amsl

AMSL stands for Above Mean Sea Level, a standard reference used to express elevation or altitude. It indicates how high a point lies relative to the average level of the world’s oceans, calculated as mean sea level over a long period. AMSL is widely used in aviation, surveying, cartography, construction, and flood-risk assessments because it provides a consistent baseline for heights across regions.

AMSL is distinct from AGL, or above-ground level, which measures height above the local terrain at a specific location. Elevations on topographic maps and altitudes for aircraft are often given as meters or feet AMSL, sometimes tied to a particular vertical datum for consistency. In aviation, altitudes are frequently presented as AMSL unless a different reference is specified, and flight levels (FL) are based on a standard pressure surface near sea level.

Sea level, and thus AMSL, is not uniform worldwide. The mean sea level used as a reference is affected by tides, ocean dynamics, and regional geoid variations. Modern elevation data are typically linked to a vertical datum that relates heights to a reference ellipsoid or geoid, such as NAVD88 in North America or global models like EGM2008, to ensure consistency across maps and charts.

Because AMSL values depend on the chosen datum, users should note the datum and reference epoch when using AMSL figures, as updates to geoid models or vertical datums can lead to changes in reported elevations.