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Techniques for Change Perception 
For comparative analysis, the "new” and the "old" data sources are dis 
played or laid-out (fig. 7b). If the two data sources are superimposed 
and matched globally, the geometries (inclusive scales) should be equal. 
If only small zones are matched locally, an approximate equality of scales 
suffices. In the case where the two data sources are laid-out separately, 
the scales may differ. 
In the first case, i.e., when the "new" and the "old" data sources are to 
be globally matched, either the new image must be transformed into the 
geometry of the map (or digital files) or the existing map (or digital 
files) must be transformed into the geometry of the new image (Kolbl, 
1980; Masry, 1977). Both cases require a substantial additional effort. 
If the two data sources are superimposed, the image detail may be obscured 
by the map symbols, text, hatches, shades, distortions caused by generali 
sation, etc. (Beotra, 1981). Hence, a selective display of the map data 
is desirable. 
After extracting the changes, all subsequent operations in the semantic 
realm are as in the original mapping. 
FLOW OF GEOMETRIC DATA 
Data Flow 
The main stages in this flow are the preparation of input, mensuration, 
transformations, editing and generalisation, and plotting (fig. 8). The 
operations specific for updating by means of DMP occur in the preparation 
stage, mensuration- and transformation stage. 
Fig. 8: Flow of geometric data.