Parallax is an apparent shift in the position of an object due to shift in the position of the observer camera (in aerial surveying). We experience this phenomenon when a moving body ( a shift in position when compared to a static object considering camera as eye). This is depended on the distance between the observer and the object. Nearer object move faster than that of the far distanced object, similar is the case of an aerial camera exposed to overlapping photographs which is caused by the movement of the aircraft is termed as stereoscopic parallax.
The above figure determine the images of object at points A and B; is considered as the part of the overlapping area within the two successive imagery vertical aerial images taken from the camera at the position in the space. Camera focus on point parallel to the flight line. In the side image they appear as a’ and b’. Point A movement is greater because it occurs at higher position than that of the point B. The stereoscopic parallax at A and B are parallax a(Pa) and parallax b(Pb).
Pa = Xa-Xb’
This is consider as the x direction parallax, similarly, to the x, y is also calculated to the relative position of the object in the imagery. As explained in the second figure Xa and Xa’ are the measuring photo coordinates of object A an the left and right respectively. These coordinates are not from the fiduciary system but are based on the flight direction of point X and X’ for measuring the parallax. It can be along the flight line for every imagery which is captured during the data acquisition of a stereoscopic pair.