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The road gradient in non flat terrain 
Usually road planning is carried out with the help of large scale 
maps or orthophotomaps with a small contour interval. The maximum 
road gradient is determined by the terrain conditions and the 
type of vehicles. Generally the gradient does not exceed 12 percent 
(Austria), although in Cyprus for example the maximum gradient is 
8 percent in order to avoid too much erosion. 
The gradient of a road is determined by the ratio of the rise in 
elevation and the corresponding horizontal distance: 
SA (D:and dH both in meters) (2) 
G = 
Where maps are lacking the gradient of a road may be measured on 
aerial photographs. The elevation difference is found through 
parallax measurements, and the distance through length. measure- 
ments in the aerial photographs. Combining both measurements 
the gradient can be determined directly from: 
c dP 
G = UT "EUER (for the symbols see (1) (3) 
This formula is used to check if the planned road is not too steep 
over a certain section. When the calculated gradient for a straight 
road section between two points does not exceed the maximum value, 
then the traverse can directly be delineated on the aerial photo- 
graphs, thus making fully use of the advantages of stereo rhoto 
interpretation. 
Often the terrain has slopes too steep to build a road in the nlan- 
ned direction, since then the gradient is exceeding the allowable 
maximum. 
A new technique has been developed (Remeijn, 1969) to design the 
road with the help of aerial photographs. Basicallv the concept 
is similar to the tic procedure using a topographic map. 
A road with a constant gradient intersects the contour lines at 
a fixed disbtanee. In the new method the road is designed with the 
help of aerial photographs of medium to large scale, replacing the 
contour interval by parallax differences measured in the stereo 
model. 
Since the photoscale changes with the elevation of the terrain, it 
is recommended to calculate the parallax difference over small 
sections only. When dP is relative small with respect to the 
stereoscopic parallax P of the starting point of each section, 
then formula (3) may be simplified to 
c dP P.d.G 
Qm mt . P or dP 2 dein (4) 
Keeping the road gradient and the horizontal distance at constant 
values, we see that dP varies directly with P. However P is 
subject to small changes depending on the elevation differences 
visible in the stereo model. Over short distances however F 
can be considered as having a cónstant value.