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Results of photogrammetric studies have been published on work undertaken 
using Wild cameras (RC-8, RC-10 cameras) and 152 mm, 203 mm and 305 mm focal 
length lenses. Bock's study (1968) showed that a very favourable C-factor was 
obtained and the film resolution was as fine as 0.0032 mm for high contrast 
detail (e.g. white lines 0.3 m in width on an aircraft runway). 
From the recent study of Gutt and Höhle (1977), more detailed information 
is available, They reported very small errors during flight due to atmospheric 
refraction, changes of focal length with temperature and tip and tilt. However, 
it was found necessary to correct the stereoscopic model for warp (about 0.02% 
of flying height), due mainly to curvature of the earth; but the accuracy of 
elevation (vertical) measurements was as good as about 0.004% of the flying 
height, which was a new achievement in analogue photogrammetry. The error in 
the ground resolution (with panel targets) and a flying height of 13,400 m was 
better than 2.5 m. With increased flying height and a wide-angle lens (i.e. 15 cm 
focal length) distances of about 60 km between ground control points were 
bridged across the stereoscopic model. They concluded that planimetric maps 
and ortho-photomaps can be produced at scales of 1:25,000 and 1:50,000 from 
photographs at a scale of 1:100,000. It would also appear that topographic 
maps at scale 1:25,000 with an altimetric and planimetric accuracy falling 
within international standards can be very economically produced too from the 
IRC HFAP coverage of Sierra Leone (IGN, personal communication, 1977). 
V. PLANNING HIGH ALTITUDE AERIAL PHOTOGRAPHY 
IN THE MOIST TROPICS 
General 
The unsuccessful attempt by NASA to obtain complete world coverage by Landsat 
draws attention to the cloud problem in the tropics for satellite sensing, 
Indeed, between August 1972 and August 1977, only about 73% of the land surface 
within the tropics had been covered by Landsat MSS imagery having less than 
10% cloud cover and within the tropical rainforest zones only about 45% was 
covered, 
This immediately suggests for these cloud-prone areas, the use of side-looking 
airborne radar (SLAR) with its cloud penetrating and almost all weather 
capabilities (also see Allen, 1975 and Schwaar, 1978 a). A third advantage of 
SLAR, namely its ability to partly penetrate the vegetal cover and hence provide 
information on sub-surface features is not so important for forestry as in 
mineral exploration etc. Unfortunately, the resolution of SLAR is either 10-20 m 
or 40-60 m depending on the type of equipment used. In comparison, the 
instantaneous field of view (IFOV) of the Landsat-3 MSS system is about 80 m 
and using RBV, about 40 m, whilst the resolution of a modern aerial camera 
system may be better than one meter for lower flying heights and wide-angle camera 
lens; and in the case of high flight photography can be expected to be better 
than about 3 me 
This, therefore, favours consideration of aerial photography, provided the 
problem of cloud cover can be overcome by better information on the extent and 
periods of cloud cover and by inoreased aircraft ground speed and reduced time 
for photographic coverage. From what has already been said it will be appreciated 
that the latter constraints can be largely eliminated by high altitude aerial 
photography, In this respect, however, it must be borne in mind that aemial 
photographic contracts usually specify a maximum of 5% of clouds and their 
    
  
  
  
  
  
   
    
    
   
  
   
  
  
  
   
  
  
   
   
    
    
   
  
   
  
  
  
   
  
  
  
  
  
  
  
  
   
   
   
  
   
  
  
  
     
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