# Compute Change function

## Overview

The Compute Change function computes the differences between two categorical or continuous raster datasets.

## Notes

This function can be used to enumerate the differences between two classified rasters for land-cover change analysis, or it can be used to examine changes between two single-band, continuous rasters such as elevation, temperature, canopy cover, and so on.

There are six computation methods:

• Difference—Calculates the mathematical difference (subtraction) between two raster layers.

``To Raster - From Raster``

• Relative Difference—Calculates the difference between the raster datasets while accounting for the magnitude of the pixel values.

``(To Raster - From Raster)/max(From Raster, To Raster)``

• Categorical—The difference between two categorical or thematic rasters, where the output shows every class transition that occurred between the two rasters. For example, a class labeled Forest > Developed indicates a change from Forest in From Raster to Developed in To Raster.
• Spectral Euclidean Distance—Calculates the Euclidean distance between two multiband rasters by treating each pixel as a vector. Larger values indicate more change between the images.
• Spectral Angle Difference—Calculates the spectral angle between two multiband rasters by treating each pixel as a vector. Larger angles indicate more change between the images.
• Band With Most Change—Calculates the band that accounts for the most change in each pixel between two multiband rasters.

To visualize only the pixels that changed from one class to another, use the From Classes and To Classes parameters, and check the Changed Pixels Only box. For example, to see only those pixels that changed from Forest to Developed, enter the class value for Forest in the From Class Values parameter, enter the Developed class value in the To Class Values parameter, and check the box to include changed pixels only. Only the pixels that changed from Forest to Developed are symbolized in the output layer. All other pixels are grouped in a class named Other, which is transparent by default.

If Compute Change Method is set to Categorical and one of the inputs is not categorical, a simple difference is calculated.

##### Note:

When analyzing categorical change, both From Raster and To Raster must have an associated raster attribute table. The attribute table must have either a VALUE field, CLASS_NAME field, CLASSNAME field, or one string field. The data in these fields is used to compare between the two rasters.

## Parameters

Parameters for the Compute Change function are detailed below.

ParameterDescription

From Raster

The first raster to be used in the computation. To evaluate change from time 1 (earlier) to time 2 (later), enter the time 1 raster.

To Raster

The second raster to be used in the computation. To evaluate change from time 1 (earlier) to time 2 (later), enter the time 2 raster.

Compute Change Method

The method used for computation.

• Single-band rasters
• Difference—The mathematical difference, or subtraction, between the pixel values in the input rasters is calculated. This is the default.
• Relative Difference—The difference in pixel values, accounting for the magnitudes of the values being compared, is calculated.
• Categorical—The difference between two categorical or thematic rasters is calculated, where the output contains class transitions that occurred between the two rasters.
• Multiband rasters
• Spectral Euclidean Distance—The Euclidean distance between the pixel values of two multiband rasters is calculated.
• Spectral Angle Difference—The spectral angle between the pixel values of two multiband rasters is calculated. The output is in radians.
• Band With Most Change—The band that accounts for the most change in each pixel between two multiband rasters is calculated.

From Class Values

The class values from the From Raster parameter to be included in the computation, if comparing two categorical rasters. This is a space-delimited list of integer values corresponding to the ClassValue field in your input raster dataset.

To Class Values

The class values from the To Raster parameter to be included in the computation, if comparing two categorical rasters. This is a space-delimited list of integer values corresponding to the ClassValue field in your input raster dataset.

Field Name For Class Names In From Raster

A field that stores class names in the From Raster input. The tool automatically searches for the ClassName field or Class_Name field to use.

Use this option if the input does not contain these standard field names.

Field Name For Class Names In To Raster

A field that stores class names in the To Raster input. The tool automatically searches for the ClassName field or Class_Name field to use.

Use this option if the input does not contain these standard field names.

Filter Method

Choose the filter method for categorical change computation.

• Keep All—All classes are included in the output, and no filtering occurs. Choose this option to see change and persistence in a single dataset.
• Keep Changed Pixels Only—Only the classes that have changed from one class type to another class type are included in the output. Choose this option if you are only interested in change.
• Keep Unchanged Pixels Only—Only the classes that have not changed are included in the output. Choose this option if you are only interested in persistence.

Define Transition Colors

Specifies the color to use to symbolize the output classes. When a pixel changes from one class type to another, the output pixel color represents the initial class type, the final class type, or a blend of the two.

• Average From And To Colors—The color of the output class is the average of the from (initial) and to (final) class colors. This is the default.
• Use From Colors—The color of the output class matches the color of the from (initial) class.
• Use To Colors—The color of the output class matches the color of the to (final) class.

Cellsize Type

Choose which cell size to use in the output raster. If all the input cell sizes are the same, all the options will yield the same results.

• First Of—Use the first cell size of the input rasters.
• Min Of—Use the smallest cell size of all the input rasters.
• Max Of—Use the largest cell size of all the input rasters. This is the default.
• Mean Of—Use the mean cell size of all the input rasters.
• Last Of—Use the last cell size of the input rasters.

Extent Type

Choose which extent should be used in the output raster:

• First Of—Use the extent of the first input raster to determine the processing extent.

• Intersection Of—Use the extent of the overlapping pixels to determine the processing extent. This is the default.
• Union Of—Use the extent of all the rasters to determine the processing extent.
• Last Of—Use the extent of the last input raster to determine the processing extent.