It may be worthwhile to convert the data from one format to the other to perform the analysis. Of course, this also depends on whether your data is already stored as features or rasters. Overlay analysis to find locations meeting certain criteria is often best done using raster overlay (although you can do it with feature data). Some types of overlay analysis lend themselves to one or the other of these methods. In general, there are two methods for performing overlay analysis-feature overlay (overlaying points, lines, or polygons) and raster overlay. Often, overlay is one step in an analysis process or model and may occur at various points in the process. Similarly, you'll likely perform additional analysis on the results of the overlay, such as extraction to select a subset of features, or generalization (to dissolve polygons, for example). For example, you might include datasets derived from proximity analysis (such as the Buffer tool) or surface analysis (the Slope or Aspect tool). Overlay analysis is often used in conjunction with other types of analysis. The resulting polygons have all the attributes of the original polygons. New polygons are created by the intersection of the input polygon boundaries. For example, you'd overlay layers of vegetation type, slope, aspect, soil moisture, and so on, to find areas susceptible to wildfire.īelow is an example of an overlay of steep slopes, soils, and vegetation. This approach is often used to find locations that are suitable for a particular use or are susceptible to some risk.
You can then find specific locations or areas that have a certain set of attribute values-that is, match the criteria you specify. You can use overlay analysis to combine the characteristics of several datasets into one. The lines are shown symbolized by the vegetation type associated with each. The lines have been split where they were intersected by polygons, and each line feature has been assigned the attributes of both original layers. In the illustration below, logging roads (lines) and vegetation types (polygons) are overlaid to create a new line feature class. Similarly, you'd overlay watershed boundaries with a vegetation layer to calculate the amount of each vegetation type in each watershed. Following is a chart illustrating the result of this calculation. The total area of each land-use type in the flood zone can be calculated by selecting all polygons within the flood zone (using the Select Layer By Attribute tool, for example) and summarizing the area by land-use type (using the Frequency tool). The FID_flood value indicates whether polygons are outside (-1) or inside the flood zone, and all polygons retain their original land-use category values.
The parcels are split where they are crossed by the flood zone boundary, and new polygons created. Because overlay yields such valuable information, it was paramount to the development of GIS.Īn overlay operation is much more than a simple merging of line work all the attributes of the features taking part in the overlay are carried through, as shown in the example below, where parcels (polygons) and flood zones (polygons) are overlaid (using the Union tool) to create a new polygon dataset. To answer such questions before the days of GIS, cartographers would create maps on clear plastic sheets and overlay these sheets on a light table to create a new map of the overlaid data.