Abstract: A multispectral data classification system has an improved apparatus for the identification of training samples of multispectral image data useful in the computation of spectral signatures.A set of multispectral image data is displayed on a user-interactive device in a two-dimensional ordered array. The training sample is defined by the sequential definition of the vertices of a polygon representing the training sample. A rectangle is closely superscribed about the polygon. The data elements within the rectangle are examined to determine if they are within the polygon.The apparatus includes a multispectral data classification system incorporating a color, CRT display for displaying the set of data; a manually-operable trackball cursor interactive with the color, CRT display for defining the polygon and rectangle; an alphanumeric keyboard and monochrome CRT for providing communication between the operator and system; and a random access memory device.