Discussion
Which is the point on the ground vertically beneath the center of the camera lens at the time the aerial photograph was taken ?
Answer: D
The point on the ground vertically beneath the lens centre is the Ground Nadir (printed 'Gound Nadir').
In aerial photography the plumb line through the camera's perspective centre meets the photograph at the photo nadir and the ground at the ground nadir. On a truly vertical photograph the nadir, principal point and isocentre coincide, but on a tilted photograph they separate.
Fiducial marks are reference marks on the frame used to locate the principal point, the camera lens centre is the exposure station in the air, and the isocentre lies on the bisector of the tilt angle, not directly beneath the lens.
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