Sea level is generally used to refer to mean sea level (MSL), an average level for the surface of one or more of Earth's oceans from which heights such as elevations may be measured. MSL is a type of vertical datuma standardised geodetic reference pointthat is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured in order to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location.〔(''What is "Mean Sea Level"?'' ) (Proudman Oceanographic Laboratory).〕
Sea levels can be affected by many factors and are known to have varied greatly over geological time scales. The careful measurement of variations in MSL can offer insights into ongoing climate change, and the current rise in sea levels has been widely quoted as proof of ongoing global warming.〔Solomon ''et al.'', (Technical Summary, Section 3.4 Consistency Among Observations ) in ; Hegerl ''et al.'', (Executive summary, Section 1.3: Consistency of changes in physical and biological systems with warming ) in .〕
The term above sea level generally refers actually to above mean sea level (AMSL). It is commonly used in feet and metres above mean sea level (MAMSL).
== Measurement ==
Precise determination of a "mean sea level" is a difficult problem because of the many factors that affect sea level.〔US National Research Council, ''Bulletin of the National Research Council 1932'' page 270〕 Sea level varies quite a lot on several scales of time and distance. This is because the sea is in constant motion, affected by the tides, wind, atmospheric pressure, local gravitational differences, temperature, salinity and so forth. The best one can do is to pick a spot and calculate the mean sea level at that point and use it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point.
To an operator of a tide gauge, MSL means the "still water level"—the level of the sea with motions such as wind waves averaged out—averaged over a period of time such that changes in sea level, e.g., due to the tides, also get averaged out. One measures the values of MSL in respect to the land. Hence a change in MSL 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.
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. Prior to 1921, the datum was MSL at the Victoria Dock, Liverpool.
In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level. It is used for a part of continental Europe and main part of Africa as official sea level. Elsewhere in Europe vertical elevation references (European Vertical Reference System) are made to the Amsterdam Pile 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.
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