In stereoscopic photogrammetry, absolute stereoscopic parallax is used to compute the elevations of terrain features. If the flying altitude of the aircraft is increased while maintaining the same air base and camera’s focal length, what is the effect on the measured parallax difference for a building of a given height?
- AThe parallax difference increases exponentially allowing for better height estimation.
- BThe parallax difference remains constant as it is an intrinsic property of the object's height.
- CThe parallax difference decreases leading to lower vertical measurement precision.Correct
- DThe parallax becomes negative requiring an optical inversion of the stereo model.
Solution
Concept
Parallax difference variation with flying altitude
Formula
Δp approx (B × f × h) / H2
Step-by-step solution
1. Parallax difference measures the stereo shift between the top and bottom of an object: Delta p = (B × f × h) / (H × (H - h)). 2. If air base B and focal length f remain constant while altitude H increases, the denominator increases significantly. 3. Consequently, the measured parallax difference decreases, diminishing vertical measurement precision. Answer: C
Explanation
In stereoscopic photogrammetry, the parallax difference (Delta p) for a given object height (h) is related to flying height (H), focal length (f), and air base (B) by Delta p = (B × f × h) / (H2). As flying height increases, the parallax difference decreases quadratically, resulting in smaller image shifts and reduced precision when estimating object heights. Thus, option C is correct.
Why the other options are wrong
A) Parallax difference decreases with height rather than increasing. B) Parallax difference is strongly dependent on flying height H. D) Parallax remains positive for features above ground.
Shortcut
Higher Altitude H -> Smaller Parallax Difference Delta p -> Lower Vertical Accuracy.
Common mistake
Believing parallax difference is independent of altitude; parallax difference drops rapidly with increased flying height.
Correct answer: C) The parallax difference decreases leading to lower vertical measurement precision.