Geographic Information Systems: An Introduction, 3rd Edition

14.13 Raster Data Overlay

Raster data may also be overlaid. Indeed, raster overlay is often more efficient than vector overlay (Figure 14.21), because the extent of the calculations is much less. The positions of the overlaid thematic layers need be tested only to see whether they contain cell values. The resultant cell-to-cell comparison presupposes that all cells in each thematic layer are queried, regardless of their values. The total number of cells therefore has an effect on processing time. The new composite cells are assigned attributes composed of those from the original cells. These new cells are registered as a new thematic layer.

Raster data consist of equally spaced cells of equal size (assuming that the various thematic layers cover the same area or have been modified to do so). Consequently, there is no formation of smaller erroneous polygons as with vector data overlays, and there is no need to distinguish between polygons, lines, and points, because all raster data comprise cells. However, an important condition is that both raster layers have identical geometry. That is, cell size must be the same and there must be no relative rotation or transfer between the grids. Deviation is corrected by transformation and resampling to the same cell size.

In raster data, attributes are not usually listed in tables as in vector data, but are represented by thematic layers. Therefore, arithmetic operations and some logical and statistical operations may be performed directly ...