well as in areas of secondary growth. Rivers and streams 
appear black and intermittent drainage can be easily recognized. 
Streambeds which usually have higher moisture content appear 
darker than the surrounding ground and can be interpreted 
without difficulty. These features are of particular import- 
ance since the drainage pattern is the predominant detail of 
many map sheets. Furthermore, photographs and maps which 
accurately portray drainage patterns are more useful for 
projects such as watershed evaluation, hydro electric develop- 
ment, agricultural and irrigation planning. In addition, the 
infra-red photography shows more differentiation in the tropical 
vegetation than panchromatic does, thus substantially improving 
the stereoscopic image. 
NAVIGATION FOR AERIAL PHOTOGRAPHY 
The unexplored areas lack maps suitable for navigation 
of aerial photographic flights. Since the number of days 
favourable for aerial photography is very limited every 
effort must be made that each photographic mission produces 
pictures perfect in every respect. Proper navigation must be 
ensured so that there are no gaps. An effective way of ensuring 
proper navigation is to make us of a Doppler navigation system. 
This instrument is installed in the aircraft and does not