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c keeping the surveys up-to-date. In urban areas there are frequent changes: 
old buildings being demolished, new buildings erected, sudden access of 
squatters, etc. 
The application of photogrammetry is not hampered by the first two 
difficulties, and the third can usually be solved more economically by re- 
photographing the area than by field revision methods. 
In urban areas, photogrammetry also meets with difficulties, as follows. 
a In urban areas there are —more than anywhere else—objects hidden behind 
others. To reduce these "dead" areas as much as possible, narrow angle 
photography should be used. 
b Besides the “dead” areas mentioned above, there are also many objects 
obscured by trees etc. Consequently, a large amount of field completion 
survey will be needed. This, however, is largely facilitated by the fact 
that in urban areas there is a multitude of objects--usually man-made, 
such as manholes in side walks, house corners, walls, fences, etc—of a 
“permanent” character (at least during the period between photo flight 
and field completion survey) which are sharply identifiable (both in situ 
and in the photographs), so that they can be used as reference points for 
the completion survey. 
The field completion surveys can therefore usually be reduced to 
simple measurements only, thereby fixing the positions of non-visible objects 
relative to close-by reference points. When photo maps are produced the 
reference points will automatically appear on the map and the invisible 
objects can be plotted on the photo map (or onto an overlay) by simple off- 
sets of the field completion measurements. When line maps are produced by 
stereo-restitution, or when numerical stereo-restitution is carried out, the 
reference points selected in the field should of course be included in the 
stereo-restitution in order to be able to plot, from them, the completion data. 
Where no clearly identifiable objects of a permanent character are 
available, points need to be monumented and signalised before the photo 
flight. This requires careful reconnaissance, for which a pre-existing photo 
cover would be very helpful.