Above ground level (AGL)

Above ground level (AGL) is the vertical distance between an aircraft and the ground or other surface directly below it. An AGL altitude changes as terrain elevation changes. It differs from mean sea level, which uses a common sea-level reference.

How it works

An aircraft altimeter normally displays altitude referenced to mean sea level when set to the local altimeter setting. To determine AGL, a pilot compares that indicated altitude with the elevation of the surface below. A radio or radar altimeter can measure height above the surface directly, but those systems have operating limits and are not interchangeable with the barometric altimeter.

A chart may publish obstacle heights in AGL and elevations in MSL. Reading the reference carefully prevents mixing two different vertical measurements. Regulations and procedures can also specify an altitude in MSL, AGL, or both.

Example

An aircraft is at 3,500 ft MSL above terrain that is 1,200 ft MSL. Its approximate height above that terrain is 2,300 ft AGL. If the aircraft crosses a ridge at 2,800 ft MSL, its AGL height becomes about 700 ft, even though the altimeter indication has not changed.

Why it matters

AGL is useful when describing cloud bases, obstacle clearance, pattern altitude, and terrain-relative height. A cloud report such as BKN020 uses hundreds of feet above the station, while many operating rules use height above the surface. Always check the stated reference before applying a number.

AGL is not a substitute for current terrain, obstacle, or chart information. Use approved charts, current notices, and applicable operating rules for flight planning.

Related terms and tools

• Mean sea level
• Pressure altitude
• FAA Aeronautical Information Manual