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.