Geographic Information Systems: An Introduction, 3rd Edition

14.11 Overlay

Overlay is used in data integration and is a technical process, the results of which can be used in realistic forms of spatial analysis.

14.11.1 Polygon overlay

Polygon overlay is a spatial operation in which a first thematic layer containing polygons is superimposed onto another to form a new thematic layer with new polygons. This technique may be likened to placing map overlays on top of each other on a light table (Figure 14.14). The corners of each new polygon are at the intersections of the borders of the original polygons; hence, computing the coordinates of border intersections is a vital function in polygon overlay. The computations are relatively straightforward, but they must be able to cope with all conceivable geometric situations, including vertical lines, parallel lines, and so on. Computing the intersections of a large number of polygons can be very time-consuming.

If areas are stored as links in a topological model, fewer intersections need to be computed, thus reducing the computing time. The new intersections are identified as nodes and the lines between the nodes as links. The new nodes and links then constitute a new topological structure. Consider as an example polygon C4 in Figure 14.15, which is a combination of polygons C and 4. The system cannot associate attributes with C4 unless topology is associated with either the original or the new data. With topology for the new data, the system recognizes that polygon C4 comprises line 22 and parts ...