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persuaded to use the photo maps instead of the conventional line maps. Sub- 
sequent experience showed that the map users became enthusiastic and were 
able to extract much more information from the orthophoto maps than from 
the line maps. At present about 45 private photogrammetric companies, each 
with one or more SFOM-693 differential rectifiers, are taking part in this 
production. The output is reported to be 10 to 15 times faster than that of 
line maps. 
The disadvantages of photo maps, as compared with line maps, on the 
other hand, are as follows. 
a Objects appear in the form of their photographic image. Buildings, streets, 
etc, are shown with their traditional outlines and symbols. There are many 
irrelevant images such as those of cars, heaps of building construction 
materials, etc. The map user has to carry out some photo interpretation 
himself (whereas before the photogrammetric operator does this, in the 
course of the plotting). At the large scale concerned, however, the inter- 
pretation does not require special skill. 
b Objects standing above the ground (houses are of particular importance, 
of course) become radially displaced (from the nadir point—approximately 
the centre of the photograph) due to the photograph being a central pro- 
jection instead of an orthogonal one as the map should be. 
Maximum radial displacements, e.g. in the corners of the photograph, 
are equal to H. tan (y. where H is the height above the ground and QY half 
the opening angle of the aerial camera. In practice Qv is 22? for a narrow angle 
camera (9" x 9", 30 cms) but 40? for a wide angle camera (9" x 9", 15 cms). 
Where H = 10 m, for instance, the maximum radial displacement to be ex- 
pected is thus IO tan Qr, or 4 metres for a narrow angle camera. At a map scale 
of 1:2,000 this is as large as 2 mm ! 
2.3.2 A Possible System for Urban Mapping 
When covering the entire urban area with 1:5,000 aerial photography 
—preferably with a narrow angle camera to reduce dead areas in stereo models 
and radial displacements in photo maps—the minor control points, as well as 
a number (or all) of the reference points (see section 2.1 above), can be 
determined to satisfactory accuracy by aerial triangulation and subsequent