Using the Image Algebra process

The Image Algebra process allows you to perform algebra on the spatially overlapping pixels from two raster bands or two raster datasets with up to four bands. For example, you can add two spatially overlapping, single-band raster datasets to form one output single-band raster, and the pixel values of the output would be combined values from pixels in each band: band 1 + band 2. Calculations such as the Normalized Difference Vegetation Index (NDVI) are actually an implementation of image algebra, and many other similar calculations can be executed using this process. You can perform equations, such as vegetation indexes, by chaining together two or more Image Algebra processes.

There are three rasters involved in this process: primary, auxiliary, and output. The primary and auxiliary rasters are the processed inputs, and the result is the output raster. The primary and auxiliary rasters must overlap spatially and must have the same bit depth and bands per pixel. The Convert Pixel Type and Stack Bands processes should be used prior to the Image Algebra process if the bit depth and bands are not in the required format. The output will be truncated to the limits of the bit depth. Note that float values are scaled by 214 when read into ArcGIS Image Server and handled as signed, 32-bit long values.

The methods in this process can also be affected by setting offset and scale values for the input data. The scale value is always applied first, then the scaled value is shifted using the defined offset value. The output can also be scaled and offset. These scale and offset values are often used to normalize pixel values or bring them into a suitable range, as shown in the following example:

Output Value = (((Primary * Scale) + Offset) [function]
((Auxiliary * Scale) + Offset) * Scale) + Offset
Image Algebra Process Definition dialog box

The Resample option on the Auxiliary tab specifies whether to resample the auxiliary image. Resampling ensures that the auxiliary image is in the same spatial dimension. Checking the Resample check box is required if the auxiliary image is from a different image and its pixels are not aligned with the primary image.

The Auxiliary raster ID is a combination of two identifier values: "<RasterID>.<RasterIDinRPDef>". The RasterID value is listed in the RasterID column of the service table. The RasterIDinRPDef value appears on the Raster Properties dialog box. When you open this dialog box, click the Rasters node and identify the correct ID value from the displayed table.

There are six algebra methods that can be applied to the raster datasets:

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7/10/2012