1. In a system for the classification of a body of multispectral data vectors which represents a multivariate scene having a plurality of subject classes, the system having means for determining the spectral signature of each subject class and means for classifying the body of multispectral data vectors in accordance with the spectral signatures, the improvement comprising the inclusion in the means for determining the spectral signature of each subject class of: storage means for storing the body of multispectral data vectors; display means for displaying a data set from the body of data vectors which contains a subject class in a format forming a two-dimensional, ordered array; operator control means responsive to an operator identification of the vertices of a polygon representing a subject class contained within a displayed set of data vectors; means for identifying data vectors contained within the polygon defind by the operator; and means for operating on data vectors identified as being contained within the polygon to compute a spectral signature.