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Small scale aerial photographs (scales between 1 s 30,000 and 
1 : 50,000) are used to mark the potential areas for road 
passage. The various possibilities are then compared and one or 
two of the most promising traverses are then selected for close 
inspection. 
The selected routes will be covered by large scale aerial photo- 
graphy (scales between 1 : 10,000 and 1 : 15,000) in a strip wide 
enough to allow shifts in the final traverse.(Pryor, 1960). 
Detailed interpretation includes also slope measurements for topo- 
graphic studies, 
The topography 
A study of the landform of the region allows the planner to 
indictae a wide band €nà the maps and photogranhs within which 
the road has to be situated in order to connect the cardinal 
points. 
The area within this strip can be divided in sections of uniform 
topography. A division in three slope categories will be sufficient. 
- nearly flat areas with slopes less than 5 percent. Hardly any 
earth mass movement will be necessary. 
- areas with gentle slopes between 5 and 15 percent. Small cuts 
and fills cause a small increase of the road construction cost. 
The direction of the planned road is not restricted by the 
steepness of the terrain. 
- Areas with slopes steeper than 15 percent. The roads follow 
more or less the direction of the contour lines, otherwise the 
maximum allowable road gradient will be exceeded. 
The three slope classes can be delineated on the maps, and then 
transferred to the aerial photographs for further studies. 
In case no topographic maps are available then it is not necess- 
ary to construct them for the single purpose of contouring, since 
the slopes may be directly measured or estimated on the large 
scale aerial photographs. 
Slope measurements on aerial photographs 
The required tools for such measurements are a mirror stereoscope, 
a parallax bar and tracing paper. The slope percent is determined 
using a formula which combines the known methods for the determ- 
ination of elevation differences and the horizontal distance 
between slope points (Stellingwerf, 1966). 
slope % = EH : cuoc (1) 
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