How Weighted Sum works
The Weighted Sum tool provides the ability to weight and combine multiple inputs to create an integrated analysis. It is similar to the Weighted Overlay tool in that multiple raster inputs, representing multiple factors, can be easily combined incorporating weights or relative importance.
There are two major differences between these tools:
- The Weighted Sum tool does not rescale the reclassified values back to an evaluation scale.
- The Weighted Sum tool allows floating-point and integer values, whereas the Weighted Overlay tool only accepts integer rasters as inputs.
By not rescaling the reclassified values back to the evaluation scale, the analysis maintains its resolution. For example, in a suitability model, if there are 10 input criteria that were reclassified to a 1 to 10 scale (10 being the most favorable) and no weights were assigned, the values on the output from Weighted Sum could range from 10 to 100. For the same input, Weighted Overlay would normalize the 10 to 100 reclassified analysis range to the evaluation scale, for example, back to the 1 to 10 scale. Maintaining the model resolution in Weighted Sum can be useful when you want to identify only the few top favorable locations or a specified number of sites.
Combining multiple continuous data layers with different ranges is not always meaningful.
Generally, the values of continuous rasters are grouped into categories. For example, the various slope values can be grouped into flat, moderate, steep, and very steep. Each slope value can be assigned to one of these categories and the category assigned a reclass value identifying the preference for the class relative to a criterion in the overlay analysis. The Reclassify tool allows such rasters to be reclassified.
The Weighted Overlay tool is used most commonly for suitability modeling and should be used to ensure correct methodologies are followed. The Weighted Sum tool is useful when you want to maintain the model resolution or when floating-point output or decimal weights are required.
Weighted Sum works by multiplying the designated field values for each input raster by the specified weight. It then sums (adds) all input rasters together to create an output raster.
The steps for running Weighted Sum tool are as follows:
- Add rasters.
Click the Input rasters arrow and click an input raster, or browse to an input raster and click Add. The raster is added to the Weighted Sum table. Repeat to enter the next raster, and so on.
- Select the field.
For each input raster, click the field column to specify the field to be used in the overlay analysis. A drop-down list -appears displaying all valid fields for the input. The field must be a numeric field.
- Assign weights for input rasters.
Each input raster can be weighted, or assigned a percentage influence, based on its importance. For each input raster, click in the weight column to specify a value. The weights can be any floating-point value (negatives included).
- Run the tool.
The cell values of each input raster are multiplied by the raster's weight. The resulting cell values are added to produce the final output raster.