1. The method of classifying fingerprints comprising the steps of: making an impression of a fingerprint on a two-dimensional surface, transforming the two-dimensional surface representation of the fingerprint impression into a representation of a three-dimensional surface with a predetermined set of first and second coordinates and with a variable third coordinate for each set of first and second coordinates, the third coordinate having a value dependent upon the frequency of occurrence or ridge lines at a specific set of first and second coordinates when the origin of the first and second coordinates is taken at multiple positions over a prescribed part of the fingerprint impression, determining the values of local spatial derivatives of a plurality of different areas of the three-dimensional surface, and concatenating said values in a predetermined order to construct a descriptor code for the fingerprint.