An aerial photograph is taken over undulating terrain. How does relief displacement alter the apparent position of elevated ground features relative to the photograph's principal point and what governs its magnitude?
- ADisplaces points radially inward; inversely proportional to the aircraft's flying height.
- BDisplaces points radially outward for features above the datum; inversely proportional to the flying height.Correct
- CDisplaces points parallel to the flight line; directly proportional to the aircraft's flying height.
- DDisplaces points radially outward; directly proportional to the flying height.
Solution
Concept
Relief displacement formula and direction in aerial photographs
Formula
d = (r × h) / H
Step-by-step solution
1. Relief displacement d represents the image position shift caused by terrain elevation variations. 2. Features above datum are shifted radially away from the principal point (radially outward). 3. The governing formula d = (r × h) / H demonstrates that displacement is inversely proportional to flying height H. Answer: B
Explanation
On a vertical aerial photograph, relief displacement of points situated above the datum occurs radially outward from the principal point (or nadir). The magnitude of relief displacement is directly proportional to the height of the object (h) and its radial distance from the center (r), but inversely proportional to the aircraft flying height above datum (H). Hence, option B is correct.
Why the other options are wrong
A) Points above datum displace outward, not inward (points below datum displace inward). C) Relief displacement is radial from the principal point, not parallel to flight line. D) Relief displacement is inversely proportional to flying height, not directly.
Shortcut
Relief Displacement d = r·h / H -> Radially OUTWARD above datum, INVERSE to flying height H.
Common mistake
Stating displacement is directly proportional to flying height; higher flying altitude actually compresses and reduces relief displacement.
Correct answer: B) Displaces points radially outward for features above the datum; inversely proportional to the flying height.