An area feature is determined by its outer boundary and possibly by a set of inner boundaries. Outer boundary, as well as all inner ones, represents closed polylines. The area occupied by a feature is determined by the polygon area, specified by outer boundary, minus the total area of inner regions, that is, polygons set by inner boundaries. The feature boundary should not cross itself and other boundaries. Every inner boundary should be lying entirely inside the outer boundary but it cannot lie inside other inner boundary. Every boundary should contain not less than three points.
Buildings, blocks of buildings, parcels can be examples of area feature forms.
At figure 9-3 examples of area feature forms are shown.
Figure 9-3. Area feature forms
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