Stereophotogrammetry parallax measurement: the standard parallax equation is valid across the entire overlap only if the parallax is measured in which direction?

Introduction / Context:
In aerial stereophotogrammetry, horizontal parallax (p) is used to derive elevations and relative positions from overlapping photographs. The direction of parallax measurement relative to the flight base line is crucial for the validity of standard equations.

Given Data / Assumptions:

• Standard vertical photography with known base line (B) along the flight direction.
• Parallax is measured between conjugate image points in the overlap region.

Concept / Approach:
The classical parallax equation relates parallax to camera geometry when parallax is measured along the base line direction. Measuring parallax parallel to the base simplifies relationships and ensures consistency across the entire overlap area, where the geometry aligns with flight direction.

Step-by-Step Solution:
Identify the base line as the projection of flight path between exposure stations.Note that parallax fundamentally manifests along this direction in vertical photography.Therefore, measure parallax parallel to the base line for the standard parallax formulations to hold across the overlap.

Verification / Alternative check:
Photogrammetry texts emphasize measuring x-parallax along the base line; y-parallax (normal) is typically minimized by orientation.

Why Other Options Are Wrong:

• Normal to base line: corresponds to y-parallax and is minimized; equations are not derived for this direction.
• Both/neither/45°: do not reflect the standard geometric derivation.

Common Pitfalls:
Failing to eliminate tilt/roll during relative orientation; mixing x- and y-parallax components.

Final Answer:
Parallel to the base line