# Weighted Sum (Spatial Analyst)

## Summary

Overlays several rasters, multiplying each by their given weight and summing them together.

## Illustration In the illustration, the cell values are multiplied by their weight factor, and the results are added together to create the output raster. For example, consider the top left cell. The values for the two inputs become (2.2 * 0.75) = 1.65 and (3 * 0.25) = 0.75. The sum of 1.5 and 0.75 is 2.4.

## Usage

• A useful way to add several rasters together is to input multiple rasters and set all weights equal to 1.

• Input rasters can be integer or floating point.

• The weight values can be any positive or negative decimal value. It is not restricted to a relative percentage or equal to 1.0.

• The weight will be applied to the specified field for the input raster. Fields can be of type short or long integer, double or float.

## Syntax

WeightedSum (in_rasters)
 Parameter Explanation Data Type in_rastersin_weighted_sum_table The Weighted Sum tool overlays several rasters, multiplying each by their given weight and summing them together. An Overlay class is used to define the table. The WSTable object is used to specify a Python list of input rasters and weight them accordingly.The form of the WSTable object is:WSTable ([[inRaster, field, weight],...]) WSTable

#### Return Value

 Name Explanation Data Type out_raster The output suitability raster.It will be of floating point type. Raster

## Code Sample

WeightedSum example 1 (Python window)

This example creates a suitability raster for locating a ski resort by combining multiple rasters together, and applying appropriate weight factors.

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

# Execute WeightedSum
outWeightedSum = WeightedSum(WSTable([["snow", "VALUE", 0.25], ["land", "VALUE",0.25],
["soil", "VALUE", 0.5]]))
outWeightedSum.save("C:/sapyexamples/output/outwsum")```
WeightedSum example 2 (stand-alone script)

This example creates a suitability raster for locating a ski resort by combining multiple rasters together, and applying appropriate weight factors.

```# Name: WeightedSum_Ex_02.py
# Description: Overlays several rasters multiplying each by their given
#    weight and summing them together.
# Requirements: Spatial Analyst Extension
# Author: ESRI

# 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"
WSumTableObj = WSTable([[inRaster1, "VALUE", 0.25], [inRaster2, "VALUE", 0.25],
[inRaster3, "VALUE", 0.5]])

# Check out the ArcGIS Spatial Analyst extension license
arcpy.CheckOutExtension("Spatial")

# Execute WeightedSum
outWeightedSum = WeightedSum(WSumTableObj)

# Save the output
outWeightedSum.save("C:/sapyexamples/output/weightsumout")```

## Licensing Information

ArcView: Requires Spatial Analyst
ArcEditor: Requires Spatial Analyst
ArcInfo: Requires Spatial Analyst

6/29/2011