22 
block adjustment. Planimetric ground control is then required along the 
periphery of the city area only, where this can be easily obtained without 
much hindrance from traffic. From this 1:5,000 photography, (ortho)photo 
maps at a scale of 1:2,000 (or even 1:1,250) may be produced of the whole 
area in a reasonably short time. 
If necessary, from the same photography, line maps at a scale of 
1:1,000 (or even 1:500) can be made by conventional graphical stereo- 
restitution on a precision plotter. This may, however, be restricted to fhose 
areas where such time-consuming and expensive mapping is really needed. 
In order to avoid the extra field completion survey that would be needed to 
correct the photogrammetric line plot for roof-eaves etc, an annotation 
could be printed on the map sheets, saying: ‘situation as seen from the air”. 
As for heights, the departments of water supply and road construction 
etc of the Municipality usually require height reference points all along the 
streets to such an accuracy as would normally require field spirit levelling. 
These heights, subsequently, provide sufficient ground control for the 
levelling of the stereo models so that in open areas contours (and spot heights) 
can be produced photogrammetrically when desired. With 1:5,000 aerial 
photography, taken with a narrow angle camera (9" x 9", 12"), contours at 
1 metre interval are feasible. 
As mentioned above, the use of a narrow angle camera is strongly 
recommended for urban mapping in order to reduce dead areas and radial 
displacement. Where only a wide angle camera is available, the use of 60% 
side-lap and 80% forward-overlap, and restitution of the nett stereo models 
only, would lead to the same reduction. Dead areas can then even be reduced 
by plotting otherwise obscured areas from neighbouring stereo models 
(ref. Visser: Some Remarks on Urban Mapping, Proceedings of the Inter- 
national Symposium on Photogrammetry, Prague 1966). This, however, 
would be at a cost of twice the number of photographic strips and four times 
the number of stereo models to orientate for plotting (though the number of 
stereo models for aerial triangulation remains the same). 
In order to produce good 1:5,000 aerial photography, an aircraft of 
slow speed (preferably slower than 200 km/hour whilst photographing) and 
a camera with a short exposure time (preferably less than 1/300 second) and 
a fast film cycling speed (preferably less than 3 seconds), would be required. 
9 >