1. A computer method for using the Second Homotopy Group to assess the similarity of sets of two dimensional digital data (S2DDD) comprising: a) receiving, into a memory by a processor, one or more sets of digital data for presentation in two-dimensions each S2DDD comprising: i) at least two data values per data point organized in sequential addresses or at set intervals of addresses, ii) the number of data values per data point, iii) a specified starting address, iv) a specified number of data points, called the width, in the horizontal or x-direction, and v) a specified number of data points, called the height, in the vertical or y-direction, b) computing a numerical similarity signature for the received S2DDD, referred to as the equivalence signature, as the Second Homotopy Group's invariant for the values of said S2DDD interpreted as a map from the two-dimensional presentation space into the space of the S2DDD, c) querying a database for similarity signature that is equivalent to the computer similarity signature of the received S2DDD, d) computing a similarity distance between the computed similarity signature of the received S2DDD and any S2DDD in said database as the absolute value of the difference of their equivalence signatures, and e) outputting results of the query, and wherein computing an equivalence signature as the Second Homotopy Group's invariant of a pair of planes of normalized digital data further comprises: looping over each y-position, from the first x-position to the second to last y-position, and over each x-position, from the first x-position to the second to last x-position, and for each position, a) computing the difference of the data value at said position in the first plane and the data value at said position shifted by one in the x-direction in the first plane, b) repeating the step a but with the first plane replaced by the second plane and the x-direction replaced by the y-direction, c) multiplying the results from steps a and b, d) repeating steps a-c but with the first plane replaced by the second plane and vice versa, e) computing the difference of the results from step c and step d and multiplying the result with the sine of the data value at said position in the second plane, f) adding the result from step e to the result computed in the last roll of the inner loop to form the result for the current roll of the inner loop with the value set to zero in the initial roll, and g) upon completion of both loops, diving the result from the last roll of the inner loop by negative four times the value of the constant Π to form the value of the equivalence signature.