Definition
The average height of the surface of the sea, calculated from many measurements over a long period, used as a fixed reference point for measuring altitude in aviation. Altitudes given in MSL are measured upward from this reference, regardless of the height of the ground below the aircraft.
Plain English
A standard zero point set at the average level of the ocean. When an altitude is given in MSL, it tells you how high you are above that ocean-level reference, not how high you are above the ground beneath you.
Context Anchor
You will see MSL on charts, in airspace descriptions, in airport elevation information, and when reading or flying assigned altitudes.
Derivation
Mean' here means 'average' (from the Latin medianus, middle). The sea rises and falls with tides, so a single moment's sea level is not stable enough to use as a reference. Averaging it over time gives a fixed, reliable zero point that everyone can agree on.
Why Pilots Care
Provides a uniform altitude reference for terrain clearance, obstacle avoidance, and ATC instructions regardless of local geography or tide.
Analogy
MSL is like using the same zero mark on a ruler. Once everyone agrees where zero is, heights can be compared clearly even in different places.
Grounding Statement
If an airport is listed as 800 feet MSL, it means the airport sits 800 feet above the standard sea-level reference.
Intuition Check
MSL does not mean height above the ground. It means height above the standard sea-level reference.
Example Sentence 1
Example Sentence 2
Sectional charts list the highest terrain in each quadrant as elevation above mean sea level.