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Weighted Overlay

Available with Spatial Analyst license.

Summary

Overlays several rasters using a common measurement scale and weights each according to its importance.

Learn more about how Weighted Overlay works

Illustration

Weighted Overlay illustration
In the illustration, the two input rasters have been reclassified to a common measurement scale of 1 to 3. Each raster is assigned a percentage influence. The cell values are multiplied by their percentage influence, and the results are added together to create the output raster. For example, consider the upper left cell. The values for the two inputs become (2 * 0.75) = 1.5 and (3 * 0.25) = 0.75. The sum of 1.5 and 0.75 is 2.25. Because the output raster from Weighted Overlay is integer, the final value is rounded to 2.

Usage

  • All input rasters must be integer. A floating-point raster must first be converted to an integer raster before it can be used in Weighted Overlay. The Reclassification tools provide an effective way to do the conversion.

  • Each value class in an input raster is assigned a new value based on an evaluation scale. These new values are reclassifications of the original input raster values. A restricted value is used for areas you want to exclude from the analysis.

  • Each input raster is weighted according to its importance or its percent influence. The weight is a relative percentage, and the sum of the percent influence weights must equal 100. Influences are specified by integer values only. Decimal values are rounded down to the nearest integer.

  • Changing the evaluation scales or the percentage influences can change the results of the weighted overlay analysis.

  • By default, this tool will take advantage of multi-core processors. The maximum number of cores that can be utilized is limited to four.

    If you want the tool to use fewer cores, use the parallelProcessingFactor environment setting.

  • See Analysis environments and Spatial Analyst for additional details on the geoprocessing environments that apply to this tool.

Syntax

WeightedOverlay (in_weighted_overlay_table)
ParameterExplanationData Type
in_weighted_overlay_table

The Weighted Overlay tool allows the calculation of a multiple-criteria analysis between several rasters.

An Overlay class is used to define the table. The WOTable object is used to specify the criteria rasters and their respective properties.

The form of the object is:

  • WOTable(weightedOverlayTable, evaluationScale)

WOTable

Return Value

NameExplanationData Type
out_raster

The output weighted raster.

Raster

Code sample

WeightedOverlay example 1 (Python window)

This example creates a suitability IMG raster that identifies potential site locations for a ski area.

import arcpy
from arcpy import env  
from arcpy.sa import *
env.workspace = "C:/sapyexamples/data"

outsuit = WeightedOverlay(WOTable(
           [
            ["snow", 50, 'VALUE', RemapValue([[1,"Nodata"],[5,3],[9,10],["NODATA","NODATA"]])], 
            ["land", 20, '', RemapValue([["water","1"],["forest",5],["open field",9],["NODATA", "NODATA"]])],
            ["soil", 30, 'VALUE', RemapValue([[1,"Restricted"],[5,5],[7,7],[9,9],["NODATA", "Restricted"]])]
           ],[1,9,1]))
outsuit.save("C:/sapyexamples/output/outsuit.img")
WeightedOverlay example 2 (stand-alone script)

This example creates a suitability IMG raster that identifies potential site locations for a ski area.

# Name: WeightedOverlay_Ex_02.py
# Description: Overlays several rasters using a common scale and weighing 
#    each according to its importance.
# Requirements: Spatial Analyst Extension

# Import system modules
import arcpy
from arcpy import env
from arcpy.sa import *

# Set environment settings
env.workspace = "C:/sapyexamples/data"

# Set local variables
inRaster1 = "snow"
inRaster2 = "land"
inRaster3 = "soil"

remapsnow = RemapValue([[0,1],[1,1],[5,5],[9,9],["NODATA","NODATA"]])
remapland = RemapValue([[1,1],[5,5],[6,6],[7,7],[8,8],[9,9],["NODATA","Restricted"]])
remapsoil = RemapValue([[0,1],[1,1],[5,5],[6,6],[7,7],[8,8],[9,9],["NODATA", "NODATA"]])

myWOTable = WOTable([[inRaster1, 50, "VALUE", remapsnow],
                     [inRaster2, 20, "VALUE", remapland], 
                     [inRaster3, 30, "VALUE", remapsoil]
					          ], [1, 9, 1])    

# Execute WeightedOverlay
outWeightedOverlay = WeightedOverlay(myWOTable)

# Save the output
outWeightedOverlay.save("C:/sapyexamples/output/weightover2")

Licensing information

  • ArcGIS Desktop Basic: Requires Spatial Analyst
  • ArcGIS Desktop Standard: Requires Spatial Analyst
  • ArcGIS Desktop Advanced: Requires Spatial Analyst

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