GIS Dictionary
Definitions for GIS terms related to operations such as analysis, GIS modeling and web-based GIS, cartography, and Esri software.
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nadir
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[aerial photography]
In aerial photography, the point on the ground vertically beneath the perspective center of the camera lens. 
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[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. 
