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Mean sea level ( MSL, often shortened to sea level) is an mean surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of standardised is used, for example, as a chart datum in cartography and Navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to Calibration altitude and, consequently, aircraft . A common and relatively straightforward mean sea-level standard is instead a long-term average of tide gauge readings at a particular reference location. What is "Mean Sea Level"? Liverpool, UK: National Oceanography Centre. Retrieved 29 January 2024.
The term above sea level generally refers to the height above mean sea level (AMSL). The term APSL means above present sea level, comparing sea levels in the past with the level today.
Earth's radius at sea level is 6,378.137 km (3,963.191 mi) at the equator. It is 6,356.752 km (3,949.903 mi) at the poles and 6,371.001 km (3,958.756 mi) on average. This flattened spheroid, combined with local gravity anomalies, defines the geoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant depression in the Indian Ocean, whose surface dips as much as below the global mean sea level (excluding minor effects such as tides and currents).
Still-water level or still-water sea level (SWL) is the level of the sea with motions such as wind waves averaged out.
Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the , also have zero mean.
Global MSL refers to a spatial average over the entire ocean area, typically using large sets of tide gauges and/or satellite measurements.
One often measures the values of MSL with respect to the land; hence a change in relative MSL or (relative sea level) can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates, or both.
In the UK, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Before 1921, the vertical datum was MSL at the Victoria Dock, Liverpool.
Since the times of the Russian Empire, in Russia and its other former parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge.
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of above chart datum "Tide: Notes", Hong Kong Observatory. . and below the average sea level.
In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collated data about the sea level. It is used for a part of continental Europe and the main part of Africa as the official sea level. Spain uses the reference to measure heights below or above sea level at Alicante, while the European Vertical Reference System is calibrated to the Amsterdam Peil elevation, which dates back to the 1690s.
Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008.
When referring to geographic features such as mountains, on a topographic map variations in elevation are shown by . A mountain's highest point or summit is typically illustrated with the AMSL height in metres, feet or both. In unusual cases where a land location is below sea level, such as Death Valley, California, the elevation AMSL is negative.
The melting of at the end of results in isostatic post-glacial rebound, when land rises after the weight of ice is removed. Conversely, older volcanic islands experience relative sea level rise, due to isostatic subsidence from the weight of cooling volcanos. The subsidence of land due to the withdrawal of groundwater is another isostatic cause of relative sea level rise.
On planets that lack a liquid ocean, can calculate a "mean altitude" by averaging the heights of all points on the surface. This altitude, sometimes referred to as a "sea level" or zero-level elevation, serves equivalently as a reference for the height of planetary features.
Some land movements occur because of isostasy adjustment to the melting of at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the geoid and true polar wander. Atmospheric pressure, and local ocean temperature changes can affect LMSL as well.
Eustatic sea level change (global as opposed to local change) is due to change in either the volume of water in the world's oceans or the volume of the . Two major mechanisms are currently causing eustatic sea level rise. First, shrinking land ice, such as mountain glaciers and polar ice sheets, is releasing water into the oceans. Second, as ocean temperatures rise, the warmer water expands.
Height above mean sea level
Height above mean sea level ( AMSL) is the elevation (on the ground) or altitude (in the air) of an object, relative to a reference datum for mean sea level (MSL). It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (contrast with flight level), and in the atmospheric sciences, and in land surveying. An alternative is to base height measurements on a reference ellipsoid approximating the entire Earth, which is what systems such as GPS do. In aviation, the reference ellipsoid known as WGS84 is increasingly used to define heights; however, differences up to exist between this ellipsoid height and local mean sea level. Another alternative is to use a geoid-based vertical datum such as NAVD88 and the global EGM96 (part of WGS84). Details vary in different countries.
Difficulties in use
It is often necessary to compare the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid. In the absence of external forces, the local mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits gravity anomalies such as those measured by NASA's GRACE satellites. In reality, the geoid surface is not directly observed, even as a long-term average, due to ocean currents, air pressure variations, temperature and salinity variations, etc. The location-dependent but time-persistent separation between local mean sea level and the geoid is referred to as (mean) ocean surface topography. It varies globally in a typical range of ±.
Dry land
Several terms are used to describe the changing relationships between sea level and dry land.
(1997). 9780922152346, American Geological Institute. ISBN 9780922152346
(1997). 9780922152346, American Geological Institute. ISBN 9780922152346
(1997). 9780922152346, American Geological Institute. ISBN 9780922152346
Change
Local and eustatic
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time long enough that fluctuations caused by waves and are smoothed out, typically a year or more. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can occur at rates similar to sea level changes (millimetres per year).
Short-term and periodic changes
Many factors can produce short-term changes in sea level, typically within a few metres, in timeframes ranging from minutes to months:
0.1–10+ m <0.1 m 5 mm −0.7 to 1.3 m Up to 5 m <0.1m Up to 1 m Up to 0.6 m 1 m 0.2 m Up to 2 m 0.1–10+ m Up to 10 m
Recent changes
Aviation
Pilots can estimate height above sea level with an altimeter set to a defined barometric pressure. Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSL in the region being flown over. This pressure is referred to as either QNH or "altimeter" and is transmitted to the pilot by radio from air traffic control (ATC) or an automatic terminal information service (ATIS). Since the terrain elevation is also referenced to MSL, the pilot can estimate height above ground by subtracting the terrain altitude from the altimeter reading. are divided into boxes and the maximum terrain altitude from MSL in each box is clearly indicated. Once above the transition altitude, the altimeter is set to the international standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.US Federal Aviation Administration, Code of Federal Regulations Sec. 91.121
See also
External links
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