A digital image is a 2D array of pixels. The value of a pixel, i.e. its DN, is in the case of an 8-bit record in the range 0 to 255. Each DN corresponds to the EM radiation reflected or emitted from a ground resolution cell—unless the image has been resampled. The spatial distribution of the DNs defines the image or image space. A multispectral sensor records the radiation from a particular GRC in different channels according to its spectral band separation. A sensor recording in three bands yields three pixels with the same row and column tuple (i, j) since they stem from one and the same GRC.