Altitude

Altitude, in astronomy, the angle in degrees of a heavenly body above the horizon. An altitude measurement is not a linear distance, but rather the angle between the plane of the horizon and a line drawn from the eye to the heavenly body, or the arc of a vertical circle intercepted between the heavenly body and the horizon.

In physical geography, altitude refers to elevation above mean sea level. This measurement is determined most accurately by triangulation with optical instruments. The altitude of an airplane, however, is measured most accurately by means of a radio altimeter.