altitude datum


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altitude datum

[′al·tə‚tüd ‚dad·əm]
(engineering)
The arbitrary level from which heights are reckoned.

altitude datum

altitude datumclick for a larger image
The arbitrary level from which vertical displacement is measured. The datum for height measurement is the terrain directly below the aircraft or some specified datum; for pressure altitude, the level at which the atmospheric pressure is 1013.2 hectopascals/millibars (29.92 in or 760 mm of mercury); and for true altitude, mean sea level.