Question

The point where vertical line passing through the perspective centre intersects the plane of the photograph, is known as

• A. Photo Plumb Point
• B. Plumb Point
• C. Nadir Point
• D. Isocentre

Hint:

For vertical aerial photographs, the Principal Point, Nadir Point, and Isocentre all coincide. However, for tilted photographs, these three points are distinct and lie on a line called the "principal line." Understanding their individual definitions is key for analyzing geometric distortions in tilted imagery.

Answer 1

The Correct Option is A

Step 1: Understand the terminology related to vertical lines and photographic planes in photogrammetry.
This question focuses on a specific point on the photograph that is defined by the vertical direction from the camera's exposure station.
Step 2: Analyze the definitions of the given options in this context.

• (1) Photo Plumb Point: This term specifically refers to the point on the photograph that is the intersection of the truly vertical line (plumb line) passing through the perspective center (exposure station) with the plane of the photograph. It is the photographic representation of the nadir (the point on the ground directly below the camera).
• (2) Plumb Point: While "plumb point" generally refers to a point defined by a plumb line, in the context of photogrammetry, when it's on the photo, it's more precisely called the "Photo Plumb Point" or "Nadir Point on the photo" to distinguish it from the perspective center or the ground nadir.
• (3) Nadir Point: The Nadir Point, in a broader sense, is the point on the ground vertically beneath the perspective center. However, it is also very commonly used to refer to the point on the photograph that is the projection of the ground nadir. In many photogrammetry texts, "Nadir Point" is used synonymously with "Photo Plumb Point" for the point on the image. Given the options, "Photo Plumb Point" is often considered more precise when explicitly referring to the point on the photograph.
• (4) Isocentre: As discussed in previous questions, the isocentre is related to the tilt of the photograph and is the point where the bisector of the angle of tilt intersects the photo plane. It is not defined by a vertical line from the perspective center.

Step 3: Choose the most precise and commonly accepted term.
The phrasing "vertical line passing through the perspective centre intersects the plane of the photograph" perfectly describes the Photo Plumb Point. While "Nadir Point" is also correct in this context for the point on the photo, "Photo Plumb Point" is specifically coined to refer to its location on the photograph. $$\boxed{\text{Photo Plumb Point}}$$