Photogrammetric geometry: The point where the vertical line (plumb line) through the perspective centre intersects the photograph plane is called

Introduction / Context:
Several special points are defined on an aerial photograph: the principal point, nadir (plumb) point, and isocentre. Correctly identifying these is foundational for tilt correction, orientation, and relief displacement analysis.

Given Data / Assumptions:

• Vertical or slightly tilted photograph.
• Perspective centre (exposure station) projects onto the photo plane.
• Terminology equivalences are recognized in practice.

Concept / Approach:
The nadir point on the photograph is the intersection of the plumb line through the camera perspective centre with the photo plane. Many texts synonymously call it the “photo nadir” or “plumb point.” The principal point is the orthogonal foot of the perspective centre onto the photo plane along the camera axis, and the isocentre lies on the bisector of the angle between the camera axis and vertical (midway between principal point and nadir for small tilts).

Step-by-Step Solution:
Identify the defining line: true vertical through the perspective centre.Locate its intersection with the photo plane: this is the nadir (plumb) point.Therefore, choose “Nadir point.”

Verification / Alternative check:
Under zero tilt, principal point and nadir coincide; with tilt, they separate, and the isocentre falls on the line joining them.

Why Other Options Are Wrong:
Isocentre: defined by angle bisector, not vertical.Principal point: perpendicular projection along camera axis, not along the plumb line unless tilt is zero.“Photo plumb point” is a synonym often used for the nadir; we adopt the standard term “Nadir point.”

Common Pitfalls:
Confusing principal and nadir points on tilted photos; assuming they always coincide.

Final Answer:
Nadir point.