In aviation, both AGL and MSL are elevation measurements and are very important for aircraft. The abbreviation AGL stands for above ground level. MSL refers to mean sea level. Pilots use these two measurements at different times during the course of a flight; the measurements are vital for pilots to fly a steady course and land safely.
Above Ground Level
An AGL measurement determines the height above the ground. This measurement changes as the topography of the earth changes, when a plane flies above it. For example, if a plane flies a steady course at 10,000 feet above ground level initially, then a 10,000-foot-high mountain would make the AGL 0 when the two objects come together. In this scenario, the MSL would not change.
Median Sea Level
An MSL measurement refers to the altitude or height above the average height of the oceans and seas. An MSL is a reference point for elevations. The MSL calculation is derived from observations of tides and seasonal variations over a 19 year period to arrive at the average MSL. A plane that flies at 10,000 feet MSL and stays level will register as flying at 10,000 feet MSL -- no matter the terrain changes below the pilot.
It is very important for pilots to know which elevation measurement will be guiding the plane. Skydivers must know the AGL measurement of the area they are jumping from. If the skydiver uses the MSL, then the ground may be much closer than they anticipate.
Pilots use altimeters, which measure the AGL, when the aircraft are flying at relatively low heights and when coming in for a landing at an airport. But as the plane increases in altitude, the altimeter reading become less and less accurate. Once the plane reaches transition altitude, the aircraft begins to use the MSL along with air pressure readings to keep level flight.