Mean Sea Level

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Mean sea level (MSL) is a measure of the average height of the ocean's surface (such as the halfway point between the mean high and low tide). It plays an extremely important role in aviation, where standard sea level pressure is used as the measurement reference of altitude at flight levels.

Height above mean sea level

Not to be confused with AGL.

Height above mean sea level (AMSL) refers to the elevation of an object relative to the mean sea level.