e 
A special advantage of this procedure is given when the 
digitally registered contour lines will also be auto- 
matically plotted. This procedure, of course, requires 
an off-line orthophoto system, which has a number of 
other advantages. 
The accuracy in scale of an orthophoto has to be kept 
  
within graphical accuracy tolerance, as the planimetric 
highway design is represented in a print combined with 
the orthophotomap and exactly fitted to the orthophoto. 
This requirement in accuracy is normally met by differential 
rectification of the aerial photographs using an orthopro- 
  
jector. As in conventional rectification attention has to 
be paid to ensure that no planimetric distortions, due to 
differences in height on the ground, will arise. The height 
differences Ah on the ground therefore should not exceed 
an amount 
if di is the figure of" the scale of photographs and f the 
focal ‘distance. Since” the radial distortions are inversely 
proportional to the focal distance, the use of long focal 
distances f = 30 cm or f = 60 cm is recommended, especially 
in areas with high buildings. On the other hand the focal 
distance is also inversely proportional to the vertical 
precision which for highway design is required to be as 
high as possible. That is why normally a short focal dis- 
tance of f = 15 cm is applied. As a matter of fact, it was 
even investigated whether any advantages might result for 
highway design by using the focal distance f - 8,5 cm. 
Smaller scales of photography resulting in fewer models, 
but with the same altimetric accuracy, should increase the 
economy. Long focal distances are mainly used in production 
of inventory maps, where normally no height measurements 
are required.