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). 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. Less commonly, elevation is measured using the center of the Earth as the reference point. Due to
equatorial bulge, there is debate as to which of the summits of
Mt. Everest or
Chimborazo is at the higher elevation, as the Chimborazo summit is further from the Earth's center, while the Mt. Everest summit is higher
above mean sea level.