Using principal points for radial directions on tilted photos: if average ground relief is about 10% of the flying height, up to what photo tilt can principal points still be used as centers of radial directions for small-scale mapping?

Introduction / Context:
Photo-grammetric plotting often assumes radial directions emanate from the principal point. With tilted photographs, this assumption introduces error, but may be tolerated within small angles depending on relief and scale.

Given Data / Assumptions:

• Ground relief ≈ 10% of flying height.
• Small-scale mapping where minor planimetric error is acceptable.

Concept / Approach:
Empirical guidelines allow using the principal point as the radial center on slightly tilted photos when relief is small relative to flying height. For relief ≈ 0.1 H, tilts up to about 3° maintain acceptable accuracy.

Step-by-Step Solution:
Assess relief ratio r = relief / H ≈ 0.1.Adopt permissible tilt where radial error remains within plotting tolerance.Choose ≈ 3° as acceptable upper bound for small-scale mapping.

Verification / Alternative check:
Photogrammetry references cite 2–3° as a practical limit for using principal points as radial centers when relief is modest; here the higher bound 3° is applicable.

Why Other Options Are Wrong:

• 1° and 2° are conservative but not the maximum permissible.
• 4° often exceeds acceptable planimetric accuracy under given relief.

Common Pitfalls:

• Ignoring relief variations that can tighten the allowable tilt below the rule-of-thumb.

Final Answer:
3°