Photogrammetry fundamentals – name the special point on a photo In aerial photogrammetry, the foot of the perpendicular dropped from the optical centre (perspective centre) of the camera lens onto the picture plane is known as:

Introduction / Context:
The geometry of a photograph is described using a few standard points: the perspective centre (camera lens centre at exposure), the principal point, the nadir (plumb) point, and the isocentre. Correctly identifying these is foundational for orientation, parallax measurement, and accurate mapping from photos.

Given Data / Assumptions:

• A single exposure of a frame camera is considered.
• The picture plane is the plane of the image (negative/photograph).
• The optical centre (also called the perspective centre) is known.

Concept / Approach:

By definition, the principal point is the orthographic projection of the perspective centre onto the image plane. It is obtained by drawing a line from the perspective centre perpendicular to the image plane; the foot of that perpendicular is the principal point. Other named points are related but distinct: the nadir (plumb) point is the image of the vertical from the perspective centre, and the isocentre lies at the intersection of the bisector between the principal line and the plumb line with the image plane.

Step-by-Step Solution:

Verification / Alternative check:

In interior orientation, fiducial marks establish the principal point on the scanned photo/negative. This operational practice confirms the definitional link between the perpendicular from the lens centre and the principal point location.

Why Other Options Are Wrong:

• Isocentre: defined using the bisector between the principal line and the plumb line, not the perpendicular from the lens centre.
• Perspective centre: a point in space, not on the image plane.
• Plumb line / nadir point: projection of the vertical, not necessarily coincident with the perpendicular from the lens centre on a tilted photo.

Common Pitfalls:

Confusing the principal point with the nadir point on tilted photographs; they coincide only when the camera is perfectly vertical with no tilt.

Final Answer:

Principal point