Definition
A vertical reference based on the average level of the ocean's surface, used as the zero point for measuring altitude. An altitude expressed in feet MSL is the height above this fixed sea-level reference, regardless of the elevation of the terrain below the aircraft.
Plain English
It is height measured from sea level, not from the ground beneath you. If your altimeter reads 5,000 feet MSL, you are 5,000 feet above the average level of the ocean — even if the ground below you is a mountain at 4,000 feet.
Context Anchor
Seen on instrument charts, airway descriptions, airport information, and procedure altitudes where heights are published using a common reference.
Derivation
‘Mean’ here means ‘average’ — the average level of the sea over time, smoothing out tides and waves. It gives aviation a single, fixed reference point that everyone agrees on, no matter where they are flying.
Why Pilots Care
Provides a uniform altitude reference so that terrain clearance, air traffic separation, and charted altitudes remain consistent regardless of location.
Analogy
A town sign may say the town is 2,000 feet above sea level even while you are standing on the ground there. MSL works the same way: it uses sea level as the measuring point, not your current surface.
Grounding Statement
An aircraft at 5,000 feet MSL is 5,000 feet above the sea-level reference, even if the ground below is much higher than sea level.
Intuition Check
MSL does not mean height above the ground below you. It means height measured from the average sea-level reference.
Example Sentence 1
ATC cleared the flight to climb and maintain 9,000 feet MSL.
Example Sentence 2
The pilot reported level at 4500 feet MSL while on the arrival.