altitudes

Altitude is the vertical distance of a point or object above a reference level. In geography, aviation, and related fields, the reference is commonly mean sea level (MSL). Altitude is distinct from elevation (height above a fixed surface on the Earth) and from plain distance, as it specifies height relative to a standard reference.

There are several related concepts. Geodetic or geometric height is the distance above the reference ellipsoid used to model the Earth’s shape. Orthometric height, often called height above geoid, is measured relative to the geoid, which approximates mean sea level. The geopotential height relates to the Earth’s gravity field and is important in physics and meteorology. Barometric altitude, derived from air pressure readings using a standard atmosphere, changes with temperature and pressure. In aviation and surveying, additional terms such as indicated altitude (uncorrected by instrument error), calibrated altitude (instrument-corrected), pressure altitude (based on a standard pressure setting), and density altitude (accounting for air density) are commonly used.

Measurement methods include barometric sensors, radar or radio altimeters, and satellite-based systems like GPS, which provide ellipsoidal height that may be converted to orthometric height with a gravity model. In aviation, maintaining appropriate altitude ensures safe routing and separation of aircraft, often using flight levels above a transition altitude. In meteorology, altitude influences atmospheric properties such as temperature and humidity through the lapse rate. In astronomy, altitude also denotes the angular distance of a celestial object above the observer’s horizon.

See also: mean sea level, ellipsoid, geoid, standard atmosphere, flight level.