As an example may serve a photopair with 60 percent overlap in the 
23 cm format, taken with a 150 mm lens system. The stereoscopic 
parallax of the starting point of the new road in the stereo model 
is 91.2 mm. The parallax difference over a road section of 14 mm 
length at photo scale can be calculated for a maximum gradient of 
8 percent,The photoscale is 1 : 10,000. 
91.2 £15 0,08 
150 
Assuming that the starting point gives a reading in the parallax 
bar of 17.95, then the reading of the bar at the next point 
uphill will be 17.95 + 0.69 = 18.64, and the stereoscopic parallax 
of the new point is then 91.89 mm. 
aß = 0.69 mm 
  
The new reading is set in the parallax bar, and then the bar is 
moved along a circle of 14 mm radius aroung the starting point, 
until the floating mark makes contact with the ground in the model. 
The landing position is the new road point separated from the first 
point by a correct distance and gradient. This procedure is then 
repeated for the next road point, adapting the horizontal distance 
d to the configuration of the terrain as visible in the aerial 
photographs (see figure 2). 
pre-set 
reading 
of the 
parallax bar 7 
Figure 2 
  
Road design on aerial 
photographs. 18.63 
In the example the points 1 and 12 were to be connected with a 
road of the shortest possible length. The difference in elevation 
is nnly ll m, but making use of an maximum allowable gradient of 
8 percent, the road is not level at all. A11 the data resulting 
from the measurements and calculations are recorded in table 1, 
in order to keep track on the progress in the design of the 
traverse. The record includes the road point number, the distance, 
the parallax reading and the stereoscopic parallax of each point. 
For the construction of the profile along the traverse a record 
is kept on the calculated elevations, and the total distance in 
the field. The exact length can either be calculated from scale 
measurements in the photozraphs or determined from a radial line 
triangulation on the various road points, although the last method 
is less accurate, when the traverse is situated near the flight 
line in the stereo model.