Aerial photography flight planning: What is the normal (forward) longitudinal overlap between consecutive vertical photographs in a flight line?

Introduction / Context:
Forward (longitudinal) overlap is the percentage of common ground area captured in consecutive photographs along a flight line. It is essential for stereoscopic coverage and accurate photogrammetric measurements.

Given Data / Assumptions:

• Standard vertical aerial photography for topographic mapping.
• Normal side overlap is typically about 20–30% (not asked here).
• Good weather and stable platform assumed.

Concept / Approach:
To achieve reliable stereoscopic vision and ensure that every ground point is photographed at least twice, photogrammetric specifications commonly adopt about 60% forward overlap. This allows robust stereo-model formation even with minor drift or tilt. Higher overlaps (e.g., 70–80%) may be used for special applications (dense matching, small-scale sites, or UAVs), but 60% is the textbook “normal.”

Step-by-Step Solution:
Identify the standard mapping specification for forward overlap.Select 60% as the normal value.

Verification / Alternative check:
Flight planning guides and specifications for national mapping consistently state forward overlap ≈ 60% with side overlap ≈ 25%.

Why Other Options Are Wrong:
50%: marginal for stereo security; 70–75%: higher-than-normal, used only for special cases; 40%: inadequate for stereo block formation.

Common Pitfalls:
Confusing longitudinal overlap with lateral (side) overlap; assuming higher overlap is always better despite increased flight time and data redundancy.

Final Answer:
60%.