In stereoscopic viewing of overlapping aerial photographs, the absolute stereoscopic parallax of a point is defined as
- AThe difference in the x-coordinates (along flight direction) of the same point on two successive photographsCorrect
- BThe difference in the y-coordinates (perpendicular to flight direction) of the same point on two successive photographs
- CThe angular difference between the two rays from the camera stations to the ground point
- DThe shift of a point due to relief displacement on a single photograph
Solution
Concept
Definition and formula of absolute stereoscopic parallax
Formula
p = x - x'
Step-by-step solution
1. On a pair of overlapping aerial photographs aligned to the flight direction, the x-axis corresponds to the line of flight. 2. The absolute parallax p is the total displacement between conjugate image points along the x-axis: p = x - x'. 3. Differences in y-coordinates represent y-parallax (crab or tilt errors), not absolute stereoscopic parallax. Answer: A
Explanation
Absolute stereoscopic parallax of a ground point is defined as the algebraic difference of the coordinates of the point measured parallel to the flight line (x-axis) on two successive overlapping aerial photographs. Because flight movement occurs along the x-axis, parallax is strictly measured in this direction. Hence, option A is correct.
Why the other options are wrong
B) y-coordinate differences represent y-parallax caused by tilt or flight misalignment. C) Angular convergence produces stereoscopic viewing but does not define parallax. D) Relief displacement on a single photograph is radial from the principal point, not stereo parallax.
Shortcut
Absolute Parallax = x-difference (along flight line); y-difference = error/tilt.
Common mistake
Confusing x-parallax (stereoscopic depth perception) with y-parallax (unwanted vertical parallax from tilt or crab).
Correct answer: A) The difference in the x-coordinates (along flight direction) of the same point on two successive photographs