16 w 
  
model is determined using the plotted position of nadir 
points. In the next step a common point between the Aerodist- 
controlled photography and the mapping photography is picked, 
plotted on the manuscript and the coordinates of this point 
are determined. The Wild PUG point transfer instrument is 
very helpful in transferring common points from the Aerodist- e ^ 
controlled photography to the mapping photography. In the 
event that the horizon camera data are not reliable, the 
Aerodist-controlled photographic strips have to be triangulated 
in order to permit absolute orientation of individual stereo 
models for determination of the nadir points. In the final 
step, aerial triangulation is carried out with the mapping 
photography utilizing the horizontal positions established 
from the Aerodist-controlled photography as "ground control" e o 
for strip and block adjustment. 
VERTICAL CONTROL 
  
The unexplored areas generally lack geodetic bench 
marks. Levelling on the ground would be time consuming and 
extremely expensive; therefore airborne methods must be used 
for establishing of elevations in the project area. The 
Airborne Profile Recorder is an excellent tool for this 
purpose. A grid of APR lines, flown along the rivers and 
adjusted as one block, will provide uniform vertical control