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Bethel, James 
  
different from directly illuminated areas; because they are quite dark, they would not be that different spectrally from 
water in the scene. It is a fundamental requirement that the list of classes must be reasonably exhaustive. 
To illustrate the role that the human analyst must play in the process, note that some rooftops are made of gravel/asphalt 
mixtures, the same materials used in some roads. In this case it is desired to discriminate between these two classes of 
use, and it would be necessary for the analyst to label examples of each if a classifier is to successfully discriminate 
between them. 
  
Figure 3 Image Sequence: Raw Image, Thematic Classes, Rectified Thematic Classes 
Background 
> Roof 
8» Road 
8 Grass 
E88 Trees 
Trail 
& Water 
E Shadow 
Figure 4. Class labels for the Thematic Image 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 189