Principal point (PP) Nadir point) Isocenter (IS) Fiducial marks Photo centre Flight line

Principal Point (PP): The point where the optical axis of the camera meets the photograph. In a perfectly aligned camera, it is near the geometric centre of the photo.
Nadir Point (N): The point directly vertically below the camera lens at the moment the photograph is taken.
Isocenter (I): The point located midway between the principal point and nadir point on a tilted aerial photograph.
Photo Centre: Generally refers to the centre of the photograph; it is often treated as the principal point in basic explanations.
Fiducial Marks: Small reference marks fixed along the edges/corners of an aerial photograph. Lines joining opposite fiducial marks are used to determine the principal point.
Flight Line: The path followed by the aircraft while taking a sequence of aerial photographs.

Easy relationship:
Camera → Optical axis → Principal Point
Camera → Vertical/plumb line → Nadir Point
PP ↔ Isocenter ↔ Nadir (Isocenter lies halfway between PP and N on a tilted photograph.)

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