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Related to parallax difference: Parallax error
object and at the bottom of the image. It is represented customarily by the symbol dp. The absolute stereoscopic parallax on truly vertical photographs, taken at the same height, is the algebraic difference, parallel to the line of flight, of the distances of the two images of a given object at their base from their respective principal points. Used customarily in the determination of the difference between the elevations of two objects.