Geographic Information Systems: An Introduction, 3rd Edition

4.4 Automatic Conversion Between Vector and Raster Models

GIS applications sometimes require data in a form differing from that which is available. As a result, many GISs now have facilities for automatic conversion between vector and raster models. Raster data are converted to vector data through vectorization. The reverse process, which is just as common, is rasterization. In vectorization, areas containing the same cell values are converted to polygons with attribute values equivalent to the preconversion cell values (Figure 4.33). In the reverse process of converting polygons to cells, each cell falling within a polygon is assigned a value equal to the polygon attribute value (Figure 4.34).

Figure 4.33

Conversion of raster data to vector data: (a) each raster cell is assigned an attribute value; (b) boundaries are set up between different attribute classes; (c) a polygon is created by storing x and y coordinates for the points adjacent to the boundaries.

Various routines are available for converting raster data to vector data, and vice versa. The former is the more complex and time-consuming of the two processes, and different conversion programs can yield differing results from the same set of raster data. In rasterization the following formulas can be used to place a coordinate defined pair in a raster (see also Figure 12.6):

Figure 4.34

Conversion of vector data to ...