Stereoscopic measuring tools — what does a parallax bar directly read? In aerial photogrammetry with a mirror stereoscope, the parallax bar is used to determine which quantity directly from a stereo pair?

Introduction / Context:
Heighting in photogrammetry relies on differences in stereoscopic parallax measured on overlapping photographs. The parallax bar, used with a stereoscope, aids precise measurement of the relative displacement of conjugate image points.

Given Data / Assumptions:

• Two overlapping vertical photos with adequate base-to-height ratio.
• Mirror or lens stereoscope for fused viewing.
• Calibrated parallax bar with reading scale along the x-direction.

Concept / Approach:

Absolute parallax p for a point depends on its radial position and camera geometry. The quantity that links directly to elevation difference is the parallax difference dp between two nearby points. The parallax bar allows the operator to bring two stereoscopic marks to coincidence sequentially and read the difference in their parallax coordinates, which can then be converted to height difference using calibration or known relationships.

Step-by-Step Solution:

Verification / Alternative check:

Formulae such as dp ≈ b * Δh / H (using mean photo base b and flying height H) show that dp is proportional to height difference, confirming the measurement objective of the parallax bar.

Why Other Options Are Wrong:

• (a) Absolute parallax requires a reference; the bar measures differences more reliably.
• (b) and (d) speak to derived quantities; the bar does not provide height directly without calibration.
• (e) Orientation is unrelated to the bar’s linear reading.

Common Pitfalls:

• Using widely separated points increases model distortion effects; choose nearby points where possible.

Final Answer:

Parallax difference between two image points.