(1) The algebraic difference of the distances of two images of a point from the respective photograph nadirs, measured in a horizontal plane and parallel to the air base. (2) The same as the preceding definition except that the distances are measured in the plane of the photograph and in the direction of flight. The term is often shortened to parallax. Other names are absolute parallax, horizontal parallax, linear parallax, stereoscopic parallax and x-parallax. Usually, absolute stereoscopic parallax is used, to denote similar measurements made when the ideal conditions of truly vertical photographs are not attained, as, for example, when measuring parallax on unrectified aerial photographs.