Above Ground Level
In aviation and atmospheric sciences, an altitude is said to be above ground level (AGL) when it is measured with respect to the underlying ground surface.
A pilot flying a plane under instrument flight rules (typically under poor visibility conditions) must rely on its altimeter to decide when to deploy the aircraft carriage and prepare for landing. Therefore the pilot needs reliable information on the altitude of the plane with respect to the air strip. The altimeter, which is normally a barometer calibrated in units of distance instead of atmospheric pressure, must therefore be set in such a way as to indicate the altitude of the craft above ground. This is done by communicating with the control tower of the airport (to get the current surface pressure) and setting the altimeter so as to report a null altitude AGL on the ground of that airport. Confusing AGL with AMSL, or improperly calibration of the altimeter may result in controlled flight into terrain, a crash under pilot control.\