Mean sea level (MSL)

Mean sea level (MSL) is a common vertical reference used to describe terrain elevations, airport elevations, obstacle elevations, and aircraft altitude. A value expressed in MSL uses the modeled average sea-level surface rather than the ground directly below the aircraft.

How it works

A barometric altimeter estimates altitude from atmospheric pressure. With the local altimeter setting entered, the displayed altitude is intended to approximate height above the reference datum used for the setting. The result is normally treated as an MSL altitude for aviation planning, but pressure and temperature errors can affect the estimate.

MSL and above ground level answer different questions. MSL describes the aircraft's position in a common vertical reference. AGL describes the distance to the surface below. A pilot needs both when considering terrain, obstacles, airspace, and cloud clearance.

Example

An airport has an elevation of 4,800 ft MSL, and an aircraft flies overhead at 6,300 ft MSL. Its approximate height above the airport is 1,500 ft, but its height above nearby terrain may be different. A mountain or valley changes the AGL value while the MSL value remains tied to the common reference.

Why it matters

Charts use MSL for most altitude and elevation information so pilots can compare locations. Airspace floors and ceilings may also be published in MSL, while some restrictions are stated in AGL. A note such as 4,000 ft AGL must not be read as 4,000 ft MSL.

Use the chart legend and the wording in the applicable regulation or procedure. Verify altimeter settings and current chart data before relying on any altitude reference.

Related terms and tools

• Above ground level
• Pressure altitude
• FAA Pilot's Handbook of Aeronautical Knowledge