Height above mean sea level
Height above mean sea level is a measure of a location's vertical distance in reference to a vertical datum based on a historic mean sea level. In geodesy, it is formalized as orthometric height. The zero level varies in different countries due to different reference points and historic measurement periods. Climate change and other forces can cause sea levels and elevations to vary over time.
Uses
Elevation or altitude above sea level is a standard measurement for:- Geographic locations such as towns, mountains and other landmarks.
- The top of buildings and other structures.
- Mining infrastructure, particularly underground.
- Flying objects such as airplanes or helicopters below a transition altitude defined by local regulations.
Units and abbreviations
- AMSL – above mean sea level
- AOD or AODN – above ordnance datum
- ASL – above sea level
- FAMSL – feet above mean sea level
- FASL – feet above sea level
- MAMSL – metres above mean sea level
- MASL – metres above sea level
- MSL – mean sea level
Methods of measurement
is the measurement of altitude or elevation above sea level. Common techniques are:- Surveying, especially levelling.
- Global Navigation Satellite System, where a receiver determines a location from pseudoranges to multiple satellites. A geoid is needed to convert the 3D position to sea-level elevation.
- Pressure altimeter measuring atmospheric pressure, which decreases as altitude increases. Since atmospheric pressure varies with the weather, too, a recent local measure of the pressure at a known altitude is needed to calibrate the altimeter.
- Stereoscopy in aerial photography.
- Aerial lidar and satellite laser altimetry.
- Aerial or satellite radar altimetry.