Definition
The average height of the surface of the sea, calculated from continuous tide measurements over a long period, used as the standard reference datum for measuring altitude in aviation.
Plain English
An agreed-on 'zero point' set at the average level of the ocean's surface. Aviation altitudes are measured upward from this fixed reference, so every pilot and chart is using the same starting line.
Context Anchor
Seen on aeronautical charts, airport information, altimeter settings, and instrument procedures when heights are given relative to sea level.
Derivation
Mean' here means 'average,' not 'middle' or 'unkind.' The sea's surface rises and falls with tides and waves, so a single 'sea level' would change by the minute. Averaging it over years gives a stable reference everyone can agree on.
Why Pilots Care
Provides a consistent altitude reference for aircraft separation, obstacle clearance, and performance calculations regardless of local terrain or tidal changes.
Grounding Statement
An airport listed at 1,200 feet mean sea level is 1,200 feet above the average sea-level reference, even if it is far inland.
Intuition Check
Mean sea level does not mean the ocean height at a specific beach right now. It means an average sea-level reference used as a common zero point for height.
Example Sentence 1
The airport elevation is 1,250 feet MSL, so an altimeter set correctly on the ground should read close to that value.
Example Sentence 2
Takeoff performance data is published for standard conditions at sea level and must be corrected when operating above mean sea level.