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approximate procedures. 
A similar distinction can be made with regard to problems of storage, 
processing and editing. Grid processing will be by far the most simple (Dueker 
and Horton 1972, Ackerknecht 1969) but is limited in its application. Point 
referencing combined with the “personal number” of the population records 
enables Scandinavian countries to use administrative data for planning. 
Mention should be made of a series of population density maps made from 
such a data base in Finland (Kajama and Vahala 1974). The absence of a 
network, however, is a rather critical drawback of this system. 
Segment referencing seems an attractive solution since it deals both with 
location and networks, while dual independent coding makes editing easier 
(van Est 1975). Some interesting planning applications have been demonstrated 
for this method, eg the location of public facilities (Salomonsson 1975). 
To be entered into the data base, information from remote sensing 
sources will have to be geocoded as well. This is relatively easy in the case of 
grid referencing but requires special methods in the case of point or segment 
referencing, as these are address-based systems. 
The use of stereo orthophoto maps showing the location of centroids, or 
segment positions, could then be considered, so that information can be 
entered via a digitiser/stereoscope combination. 
USEMAP 
An example of an information system for a situation with only limited 
administrative data has been developed by the ITC under the name of USEMAP. 
USEMAP is a grid-based system that can be operated even in the absence of 
reliable maps, designed for maximum input from photography. It can be 
used for small area population estimates and for planning tasks such as the 
definition of intervention priorities, assessment of residential quality etc. 
(figure 5). The programmes were designed in such a way that they can be used 
without any knowledge of computers. By comparing data from different 
years, changes can be mapped and analysed (de Bruijn 1974). 
Monitoring Changes in Time 
The possibility of obtaining dynamic series by interpreting photos from 
different base years seems one of the most important applications of photo- 
interpretation to urban planning (see figure 6). Especially the growth of