Question

Select the correct statement in the context of relief displacement in vertical aerial photographs.

• A. It is zero for principal point, irrespective of whether the point is above or below the datum
• B. It increases with increased flying height above datum
• C. It occurs radially from one of the image corners
• D. It has no effect on the appearance (in the aerial photograph) of the straight roads over an undulating terrain

Hint:

Relief displacement is always radial from the principal point, and is zero at that point regardless of elevation. It depends on object height and location—not photo corners or road type.

Answer 1

The Correct Option is A

Relief displacement in vertical aerial photographs is the apparent shift in the position of an object due to the object's elevation with respect to the datum. The formula for relief displacement is:
\[ d = \frac{r \cdot h}{H} \] where:
- \(d\) is the relief displacement,
- \(r\) is the radial distance from the principal point to the object on the photo,
- \(h\) is the height of the object above the datum,
- \(H\) is the flying height above the datum.

Relief displacement is zero at the principal point (\( r = 0 \)), and it increases with object height (\( h \)) and radial distance (\( r \)) from the principal point. However, it decreases with increasing flying height (\( H \)).

Therefore, option (A) is correct:
\[ \boxed{\text{It is zero for principal point, irrespective of whether the point is above or below the datum}} \]