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Finally, the possibility of a jump to satellite imagery has to be taken into 
account. 
Photography as part of an urban information system in a rapidly changing 
area should, of course, be sequential. In this respect, it is to be desired that 
urban photo-interpretation units for collecting and processing information at 
regular intervals would be developed in the same way as has happened with 
statistical offices. In the initial phase, the urban area should be photographed 
every year to enable an easy analysis of changes and to offer quantitative data 
on the speed of urban development. 
For a follow-up, the frequency could be reduced to intervals of 2-4 years, 
depending on the growth rate of the area and the experience in photo-inter- 
pretation that has been acquired. 
Photo maps are in the process of being accepted by urban planners as an 
up-to-date complement to obsolescent line maps. The information content 
(photo image) is a valuable addition to the line map information. The metric 
quality of photo maps is evidently acceptable for a number of applications. 
The map-like appearance of photo maps facilitates acceptance by planners. 
Cheap and quick reproduction of screened originals (in the case of the simplest 
type of photo map, black-and-white, no additional information except topo- 
graphical co-ordinates) is gratefully acknowledged. 
For planning and decision-making, maps are a reliable base only when 
they show the actual (most recent) situation. Preparation with the shortest 
possible delay, and revision at the shortest possible intervals, are therefore 
essential. Photo maps can fulfill these conditions better than any other type 
of map. Use by urban planners can therefore be expected to increase in the 
future, in particular in dynamic urban situations. 
Orthophoto maps, being differentially rectified (and usually enlarged) 
photographs, with topographic information added, are still under development. 
Problems of planimetric accuracy in large scale orthophotos, and of high 
buildings (relief displacement and chopping up of buildings) will perhaps not 
be solved satisfactorily for the makers, but accepted by the users. The metric 
accuracy of an orthophoto, approaching that of a map, combined with the 
wealth of unmanipulated details typical for the photo image, as well as the 
short production time, make it a desirable product for urban studies and 
planning. The orthophoto map is still relatively unknown to the majority