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The vertical distance of a level, point or object considered as a point, measured from a specified datum.
It is also used to describe the vertical dimension of an object, such as a tower or obstacle.
The distance in feet above mean sea level (AMSL)
Indicated by a pressure altimeter whose sub-scale is set to the current QNH for the location of the aircraft.
(c) mean sea level (MSL);
The average height of the sea surface, normally taken from long-term records of tide heights at any given place.
Above Mean Sea Level
Using Mean Sea Level (MSL) as the datum, any height above mean sea level is referred to as Altitude, measured in feet.
(e) ground level;
Ground Level is any ground or water surface.
Above Ground Level
The height of an aircraft above ground level.
May be determined by radio altimeter (RadAlt), or by substracting the elevation of the ground from the flight altitude.