In the above illustration, the principal points of the photographs can be accepted as the nadir positions. Both the top and base objects are clearly visible. The absolute stereoscopic parallax is 12.36 mm in this case, i.e., the sum of the distances of the corresponding images from their respective nadirs. Differential parallax in this case is the difference in the absolute stereoscopic parallax at the top and base objects being measured, i.e., 12.3−12.32 = 0.04.
The change in position of an image from one photograph to the next overlapping photograph caused by the aircraft's
motion. It is the algebraic difference between their respective nadirs, measured in a horizontal plane and perpendicular to the air base. It is also the algebraic sum of the distances of corresponding images from their respective nadirs. It is always measured parallel to the flight line. Also called x-parallax, linear parallax, horizontal parallax, absolute parallax, stereoscopic parallax
, and primary parallax