nadir—¹ That point on the celestial sphere vertically below the observer, or 180° from the zenith. ² In photogrammetry, the point on the ground or at sea level datum vertically beneath the perspective center of the camera lens, or the photographic image of that point. On the photograph it is the center of radiation for displacement of images because of ground relief. Also called “plumb point.”

nadir angle—A type of angle measurement in a vertical plane which uses the downward vertical as zero.

nadir point—See nadir.

nadir radial—See radial [PHOTOGRAMMETRY].

–

Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

Part of LearnCST’s exam text bundle.