Photogrammetric ground points – definition of the point below the optical center What is the name of the point on the ground where a vertical line through the camera lens’s optical (perspective) center intersects the terrain surface?

Introduction / Context:
Precise terminology distinguishes several special points used in photogrammetry: principal point, nadir (plumb point), isocentre, and the perspective center. Confusing these leads to errors in interpreting displacement and orientation.

Given Data / Assumptions:

• The camera optical axis may be vertical or slightly tilted.
• The perspective center is the camera's effective point of projection.
• We consider the vertical (gravity) line through the perspective center.

Concept / Approach:
The ground plumb point (also called the nadir point on ground) is the intersection of the gravity-vertical through the perspective center with the Earth's surface. The ground principal point would be the intersection of the principal ray with the ground; these coincide only for a perfectly vertical photo over level terrain. The isocentre lies midway (angularly) between principal and nadir directions in the photo geometry, not on the ground.

Step-by-Step Solution:
Identify the line: gravity vertical through perspective center.Find its intersection with ground → ground plumb point.Exclude other named points that refer to image-plane constructs or different rays.

Verification / Alternative check:
On a truly vertical photograph over level ground, the image plumb point and principal point are nearly coincident; on tilted photos they separate, validating the distinct definitions.

Why Other Options Are Wrong:

• Ground principal point refers to the principal ray, not the gravity vertical.
• Iso-centre is an image concept, not a ground intersection of the vertical.
• Perspective center is the camera point, not a ground point.

Common Pitfalls:
Using the terms “nadir”, “plumb point”, and “principal point” interchangeably; only in the special case of a vertical photo over level ground do they align.

Final Answer:
Ground plumb point