Combine and weight derived rasters with the Weighted Overlay tool
Segment 18 of 34
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After you've reclassified all of your input rasters to a common scale, you can combine the derived datasets and landuse to find the most suitable locations. The values of
the derived datasets representing slope, distance to recreation
sites, and distance to schools have all been reclassified
to a common measurement scale (more suitable cells have
higher values). The landuse dataset is still in its original
form because you can weight the cell values for this dataset
as part of the weighted overlay process. Values representing
areas of water and wetlands will be restricted. You’ll
also mark slope values that are less than 4 (the least
suitable because they are too steep) as restricted so these
values can be excluded. You’ll learn more about restricted
values and NoData later in this exercise. If all datasets
were equally important, you could simply combine them,
giving each equal influence; however, you have been informed that it is preferable to locate the new school close
to recreational facilities and away from other schools. You
will weight all the inputs, assigning each a percentage of
influence. The higher the percentage, the more influence a
particular input will have in the suitability model.
You will assign the inputs the following percentages of
Reclassed distance to rec_sites: 50%
Reclassed distance to schools: 25%
Reclassed slope: 13%
To do a weighted overlay, in the Spatial Analyst Tools toolbox, expand the Overlay toolset, click and drag the Weighted Overlay tool onto the ModelBuilder window, right-click the Weighted Overlay tool element and click Open.
The default evaluation scale is from 1 to 9 by 1. A scale of
1 to 10 was used when reclassifying datasets, so before
adding input rasters to the Weighted Overlay tool, you want
to set the evaluation scale from 1 to 10 by 1. This means
you will avoid having to update the scale values after adding
your input datasets.
Type “1”, “10”, and “1” in the From, To, and By text boxes.
To add the input rasters to the Weighted Overlay tool, click the Add raster row button. Click the Input raster drop-down arrow and click the Reclassed distance to schools variable. Accept the default Input field and click OK.
The raster is added to the Weighted Overlay tool. The
Field column displays the values of the Reclassed
distance to schools output. The Scale Value column
mimics the Field column because the evaluation scale
was set to encompass the range of values in each input
raster. You could modify the scale values for each class
at this point, but for this input, the values were already
weighted appropriately at the time of reclassifying.
Click the Add raster row button again to add the next input raster.
Continue adding the input rasters.
When you add the reclassified slope raster and landuse raster you'll change the scale values for certain input fields to Restricted, to disqualify certain landuse types and steep sloped from consideration in the analysis.
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 1 (zero in this exercise). If there are no inputs to the Weighted Overlay toool with cells of NoData, you could use NoData as the scale value to exclude certain values. However, if you have NoData cells in any of your inputs, it is safest to use Restricted. Potentially, a result from the Weighted Overlay tool could contain 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 intentionally 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 but that are actually suitable areas. If you use NoData to exclude certain cell values and there is NoData in one or more inputs, you will not know whether a cell of NoData means the area is restricted from use or there was no input data available in that location.
You’ll now weight the scale values of the landuse layer so they are comparable with the other inputs. A lower value indicates that a particular landuse type is less suitable for building. The scale values for Water and Wetlands will be set as Restricted, since they cannot be built on and should be excluded.
Change the default scale values for the landuse layer to
the following values:
You’ll now assign a percentage of influence to each raster, based on how much importance (or weight) each should have in the final suitability map.
In the % Influence column, type the following percentages
for each of the input rasters:
Reclassed distance to schools 25
Reclassed distance to recreation sites 50
Reclassed slope 13
Note: Move the mouse pointer over the name of an input raster to view the entire name.
After you run the process, examine the layer added to your ArcMap display. Locations with higher values indicate more suitable sites—areas that are on less steep slopes of suitable land-use types, closer to recreational facilities, and away from existing schools. Notice that the areas you marked as restricted have a value of zero.