In aviation, AGL and MSL represent acronyms used for elevation measurements by pilots and air traffic controllers. AGL stands for above ground level, while MSL refers to mean sea level. Pilots use these measurements at different times during the course of a flight. They are both vital for pilots to fly a steady course and land safely.
TL;DR (Too Long; Didn't Read)
AGL and MSL are both acronyms used by pilots and air traffic controllers, but they stand for different things. ACL is above ground level and MSL is mean sea level.
Above Ground Level
An AGL measurement determines the height above 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 registers as flying at 10,000 feet MSL -- no matter the terrain changes below the pilot.
Importance of Measurements
It is very important for pilots to know which elevation measurement guide the plane. Skydivers must know the AGL measurement of the area they jump from. If a skydiver uses the MSL, the ground may be much closer than they anticipate.
Application of AGL & MSL
Pilots use altimeters, which measure the AGL, when the aircraft are flying at relatively low heights landing at an airport. But as the plane increases in altitude, the altimeter reading become less accurate. Once the plane reaches transition altitude, the aircraft uses the MSL along with air pressure readings to maintain level flight.