y SN 
P 
í N 
  
  
  
  
  
  
photo I photo 2 J N 
Stereo photopair with principal points Pı Pa 
py end P,, and slope points a and b. Triangulation for the determ- 
ination of the horizontal 
Figure 1. distance d. P,P, equals Pa* 
In the formula for the slope determination is 
c = 
dP = 
P = 
the focal length of the camera lens system (mm), 
the parallax difference between the 2 slope points (mm), 
the stereoscopic parallax of the lower slope point a, 
as measured in the stereo model (mm), 
the horizontal distance between the slope points at a scale 
equal to the scale of the triangulation. 
For very short gentle slopes the horizontal distance may be 
estimated by direct measurements in the aerial photographs. 
In areas with long or steep slopes the differences in elevation 
between the slope points cause image displacements in the aerial 
photographs, which may result in wrong estimations for the 
horizontal distances. These displacements due to the relief 
however can be eliminated through a triangulation radially from 
the principal points to the images of the slope points in the 
stereo»model (see fig. 1). 
The horizontal distance d may be 
measured in the triangulation drawing, which has a scale equal 
to the photo scale in point a. 
Road layout 
Rectified aerial photographs are preferred, but in case there are 
no facilities existing for the execution of an aero triangulation 
(as may be the case in some developing countries), then contact 
prints may be used, provided the photography has been taken with 
the usual tolerances for tilt and flying altitude. 
Large scale photographs contain much information about the areas, 
and apart from certain restrictions or obstacles, which can easily 
be recognized in the photographs, the new road in flat and gentle 
sloping terrain may be designed in fairly straight lines between 
the connecting points. 
In rolling country however it may be éheaper 
for loaded timber trucks to go around a hill and drive over a slight- 
ly longer distance, than to go in one direction up and down hill. 
An analysis of the cost for road construction and truck transport, 
loaded and unloaded, may lead to the right conclusion about the 
location of the traverse.