altitude
Altitude, vertical distance of an object above some datum plane, such as mean sea level or a reference point on the earth's surface. It is usually measured by the reduction in atmospheric pressure with height, as shown on a barometer or altimeter . In surveying and astronomy, it is the vertical angle of an observed point, such as a star or planet, above the horizon plane. The altitude of a feature of the earth's surface is usually called its elevation . Recent spacecraft instrumentation has also measured vertical distances on the earth and other planets, determining the height of planetary features by means of radar and optical imaging. In astronomy, altitude is the angular distance of a heavenly body above the astronomical horizon as determined by the angle which a line drawn from the eye of the observer to the heavenly body makes with the plane of the horizon. The reading of the apparent altitude, as determined by a telescope attached to a graduated circle, must be correct...