20 
APPLICATIONS IN RECONNAISSANCE SURVEY OF ALTERNATE ROUTES 
Following the reconnaissance survey of area, a reconnaissance survey of 
alternate routes is undertaken. The ultimate objective in this stage is to 
ascertain which of all route alternatives is the best. This is achieved by 
comparing the feasible routes determined in the preceding stage for the high- 
way service, convenience, comfort, and safety each of them would provide were 
a highway constructed. The comparisons are made on the pertinent bases of: 
l. Attainable alinement, gradients, cross sections; grade separations, 
medians, and different roadway levels; structures; and how well the highway 
could be fitted to the topography and bordering land use esthetically. 
2. Best possible service for traffic in directness (length), in least 
practicable rise and fall, and in all types of weather; and for occupants of 
bordering lands. 
3. Least costs of highway rights-of-way, construction, and maintenance, 
and of vehicle operations. 
Once the comparisons are completed in sufficient detail on such bases and 
any other particularly applicable factors, designation is made of the route on 
which subsequent efforts must be specifically expended to complete the highway 
location and design. 
This is the stage in which variations in the applications of aerial sur- 
veys are greater than in any other. This is caused by several factors and/or 
practices. The greater the number and complexity of the problems to be solved, 
the more intricate and detailed the surveys and engineering work become. 
Photographic Coverage 
  
For accomplishing engineering work in the reconnaissance survey of alter- 
nate routes stage, photographs of the previous stage are utilized or new photo- 
graphs of similar scale are obtained where required by changes in land use. 
Large scale photographs of routes are taken, however, wherever sufficient detail 
cannot be seen on the small scale photographs, and wherever access to routes 
for inspection of the ground is very difficult, and/or wherever tall, dense 
vegetation will prohibit attainment of sufficient accuracy in the mapping which 
will be required in the preliminary survey stage. Whenever new, small scale 
photographs are obtained, sufficient coverage is usually of a route band no 
less than 3 miles (5 kilometers) wide. Where large scale photographs are 
obtained, the usual minimum width of photographic coverage is 1 mile (1.6 
kilometers). 
Procedures 
A thorough stereoscopic examination of the photographs on which route 
alternatives were delineated is made to ascertain, more fully than in the pre- 
vious stage, the places where field investigations, with photographs in hand, 
Will be required at places such as swamps, landslides or likelihood of land- 
slides, flood plains, questionable exposure areas, poor subsurface soils areas, 
and pertinent land use areas. 
Noncritical topography and land use 
Wherever topography and land use do not cause especially critical gradient 
and horizontal alinement problems, gradients and horizontal alinement checks 
are made on the ground with photographs of previous stage in hand. Additional 
checking is done with the use of altimeters. One stationary altimeter is kept 
on a point of known elevation and the others are carried to turning points used 
for gradient computations with photographs. As a result of this checking, it 
is determined whether an adjustment in grade is required because of the exist- 
ence of tilt in the photographs used. The position of the route is adjusted,