Metres above mean sea level (MAMSL) or simply metres above sea level (MASL or m a.s.l.) is a standard metric measurement in metres of vertical distance (height, elevation or altitude) of a location in reference to a historic mean sea level taken as a vertical datum. Mean sea levels are affected by climate change and other factors and change over time. For this and other reasons, recorded measurements of elevation above sea level might differ from the actual elevation of a given location over sea level at a given moment.
Metres above sea level is the standard measurement of the elevation or altitude of:
- Geographic locations such as towns, mountains and other landmarks.
- The top of buildings and other structures.
- Flying objects such as airplanes or helicopters.
How it is determined
The elevation or altitude in metres above sea level of a location, object, or point can be determined in a number of ways. The most common include:
- Global Positioning System (GPS), which triangulates a location in reference to multiple satellites.
- Altimeters. They typically measure atmospheric pressure, which decreases as altitude increases.
- Aerial photography.
- Surveying, especially levelling.
Accurate measurement of historical mean sea levels is complex. Land mass subsidence (as occurs naturally in some islands) can give the appearance of rising sea levels. Conversely, markings on land masses that are uplifted due to geological processes can suggest a lowering of mean sea level.
Other measurement systems
Feet above sea level is the most common analogue for metres above sea level in the US customary measurement system, abbreviated FAMSL.