Compute Change function

Available with Image Analyst license.

Overview

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 three computation methods:

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

  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.

To visualize only the pixels that changed from one class to another, use the From Classes and To Classes parameters, and check the box for Changed Pixels Only. For example, to see only those pixels that changed from Forest to Developed, enter the class value for Forest in the From Class Values, and enter the Developed class value in the To Class Values, then check the box to include changed pixels only. Only the pixels that changed from Forest to Developed will be symbolized in the output layer. All other pixels will be grouped in a class called 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 will be 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.

If either input is a multiband raster, only the first band in the raster is used for computation.

Parameters

Parameter	Description
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 here.
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. Difference—The mathematical difference, or subtraction, between the pixel values in the input rasters will be calculated. This is the default. Relative Difference—The difference in pixel values, accounting for the magnitudes of the values being compared, will be calculated. Categorical—The difference between two categorical or thematic rasters will be calculated, where the output contains class transitions that occurred between the two rasters.
From Class Values	The class values from the From Raster 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 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 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 will automatically search for 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 will occur. 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 is 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 will be the average of the from (initial) and to (final) class colors. This is the default. Use From Colors—The color of the output class will match the color of the from (initial) class. Use To Colors—The color of the output class will match 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.