GIS Dictionary

nadir

[aerial photography] In aerial photography and satellite imagery, the point on the ground vertically beneath the perspective center of the camera lens or scanner's detectors, or the point of convergence (vanishing point) for all vertical rays within a scene.
[astronomy] In astronomy, the point on the celestial sphere directly beneath an observer. Both the nadir and zenith lie on the observer's meridian; the nadir lies 180 degrees from the zenith and is therefore unobservable.