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170 THE CANADIAN SURVEYOR May, 1960 
  
  
  
Fic. 3. Contour map of a rock-knob. First 
order above, second order below. At some 
places differences in contour lines are 
caused by forest. Striking difference be- 
tween first and second order contour maps 
is visible. 
  
form of the rock-knob, as described above, is fairly represented. Often it is difficult 
to judge what is typical and essential and what is not. This difficulty may be overcome 
by using a topographical photo-interpretation. It is regrettable that in the new Manual 
of Photographic Interpretation of the American Society of Photogrammetry there is no 
special chapter devoted to this important subject. In my opinion, topographical inter- 
pretation is indispensable and must be developed not only for experimental purposes 
but for practical mapping, especially for small-scale mapping of underdeveloped coun- 
tries. In this respect these notes on topographical interpretation of the Renfrew test 
area may be regarded as an example of topographical photo-interpretation, although 
they are still provisional in some respects. I intend however to elaborate, in a further 
article, on the experience gained from the valuable Renfrew test and also on the role of 
topographical photo-interpretation in aerial photogrammetry.