is always a risk of overlooking a good alternative route in the dense jungle, and 
there is also the possibility that the subsequent detailed mapping may produce 
sudden surprises with regard to ground conditions, resulting in loss of time and 
labour. 
Application of photogrammetry can save much of the time and expense and 
weary plodding of survey groups through dense tropical jungle involved in both 
these methods and also, as far as the second method is concerned, ensure a greater 
degree of certainty of having found the right location. 
Photogrammetric mapping). 
If the area under consideration is covered by aerial photographs, stereoscopic 
study of these prints might enable a location to be worked out which will be 
suited to topographical conditions. This simple method, which may, however, 
take up a lot of time, does not readily permit the situation to be viewed as a 
whole, and may therefore lead to promising possibilities being overlooked. 
Accordingly, the most economical method of working is to prepare, with the 
help of aerial photographs, a reliable topographical map with contour lines, on 
which the alternatives for the location can be projected. The aerial photographs 
are also used for terrain studies. Further reference will be made to this later. 
Flying scheme and ground control. 
For use in the preparation of maps as mentioned above, aerial photographs 
must be made according to a good flying scheme and with adequate forward and 
side overlap. 
Ground control in jungle-covered undeveloped areas is an expensive and 
difficult item. It is of major importance to economize in this and thus reduce the 
number of costly expeditions in the area, and aerial triangulation is therefore a 
very welcome asset in preparing the maps. By applying aerial triangulation ground 
control can be reduced to the measuring of a few bases of, say, 8—10 kilometres 
long. About 4 points of each base are marked by ground marks before photo- 
graphy is carried out, or about 4 identifiable points are sought on each base and 
connected by a traverse or another ground survey method. The elevation of the 
points should be measured, as well as the azimuth and distance. 
It has been found possible in practice to bridge a distance of about 100 km. 
between ground control, using double-image instruments like the stereoplanigraph 
or another instrument of this type. 
For the elevation more ground control is desirable but if, because of field 
conditions, this additional ground control cannot be obtained economically, use 
of the water level along the sea coast and rivers, and consideration of the general 
drainage scheme of the area, will give enough ties for levelling the stereo-models 
with adequate accuracy. 
By using this latter method a slight general slope of the reference plane may 
sometimes occur, but this does not generally affect the usefulness of the prelimi- 
nary project. When the definite location of the road or other civil engineering 
1) The aerial triangulation and the plotting from aerial photographs for the preparation of the 
topographical maps referred to hereafter has been carried out at the Photogrammetric Depart- 
ment of Public Works at Delft (Netherlands). 
2