or false colour photographs (6). 
Experience has shown that colour photography yields better interpretation 
results, because colour tonal differences can usually be better distin- 
guished on colour than on black and white panchromatic photography (6). 
Photography gives an instantaneous "synoptic" view of the whole area 
visible through the lense. Simultaneous viewing with both eyes of a pair 
of photographs each covering the same terrain, but taken from different 
exposure positions offers the possibility of stereoscopic_vision. The 
central perspective causes relief displacement away from or towards the 
center of the image. The relief displacement and related scale 
differences, which disturb the geometry of the image, are stronger for 
terrain with considerable height differences especially at low flying 
altitude. As a result the transfer of photodata to a basemap may be time 
consuming and inevitably produces inaccuracies in the final map. 
Orthophotography is photography obtained by optical conversion of central 
projection to parallel projection at uniform scale without relief 
displacement (8). Transfer of orthophotodata to a basemap is a simple 
and rapid procedure. Recently also stereo orthophotography has become 
available but still has to be tested out for applicability in engineering 
projects (8). 
Resolution is expressed as the size of the smallest object that can be 
recognized by its own characteristics. It depends on the scale of the 
photos and on the type of film and lense used, but roughly can be indicated 
as follows: 0.25 m at à scale of 1:5,000 and at larger scales, to 2.5 m 
at à scale of 1:50,000. For conventional aerial photography a common range 
of scale is from 1:5,000 to 1:50,000, but snaller and larger scales can 
be achieved by varying the flying altitude and/or focal length of the 
camera lense. 
The "height resolution" (accuracy of height determination) is approximately 
0.01% of the flying altitude for normal aerial and satellite photography; 
for high altitude photography a factor 2 to 3 times better can be achieved. 
Satellite photography so far is available only from manned spacetrips in 
  
the Gemini, Apollo, Skylab and Soyoez programs, but coverage of the earth's 
surface is haphazard and scales are too small for engineering applications. 
In another session of this Symposium Doyle will introduce a program of 
systematic satellite photography with a high performance cartographic 
camera producing stereo photography at a scale of 1:1,000,000 and with a