» For other uses see Altitude (disambiguation) Altitude is the elevation of a point or object from a known level or datum (plural: data). Common data are mean sea level, local ground level (Above Ground Level, or AGL), or the surface of the WGS-84 geoid, used by GPS. In aviation, altitude is measured in feet. For non-aviation uses, altitude may be measured in other units such as metres or miles. Atmospheric pressure decreases as altitude increases. This principle is the basis of operation of th… (More on Altitude)