Editing Sea level (section)

== Measurement ==
[[File:Recent Sea Level Rise.png|thumb|Sea level measurements from 23 long [[tide gauge]] records in geologically stable environments show a rise of around {{convert|200|mm|in}} during the 20th century (2 mm/year).]]

Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level.<ref name=USNRC32>[[National Research Council (United States)|US National Research Council]], ''Bulletin of the National Research Council 1932'' page 270</ref> Instantaneous sea level varies substantially on several scales of time and space. 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 mean sea level at a particular location may be calculated over an extended time period and used as a [[datum (geodesy)|datum]]. For example, hourly measurements may be averaged over a full [[Metonic 19-year lunar cycle]] to determine the mean sea level at an official [[tide gauge]].<ref name=Terms>{{cite journal |last1=Gregory |first1=Jonathan M. |last2=Griffies |first2=Stephen M. |last3=Hughes |first3=Chris W. |last4=Lowe |first4=Jason A. |display-authors=etal |title=Concepts and Terminology for Sea Level: Mean, Variability and Change, Both Local and Global |journal=Surveys in Geophysics |volume=40 |pages=1251–1289 |date=29 April 2019 |issue=6 |doi=10.1007/s10712-019-09525-z |doi-access=free|bibcode=2019SGeo...40.1251G }}</ref>

''Still-water level'' or ''still-water sea level'' (SWL) is the level of the sea with motions such as [[wind waves]] averaged out.<ref>{{Cite web|url=http://glossary.ametsoc.org/wiki/Still-water_level|title=Still-water level - AMS Glossary|website=glossary.ametsoc.org|access-date=10 December 2018|archive-date=10 December 2018|archive-url=https://web.archive.org/web/20181210110949/http://glossary.ametsoc.org/wiki/Still-water_level|url-status=live}}</ref>
Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the [[tide]]s, 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.<ref name=Terms/>

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|ordnance datum]] (the 0 metres height on UK maps) is the mean sea level measured at [[Newlyn]] in Cornwall between 1915 and 1921.<ref>{{Cite web|url=https://www.ordnancesurvey.co.uk/benchmarks/|title=Ordnance Survey Benchmark locator|access-date=2021-12-21|archive-date=27 December 2021|archive-url=https://web.archive.org/web/20211227142006/https://www.ordnancesurvey.co.uk/benchmarks/|url-status=live}}</ref> Before 1921, the [[vertical datum]] was MSL at the [[Victoria Dock (Liverpool)|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 {{convert|0.146|m|abbr=on}} above [[chart datum]]<ref>[https://www.hko.gov.hk/en/tide/enotes.htm "Tide: Notes"], Hong Kong Observatory. {{Webarchive|url=https://web.archive.org/web/20220927095742/https://www.hko.gov.hk/en/tide/enotes.htm |date=27 September 2022}}.</ref> and {{convert|1.304|m|abbr=on}} 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 [[EUREF#European Vertical Reference System (EVRS)|European Vertical Reference System]] is calibrated to the [[Amsterdam Ordnance Datum|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.<ref>{{cite journal|title=Evaluating models of sea state bias in satellite altimetry|date=1994|doi=10.1029/94JC00478|bibcode=1994JGR....9912581G|volume=99|issue=C6|journal=Journal of Geophysical Research|page=12581|last1=Glazman|first1=Roman E|author1-link=Roman Glazman|last2=Greysukh|first2=Alexander|last3=Zlotnicki|first3=Victor}}</ref> 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.

=== {{Anchor|Above mean sea level|AMSL}} Height above mean sea level === <!-- Other articles link here. -->
{{Main|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 {{convert|100|m|0|abbr=off}} exist between this ellipsoid height and local mean sea level.<ref name="Geoid"/> Another alternative is to use a [[geoid]]-based vertical [[datum (geodesy)|datum]] such as [[North American Vertical Datum of 1988|NAVD88]] and the global [[EGM96]] (part of WGS84). Details vary in different countries.

When referring to [[geographic]] features such as mountains, on a [[topographic map]] variations in elevation are shown by [[contour line]]s. 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.

==== Difficulties in use ====
[[File:Geoida.svg|thumb|{{ordered list |Ocean |[[Reference ellipsoid]] |Local [[Plumb-bob|plumb line]] |[[Continent]] |[[Geoid]]}}]]

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 [[gravitation]]al 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 ±{{convert|1|m|ft|0|abbr=on}}.<ref>{{cite web |url=https://science.nasa.gov/earth/climate-change/sea-level-101-what-determines-the-level-of-the-sea/#h-waves-in-the-bathtub |title=Sea Level 101: What Determines the Level of the Sea? |publisher=NASA |date=3 June 2020 |access-date=17 April 2024}}</ref>