Often during classification, unique spectral classes appear that do not correspond to any of the classes you want to use. In other cases, a broad information class may contain a number of spectral sub-classes with unique variations. This can be caused by a mixture of ground cover types appearing in the image at the time it was recorded, or by shadows and variations in scene illumination.
Focus offers several methods for ensuring that your training sites are both representative and complete. You can work with training site data using the Training Site Editor, the Signature Statistics window, and the Scatter Plot window.
You can view and test the reliability of your training sites by creating a histogram in the Class Histogram Display window. The histogram shows the frequency of training site pixels as a percentage of the number of pixels in your training sites. A histogram should have a uni-modal shape displaying a single peak. A multi-modal histogram indicates the likelihood that the training sites for that class are not pure, but contain more than one distinct land-cover class.
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