496 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
The plan for the diagnosis of potential landslides would, therefore, require 
the area concerned to be photographed towards the end of the dry season. 
The photographs are examined for the location of such exposed areas which 
are then examined in the field for more definite manifestations of slide tend 
ency. This preliminary work with aerial photographs will, therefore, materially 
reduce the amount of field exploration which would otherwise be necessary 
(see fig. 1). 
Advantages and disadvantages 
What are the special advantages and disadvantages of the technique of photo 
interpretation in a tropical country like Malaya? To begin with, it must be 
pointed out that photo interpretation is not a complete substitute for the con 
ventional field methods. 
In route location through virgin jungle, as is the case in many of the roads of 
the Rural Development schemes, aerial photographic location surmounts the 
hardship of ground operations. The stereoscopic model provided by, say, a 
Gamble plotter or even a folding stereoscope allows the possible routes to be 
studied without physical restrictions. A rough trace of the most suitable route 
could first be made from the aerial photographs. By checking bearings provided 
by this rough trace, the approximate location of the route could be established 
with comparative ease on the ground. 
While the inaccessibility of the thick Malayan forests would accentuate this 
advantage offered by a stereoscopic model, the giant trees and thick under 
growth on the other hand present the disadvantage that it would not be possible 
to make photogrammetric measurements as accurately as in a terrain entirely 
devoid of vegetation. 
Where measurements have to be made to assist in photo interpretation, 
shadows cast by trees and further complicated by the scintillating effect of water 
reflection can upset photogrammetric measurements. This arose in the work 
on the proposed air field to be constructed on the bank of the Kuala Tahan. 
The site was very inaccessible and normal ground control was not available. 
The intention was to use the water level of the Kuala Tahan river which ex 
tended across the area in question as the datum for height measurements. In 
directing the floating mark along the length of the river, it was found that a 
variety of values was registered - a variation which could only be due to the 
water surface being overcast by the shadows of trees. 
Trees in Malayan forests rise to a height of more than 100 feet. The thick 
undergrowth and tree cover make the stereoscopic assessment of tree heights 
impossible. The heavy crown closure make the shadow method of height 
assessment impracticable. Pure stands and even-age stands are rare. There is, 
therefore, a wide range of divergence of individual trees from the average stand. 
Further, the dense foliage results in a high interception capacity and prevents 
the natural establishment of surface drainage and, therefore, the fluvial land 
patterns from which soil and other characteristics may be deduced.