Definition
The mean height of the surface of the sea, calculated by averaging tide levels at all stages over a long period (typically 19 years) at a given location. It serves as the standard zero reference from which altitudes and elevations are measured.
Plain English
The halfway point between high and low tides, averaged over many years, used as the 'zero' line from which heights above the ground or in the air are measured.
Context Anchor
Seen on aeronautical charts, airport elevation information, altimeter discussions, and altitude references.
Derivation
Sea level' is straightforward, but 'average' is the key word. Because the sea is never still — tides rise and fall, storms push water up, calm days let it settle — there is no single 'sea level.' Engineers and surveyors solved this by measuring the sea's height continuously for years and taking the average. That long-term average becomes the fixed reference point.
Why Pilots Care
Almost every altitude a pilot deals with — chart elevations, airspace floors and ceilings, minimum safe altitudes, altimeter settings — is given relative to mean sea level. Without a consistent zero reference, altitudes from different sources could not be compared safely.
Analogy
Think of average sea level as the zero mark on a tall measuring ruler used for aviation heights.
Grounding Statement
If an airport elevation is listed as 500 feet, that means the airport is about 500 feet above average sea level.
Intuition Check
Do not read “average sea level” as the exact ocean height at a beach today. In aviation, it means a fixed reference based on the ocean’s long-term average surface height.
Example Sentence 1
The airport elevation is 1,200 feet above average sea level, so an aircraft showing 3,200 feet on the altimeter is 2,000 feet above the field.
Example Sentence 2
Aircraft performance charts list takeoff distances based on standard conditions at average sea level.