Nadir Point (N)
(read as nay-der) (from Arabic: nazir = opposite):
That point on the celestial sphere directly beneath the observer and directly opposite the zenith (the point of the celestial sphere that is directly over the observer and 90 degrees from all points on that person’s horizon).
OR; Nadir is the True center of the photograph. In other words, nadir is that point where a perpendicular from the ground through the center of the camera-lens intersects the film plane.
The position of the nadir on the photograph is important because topographic displacement is a function of the distance of the displaced images from the nadir and this displacement is always in the line with the nadir either towards it or away from it. Simply, topographic displacement is always radial from the nadir.
Principal Point (PP)
It is the intersection of lines joining opposite fiducial marks. OR; it is the optical or geometrical center of the photograph. It is found by references of fiducial marks (these marks are used as a standard of reference. These are concentrated with the camera lens which forms images on the camera lens). See picture:
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