^ 
19 
  
Stereogram 6. Route location in rugged, eroded area. The route is located 
where it deviates from the canyon containing the main drainage channel to avoid 
heavy grading, to attain small drainage areas for culverts, and to eliminate 
need for major drainage structures. Also, the ascent is begun along an easy 
gradient to reach the required terminus at a higher elevation. In this exam- 
ple, physical dimensions, character of topography and drainage, and physical 
features of the highway were all correlated while the route location was deter- 
mined by proceeding from one stereoscopic pair to another until utilization 
had been made of all photographs of the entire area between route location 
terminal points. For lack of suitable, small scale, reconnaissance type topo- 
graphic maps of the area of survey, gradients and other essential dimensions, 
as representative cross sections, were determined from parallax measurements 
made while the vertical photographs of the area were examined stereoscopically.