Weighted Overlay is a technique for applying a common measurement scale of values to diverse and dissimilar inputs to create an integrated analysis.

Geographic problems often require the analysis of many different factors. For instance, choosing the site for a new housing development means assessing such things as land cost, proximity to existing services, slope, and flood frequency. This information exists in different raster layers with different value scales: dollars, distances, degrees, and so on. You can't add a raster of land cost (dollars) to a raster of distance to utilities (meters) and obtain a meaningful result.

Additionally, the factors in your analysis may not be equally important. It may be that the cost of land is more important in choosing a site than the distance to utility lines. How much more important is for you to decide.

Within a single raster layer, you must usually prioritize values. For example, a value of 1 represents slopes of 0 to 5 degrees, a value of 2 represents slopes of 5 to 10 degrees, and a value of 3 represents slopes of 10 to 15 degrees. If slope is a criteria in finding a new site, for example, and your evaluation scale is from 1 to 9 by 1, you might give a scale value of 9 to the input value of 1 (the most suitable areas with least steep slopes), a scale value of 6 to the input value of 2 (the second most suitable slopes), and a scale value of 3 to the input value of 3 (the least suitable, steepest slopes). If it was decided that slopes greater than 15 degrees would not be considered, all input values greater than 3 would be assigned a scale value of restricted to exclude them.

Weighted Overlay only accepts integer rasters as input, such as a raster of land use or soil types. Continuous (floating point) rasters must be reclassified as integer before they can be used. Generally, the values of continuous rasters are grouped into ranges, such as for slope, or Euclidean distance outputs. Each range must be assigned a single value before it can be used in the Weighted Overlay tool. The Reclassify tool allows for such rasters to be reclassified. You can either leave the value assigned to each range (but note the range of values to which the new value corresponds) and assign weights to the cell values in the Weighted Overlay dialog box later, or you can assign weights at the time of reclassifying. With the correct evaluation scale chosen, simply add the raster to the Weighted Overlay dialog box. The cells in the raster will already be set according to suitability or preference, risk, or some similarly unifying scale. The output rasters can be weighted by importance and added to produce an output raster.

The steps for running weighted overlay are:

1. Select an evaluation scale—In the Weighted Overlay dialog box, select an evaluation scale to use. Values at one end of the scale represent one extreme of suitability (or other criterion); values at the other end represent the other extreme. The default evaluation scale is from 1 to 9 in increments of 1 (least suitable 1, most suitable 9). If your input rasters are already reclassified to a common measurement scale using the Reclassify tool, it is important to select an evaluation scale that matches the scale used when reclassifying. For instance, if rasters were reclassified using a scale of 1 to 10 (1 being least suitable and 10 being most suitable), an evaluation scale of 1 to 10 by 1 should be entered for the evaluation scale in the Weighted Overlay dialog box.
2. Add rasters—Click the Add raster row button to open the Add Weighted Overlay dialog box. Click the Input raster drop-down arrow and click a raster, or click the Browse button to browse to an input raster and click Add. Click the Input field drop-down arrow to change the field if desired. Click OK. The raster is added to the Weighted Overlay table. Click the Add raster row button again to enter the next raster, and so on.

NOTE: If land use is one of your inputs, you might have a description field that describes each land use type. Using this field instead of the default Value field will make it easier to assign weights to this raster in the Weighted Overlay dialog box.

NOTE: Only discrete integer rasters can be used in the Weighted Overlay dialog box. Reclassify continuous rasters before adding them to the Weighted Overlay dialog box.

3. Set scale values—The cell values for each input raster in the analysis are assigned values from the evaluation scale. This makes it possible to perform arithmetic operations on rasters that originally held dissimilar types of values. You can change the default values assigned to each cell according to importance or suitability. For instance, a land-use raster added has values representing the land-use type (Forest = 7, Water = 3, Barren land = 1, Scrub land = 10). To find suitable locations on which to build, you would assign scale values depending on which land-use types are more suitable. For example, with an evaluation scale set at 1 to 9 by 9, you might assign the following scale values: Forest = 3, Water = Restricted, Barren land = 9, Scrub land = 7.
4. Assign weights to input rasters—Each input raster can be weighted, or assigned a percentage influence, based on its importance. The total influence for all rasters must equal 100 percent. For instance, it might be more important to build a shopping center on soils that are stable than to locate in a popular shopping area.
5. Run the Weighted Overlay tool—The cell values of each input raster are multiplied by the raster's weight (or percent influence). The resulting cell values are added to produce the final output raster.

If the tool was used for suitability modeling (to locate suitable areas), higher values generally indicate that a location is more suitable. If the tool was used to generate a cost surface (to find out how much it will cost to travel through the landscape for instance), high values will generally indicate higher travel costs. You must understand the scale values you apply to input rasters so you know what the values in the output raster mean.

Using Restricted and NoData for the Scale Value

Setting a scale value to Restricted assigns a value to that cell in the output weighted overlay result that is the minimum value of the evaluation scale set, minus one. If there are no inputs to the Weighted Overlay dialog box with cells of NoData, you could use NoData as the scale value to exclude certain values. However, it is safest, and essential if you have NoData cells in any of your inputs, to use Restricted instead. Potentially you could have a result from the Weighted Overlay tool that contains cells of NoData that have come from one or more of the inputs (NoData on any input equals NoData in the result) and Restricted areas that you purposely excluded. NoData and Restricted values should not be confused. Each serves a specific purpose. There may be areas of NoData where you don't know the value, which are actually suitable areas. If you use NoData instead of Restricted to exclude certain cell values, and there is NoData in one or more inputs, you will not know if a cell of NoData means the area is restricted from use or whether there was no input data available in that location.

Take care using Restricted for the scale value when creating a cost surface. Since using Restricted gives a value to the cell that is the minimum value of the evaluation scale, minus 1, your restricted areas will appear to be given the lowest cost, when they are actually excluded from the analysis. You should assign a high cost or set the scale value to NoData for areas you want exclude from the analysis instead.