Height above mean sea level

Height above mean sea level, often shortened to AMSL, is a way to measure how high a place is above the average level of the world's oceans. This helps us know the height or elevation of mountains, hills, and cities. We find this height by comparing a location to a special point called a vertical datum. This point is based on the average sea level from the past.

Different countries might use different points to measure from, so the zero level can change. Things like climate change and natural forces can make sea levels and heights shift over time. This idea is important in geodesy, where scientists study the shapes and measurements of the Earth. They call this specific height an orthometric height.

Uses

Elevation or altitude above sea level is a common way to measure how high places are. It is used for many things, like:

• Geographic locations such as towns, mountains and other landmarks.
• The top of buildings and other structures.
• Mining work, especially underground.
• Flying objects such as airplanes or helicopters below a certain height set by local rules.

Units and abbreviations

Elevation or altitude is usually described as "metres above mean sea level" in the metric system, or "feet above mean sea level" in United States customary and imperial units. Some common ways to write this in English include:

• 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 mean sea level
• MSL – mean sea level

Sometimes, people just use the abbreviation MSL, like with Mount Everest (8849 m MSL), or they might not even mention sea level and just say Mount Everest (8849 m).

Methods of measurement

Altimetry is a way to find out how high a place is above sea level. Here are some common ways to do this:

• Surveying, especially a type called levelling.
• Global Navigation Satellite System (such as GPS). A device uses signals from satellites to find a location. A special model called a geoid helps change this into a height above sea level.
• Pressure altimeter measuring atmospheric pressure. Pressure changes with height. But weather can also change pressure, so a recent local pressure reading at a known height helps make the altimeter more accurate.
• Stereoscopy in aerial photography.
• Aerial lidar and satellite laser altimetry.
• Aerial or satellite radar altimetry.

Measuring past sea levels can be hard. Land can sink naturally in some places, which might make it look like sea levels are rising. Land can also move upward naturally, which might make it look like sea levels are lower.