Question

Which of the following statement is NOT true regarding relief displacement in vertical photographs in the context of aerial photogrammetry?

• A. Relief displacement is the shift in the photographic position of an object caused by the elevation of the object (above or below the datum)
• B. Relief displacement is always in non-radial direction from the principal point
• C. Relief displacement can cause straight roads (not passing through the ground principal point) to appear crooked in undulating terrain
• D. The magnitude of relief displacement is affected by the flying height of the camera (assuming everything else to be same)

Hint:

In a vertical photo: relief displacement is {radial} and obeys $d=(h/H)r$. Larger height $h$ or larger distance $r$ increases $d$; larger flying height $H$ decreases $d$.

Answer 1

The Correct Option is B

Relief displacement in a (truly) vertical photograph is \[ d \;=\; \frac{h}{H}\, r, \] where $h$ is the height of the object above (or below, with sign) the datum, $H$ is the flying height above the same datum, and $r$ is the radial distance of the image point from the principal point.
Hence:
- The displacement is radial from the principal point (toward it if $h<0$, away from it if $h>0$). Therefore statement (B) (“always in non-radial direction”) is NOT true.
- (A) is the standard definition — true.
- (C) is true because along an undulating road $h$ varies, so $d$ varies, making a straight road appear crooked if it does not pass through the principal point.
- (D) is true since $d \propto 1/H$.