Sample set   

Prior to an image classification, sample pixels or training pixels have to be selected in a sample set. To create a sample set, first a map list and a domain have to be specified. Then, with sampling, assign class names to groups of pixels that are supposed to represent a known feature on the ground and that have similar spectral values in the maps in the map list.

A sample set contains:

When your graphics board is configured to use 256 colors, you can locate your training pixels on a background map, for instance a color composite. In case your graphics board is configured to use more than 256 colors, you will use an interactive color composite; then, a background map is not used.