Question

The orthogonal projection of the perspective centre on a tilted photograph is called as

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

Hint:

Key points on a photograph in photogrammetry:

• Principal Point: Perpendicular projection of perspective center onto the photo plane (center of the image). It represents the intersection of the optical axis with the image plane.
• Nadir Point (or Photo Nadir): Intersection of the plumb line from the perspective center with the photo plane. Coincides with the principal point only for truly vertical (non-tilted) photographs.
• Isocentre: A point used in tilt analysis, lying between the principal point and the nadir point on the principal line.

Understanding these points is crucial for geometric correction and mapping from aerial photographs.

Answer 1

The Correct Option is C

Step 1: Define key terms in photogrammetry.

• Perspective Centre (Exposure Station): The point in space where the exposure takes place; it's the effective optical center of the camera lens.
• Photograph (Image Plane): The plane on which the image is recorded (e.g., film or digital sensor).
• Tilted Photograph: A photograph where the optical axis of the camera is not truly vertical, meaning the photo plane is tilted relative to the horizontal plane.
• Orthogonal Projection: A projection in which the projection lines are perpendicular to the projection plane.

Step 2: Analyze the definitions of the given options.

• (1) Nadir: The Nadir point (or photo nadir) is the intersection of the plumb line (a truly vertical line passing through the perspective center) with the plane of the photograph. This point is relevant for geometric correction of tilt.
• (2) Isocentre: The Isocentre is the point on the photograph that lies halfway between the principal point and the nadir point. It is also the point where the axis of tilt intersects the photo plane, and it is the center of perspective for horizontal angles.
• (3) Principal Point: The Principal Point is the point where the perpendicular (orthogonal projection) from the perspective center (the front nodal point of the lens) intersects the plane of the photograph. This point is fixed relative to the camera's internal geometry and is considered the geometric center of the photograph.
• (4) Plumb Point: This term is often used interchangeably with the Nadir point, especially when referring to the point on the photo itself. It represents the point where a plumb line from the perspective center intersects the photo plane.

Step 3: Match the definition to the correct term.
The question asks for the orthogonal projection of the perspective centre on a tilted photograph. By definition, this is the Principal Point. It's the point where the optical axis (perpendicular from the perspective center) intersects the image plane. This point is intrinsic to the camera's calibration. $$\boxed{\text{Principal Point}}$$