Spatial Thinking in Planning Practice: An Introduction to GIS

48 Figure 9.2 Raster Bu#er around a Target Cell(s). http://2012books.lardbucket.org/books/geographic-informa- tion-system-basics/s12-geospatial-analysis-ii-raster-.html MULTIPLE LAYER ANALYSIS A raster dataset can also be clipped similar to a vector dataset. Here, the input raster is overlain by a vector polygon clip layer. "e raster clip process results in a single raster that is identical to the input raster but shares the extent of the polygon clip layer. Figure 9.3 Clipping a Raster to a Vector Polygon Layer. http://2012books.lardbucket.org/books/geographic-in- formation-system-basics/s12-geospatial-analysis-ii-raster-.html RASTER OVERLAYS Raster overlays are relatively simple compared to their vector counterparts and require much less computation- al power (Burroughs 1983) 4 . Raster overlay superimposes at least two input raster layers to produce an output layer. Each cell in the output layer is calculated from the corresponding pixels in the input layers. To do this, the layers must line up perfectly; they must have the same pixel resolution and spatial extent. Once preprocessed, raster overlay is 'exible, e&cient, quick, and o#ers more overlay possibilities than vector overlay. Despite their simplicity, it is important to ensure that all overlain rasters are coregistered (i.e., spatially aligned), cover identical areas, and maintain equal resolution (i.e., cell size). If these assumptions are violated, the analysis will either fail or the resulting output layer will be 'awed. With this in mind, there are several di#erent method- 4 Burroughs, P. 1983. Geographical Information Systems for Natural Resources Assessment . New York: Oxford University Press. Chapter 9: Raster Data Models