AGL, or height above ground level, is a general aviation term that refers to a height measured in relation to the main ground surface. This term is often interchangeable with absolute altitude, as they measure the same height. Simply, AGL is found by measuring the distance between the bottom of the aircraft and the surface of the ground beneath it.
IFR and AGL
An aircraft operating under instrument flight rules (IFR) must rely on the AGL found on the aircrafts altimeter when deciding when to deploy the wheels in preparation of landing. Typically, the altimeter readings are most important during poor visibility conditions.
After speaking with air traffic controllers at the landing airport tower, the pilot will request the current surface pressure. Using this measurement, the pilot will set the altimeter to read zero on the ground at that airport. In doing this, the pilot reduces any kind of confusion that can lead to a controlled flight into the local terrain.
Height, Altitude, and Elevation
Both height and altitude are frequently used general aviation terms. Altitude refers to the distance about sea level. Height refers to the distance above a particular point on the ground. Elevation is used to describe the distance from sea level to the top of the terrain in question.