Altitude

Altitude is the distance between a position and a vertical reference surface. Distance might be measured in angular or length units. The position might be the point location of an object or a specified point along a vehicle track or satellite orbit. The surface might be one of many vertical datums, such as the center of the Earth, the surface of the ocean, the topographic surface of the Earth, the top of the built environment, or a constant barometric pressure surface.

The precise distance between a surface and a position depends on the definition of the line between them. For example, the line might be perpendicular to a plane tangent to the reference surface, or it might extend from the position toward the center of the mass of the Earth.

The terms altitude, elevation, and height are sometimes used interchangeably. In different contexts, these words take on different meanings, and the modifiers attached to them can sometimes clarify their usage. Elevation is often associated with the distance from a defined surface, such as the geoid, the theoretical equipotential gravity surface of the Earth, or a physically defined gravity surface model, such as a specific mean-sea-level datum, or with respect to the actual local-level plane as measured at the position. Height is sometimes reserved for the distance between a reference ellipsoid and a position or for the distance from the bottom to the top of an entity, such as a building or a mountain peak. The height of an aircraft might be the distance above the topographic surface of the Earth, while the height of a geodetic survey monument might be its vertical distance from a reference ellipsoid.

This sentence appears in a text on surveying principles: “Therefore, the altitude at which the plane must fly is calculated by adding the elevation of the mean datum to the flying height.” The altitude of the aircraft is above mean sea level, the flying height is the distance between the aircraft and the ground, and the ground (the mean datum) has an elevation with respect to mean sea level.

Altitude is modified by words that further specify the meaning. Absolute altitude refers to the distance above the physical surface of the sea or land. Angular altitude is the vertical angle between some plane (such as local level) and a line from the observation point to an object such as a mark on a surveyor's rod or a star. Barometric altitude is the distance from one constant pressure surface (an isobaric surface) to another. Meridian altitude is the vertical angle to an object measured along a line of longitude.