are available one can be sure that a favourable location will not be neglected, 
while, furthermore, the possibility of surprises through terrain conditions is also 
reduced to a minimum. 
In item 3 of the bibliography a programme has been drawn up for the appli- 
cation of photogrammetry in stages, starting with small scale photography and 
photogrammetric mapping of a wide area between terminal points and passing on 
to the application of medium-scale photography and mapping over a narrow 
band of territory chosen from the small-scale maps. On these medium-scale maps 
a more definitive location can be made, and for the final location a third stage 
ensues in which large-scale photography and mapping are carried out over the 
narrow strip along the location chosen on the medium-scale maps. 
In many tropical areas this system will not be economically possible owing 
to the fact that photography in the tropics meets with many difficulties because 
of unfavourable weather, so that there is a great chance that no opportunity 
would be available of carrying out the 2nd and 3rd stages in time. Moreover it 
would be necessary, at least for the 3rd stage, to have the strip along the chosen 
location well cleared before large-scale photography could be of use, and weather 
conditions would very often make it impossible to carry out this photography in 
a short period, whilst too lengthy a period of time between clearing and photo- 
graphy might cause the whole strip to be covered again by brush, owing to the 
rapid growth of vegetation in the tropics. For the third stage (after clearing) the 
Radar Profile Recorder might be useful. 
Use of the Photographs. 
The aerial photographs themselves can be used in conjunction with the con- 
tour maps for investigation of ground conditions. From the nature of the vegeta- 
tion we can decide whether the terrain concerned is salt marsh, brackish swamp, 
fresh water swamp or uplands, while areas in which land slides have occurred, or 
may be expected, can very often also be identified on the photographs (Fig. 9). 
Photogeological study of the aerial photographs can also give information 
about the occurrence of terraces where sand or gravel may be presumed to exist — 
materials which can be very useful in the construction of the road. 
Examples. 
A few instances will suffice to illustrate the great value of this combined 
method of aerial photograph/contour map. In planning a road through fairly dif- 
ficult country a swamp had to be crossed. 
The aerial photographs, and a small indication on the contour map, showed 
that a narrow, dry strip could be expected in a particular place, which would 
make it possible to cross the swamp without much trouble. Survey work on the 
ground proved this strip to. be present, and to have a width of only 100-150 
metres. Naturally, it was hardly distinguishable from the surrounding country, and 
it would have been practically impossible to find this crossing-point using terres- 
trial methods only, since the whole area was covered with dense jungle. 
In another case of a road project, an estimate of the cost had to be made at 
short notice before any ground survey could be made. On the basis of informa- 
tion derived from aerial photographs and the contour map, an estimated excava- 
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