We can modify these clusters, so that their total number can vary arbitrarily when four or more bands are involved. (This multidimensional situation is, of course, not readily displayed in a 3-D plot.) When we do the separations through a computer program, each pixel in an image is assigned to one of the clusters as being most similar to it in DN combination value. Generally, in an area within an image, multiple pixels in the same cluster correspond to some (initially unknown) ground feature or class so that patterns of gray levels result in a new image depicting the spatial distribution of the clusters. These levels can then be assigned colors to produce a cluster map. The trick then becomes one of trying to relate the different clusters to meaningful ground categories. We do this by either being adequately familiar with the major classes expected in the scene, or, where feasible, by visiting the scene (ground truthing) and visually correlating map patterns to their ground counterparts. Since the classes are not selected beforehand, this latter method is called Unsupervised Classification.