Photogrammetry — name of the orthogonal projection of the perspective centre on a tilted photograph In a tilted aerial photograph, what is the point called that is obtained by dropping a perpendicular from the camera perspective centre to the photograph plane (i.e., the orthogonal projection of the perspective centre)?

Introduction / Context:
Several special points are defined on aerial photographs to aid in interior orientation, exterior orientation, and mapping geometry. On tilted photographs, distinguishing the principal point, nadir (plumb) point, and isocentre is fundamental for geometric constructions and relief displacement analysis.

Given Data / Assumptions:

• Photograph is tilted; the camera perspective centre is not directly above the principal point in plan.
• Standard photogrammetric definitions apply.

Concept / Approach:
The principal point is defined as the foot of the perpendicular from the perspective centre onto the photograph plane, regardless of tilt. The nadir (plumb) point is the intersection with the photo plane of the vertical line through the perspective centre (direction of gravity). The isocentre lies midway between the principal point and the nadir point on a tilted photo. Therefore, the orthogonal projection of the perspective centre on the photograph plane is the principal point.

Step-by-Step Solution:

Verification / Alternative check:
Standard interior orientation procedures locate the principal point via fiducial marks; constructions to locate isocentre use the midpoint of principal and nadir, reinforcing the distinct definitions.

Why Other Options Are Wrong:

• Nadir/plumb: vertical (gravity) projection, not orthogonal to the photo plane.
• Isocentre: midpoint of principal and nadir; not itself a projection from the centre.
• None of these: incorrect because the correct term is principal point.

Common Pitfalls:
Interchanging principal point with nadir on vertical photos (they coincide only when tilt is zero), leading to confusion on tilted imagery.

Final Answer:
Principal point