings to 
achieve better understanding between principals, for evaluation by land 
appraisers, for negotiation, and for condemnation proceedings. 
Some of the indirect uses of aerial photographs for highways are in noting 
physical and/or cultural controls, obstacles, and objectives; and in orienting 
each of their users through pinpointing sequentially his particular position 
and/or his area of concern with respect to a specific problem area, land use 
area, drainage area or condition, and to each segment of a possible route loca- 
tion in relation to topography, affected land use, and other segments and 
routes. In addition, they are used in photogrammetric instruments for making 
measurements. to be used graphically and mathematically, and for compiling maps. 
In this same vein, aerial photographs can be used to traverse the area of sur- 
vey despite any property-owner objections and the difficulties caused by topog- 
raphy, drainage, and ground conditions, to do such work without alarming prop- 
erty owners and alerting speculators, and to determine and compare as many 
alternatives as are necessary to achieve an adequate solution of the engineer- 
ing and right-of-way problems. 
Aerial photographs show, whenever photographs of an earlier date are 
available for comparison with those taken at a later date, changes in land use 
by character, intensity, and scope; alterations in topography, such as stream 
courses, level of flood plains and water bodies, alluvial fans, wind-blown 
sand, landslides, and subsidence; and changes in condition of ground from wet 
to dry and vice versa, and from seeming stability to instability. 
Aerial photographs serve as substitutes for maps, wherever practicable, 
and supplement maps by furnishing essential information and data omitted from 
or that cannot be delineated on maps. Also aerial photographs serve as indi- 
cators of sources of construction materials by site, type, condition, and quan- 
tity; and serve as a means of identifying trees for landscaping purposes by 
type, condition, and location. 
Moreover, such direct and indirect uses of aerial photographs make it 
possible for members of the highway engineering team to achieve continuity and 
consistency throughout all of their work. In addition, by full utilization of 
aerial photographs, it is easy to attain desirable variations and sequential 
changes in the engineering as the work progresses. Yes, nothing need be over- 
looked; only oecasionally will it be necessary to go onto the ground, with 
photographs in hand, for detailed verification and/or revision of photographic 
interpretation and analyses done previously, as the initial engineering work 
is accomplished. 
For highways, aerial photographs are used in many different forms, each 
one having its particular advantage or disadvantage, for stereoscopic interpre- 
tation and analyses, for measurement of parallax and photographic coordinates, 
for precise measurements and mapping, for indexing, and for illustrations. 
Mostly 9-inch by 9-inch (23 cm x 23 em) size contact prints on paper, and 
anaglyphs are used for stereoscopic interpretation and analyses purposes. 
Preference and particular type of stereoscope being used designates the desired 
size. The type of paper used for the contact prints is either double or single