1. A method of determining changes in the yaw, pitch, roll or elevation of an airborne sensor which moves along a flight path above the earth and records successive two-dimensional images of the earth's surface along said flight path, comprising the steps of: (A) generating a first set of data representing picture elements of a first two-dimensional image of said surface at a first point along said flight path such that the data representing all of the picture elements of said first image are generated simultaneously; (B) generated a second set of data representing picture elements of a second two-dimensional image of said surface at a second point along said flight path closely spaced from said first point such that at least a portion of said images overlap each other, and wherein the data representing all of the picture elements of said second image are generated simultaneously, said picture elements varying in intensity in accordance with their respective locations on the corresponding images, the corresponding picture elements of the first and second images in the overlapping portions thereof being offset from each other by an amount related to changes in the yaw, pitch, roll or elevation of said sensor; (C) correlating a plurality of corresponding picture elements of the first and second images in the overlapping portions thereof, the relative positions of the correlated picture elements representing the changes in the yaw, pitch, roll or elevation of said sensor, and wherein the step of correlating is performed at least in part by selecting at least three spaced apart N.times.M arrays of picture elements in one of said images, selecting at least three spaced apart reference picture elements in the other of said images which are respectively associated with said at least three N.times.M arrays, and comparing the at least three reference picture elements with the picture elements in the associated N.times.M arrays.