Assumed illumination is from the west shading is computed as a function of the east-west slope of the surface with a nonlinear exaggeration favoring low-relief areas. This image was generated by NOAA from digital data bases of land and sea-floor elevations on a 2-minute latitude/longitude grid (1 minute of latitude = 1 nautical mile, or 1.853 km). Elevation, or geometric height, is mainly used when referring to points on the Earth's surface, while altitude or geopotential height is used for points above the surface, such as an aircraft in flight or a spacecraft in orbit, and depth is used for points below the surface. The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic system, vertical datum).
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