Mean sea level (MSL, often shortened to sea level) is an
average surface level of one or more among
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's coastal
bodies of water from which heights such as
elevation may be measured. The global MSL is a type of
vertical datum
In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum, is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built str ...
a standardised
geodetic datumthat is used, for example, as a
chart datum
A chart datum is the water level surface serving as origin of depths displayed on a nautical chart. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. Common chart datums are ''lowest astr ...
in
cartography and
marine navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
, or, in aviation, as the
standard sea level Standard sea-level conditions (SSL), also known as sea-level standard (SLS), defines a set of atmospheric conditions for physical calculations.
The term "standard sea level" is used to indicate that values of properties are to be taken to be the sam ...
at which
atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
is measured to
calibrate
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...
altitude and, consequently, aircraft
flight levels. A common and relatively straightforward mean sea-level standard is instead the midpoint between a
mean low and mean high tide at a particular location.
Sea levels can be affected by many factors and are known to have varied greatly over
geological time scales. Current
sea level rise is mainly caused by human-induced
climate change. When temperatures rise,
mountain glaciers and the
polar ice caps melt, increasing the amount of water in water bodies. Because most of human settlement and infrastructure was built in response to a more normalized sea level with limited expected change, populations affected by climate change in connection to sea level rise will need to invest in climate adaptation to mitigate the worst effects or when populations are in extreme risk, a process of managed retreat.
The term ''above sea level'' generally refers to
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.
Measurement
Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level.
[ US National Research Council, ''Bulletin of the National Research Council 1932'' page 270] Instantaneous sea level varies quite a lot 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 easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and using 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.
''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
tides, also have zero mean.
''Global MSL'' refers to a spatial average over the entire ocean.
One often measures the values of MSL in respect to the land; hence a change in relative 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. Before 1921, the
vertical datum
In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum, is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built str ...
was MSL at the
Victoria Dock, Liverpool
Victoria Dock was a dock on the River Mersey, England, and part of the Port of Liverpool. Situated in the northern dock system, it was connected to Trafalgar Dock to the north and West Waterloo Dock to the south.
History
The dock was designed ...
.
Since the times of the
Russian Empire
The Russian Empire was an empire and the final period of the Russian monarchy from 1721 to 1917, ruling across large parts of Eurasia. It succeeded the Tsardom of Russia following the Treaty of Nystad, which ended the Great Northern War. ...
, in
Russia
Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...
and its other former parts, now independent states, the sea level is measured from the zero level of
Kronstadt
Kronstadt (russian: Кроншта́дт, Kronshtadt ), also spelled Kronshtadt, Cronstadt or Kronštádt (from german: link=no, Krone for " crown" and ''Stadt'' for "city") is a Russian port city in Kronshtadtsky District of the federal city ...
Sea-Gauge.
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 0.146m above
chart datum
A chart datum is the water level surface serving as origin of depths displayed on a nautical chart. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. Common chart datums are ''lowest astr ...
and 1.304m 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
, image_flag = Bandera de España.svg
, image_coat = Escudo de España (mazonado).svg
, national_motto = ''Plus ultra'' (Latin)(English: "Further Beyond")
, national_anthem = (English: "Royal March")
, i ...
uses the reference to measure heights below or above sea level at
Alicante, and another European vertical elevation reference (European Vertical Reference System) is 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
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
and
CNES
The (CNES; French: ''Centre national d'études spatiales'') is the French government space agency (administratively, a "public administration with industrial and commercial purpose"). Its headquarters are located in central Paris and it is und ...
, TOPEX/Poseidon was followed by
Jason-1 in 2001 and the
Ocean Surface Topography Mission
OSTM/Jason-2, or Ocean Surface Topography Mission/Jason-2 satellite, was an international Earth observation satellite altimeter joint mission for sea surface height measurements between NASA and CNES. It was the third satellite in a series s ...
on the Jason-2 satellite in 2008.
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 the average sea level datum. It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (MSL) (contrast with
flight level), and in the
atmospheric sciences, and
land surveying. An alternative is to base height measurements on an
ellipsoid of the entire Earth, which is what systems such as
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
do. In aviation, the ellipsoid known as
World Geodetic System 84 is increasingly used to define heights; however, differences exist between this ellipsoid height and mean tidal height. The alternative is to use a
geoid
The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
-based vertical
datum such as
NAVD88 and the global
EGM96 (part of WGS84).
When referring to
geographic features such as mountains on a
topographic map, variations in elevation are shown by
contour lines. The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both.
In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see
Amsterdam Airport Schiphol.
Difficulties in use
To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the
geoid
The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's
gravitational field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits measurable variations such as those measured by NASA's
GRACE satellites to determine mass changes in ice-sheets and aquifers. In reality, this ideal does not occur due to ocean currents, air pressure variations, temperature and salinity variations, etc., not even as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as (mean)
ocean surface topography
Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map.
These variations are exp ...
. It varies globally in a range of ±2m.
Dry land
Several terms are used to describe the changing relationships between sea level and dry land.
- "relative" means change relative to a fixed point in the sediment pile.
- "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps.
- "steric" refers to global changes in sea level due to thermal expansion and salinity variations.
- "isostatic" refers to changes in the level of the land relative to a fixed point in the earth, possibly due to thermal buoyancy or tectonic effects; it implies no change in the volume of water in the oceans.
The melting of
glaciers at the end of
ice ages is one example of eustatic
sea level rise. The
subsidence of land due to the withdrawal of
groundwater is an isostatic cause of relative sea level rise.
Paleoclimatologists can track sea level by examining the rocks deposited along coasts that are very tectonically stable, like the east coast of North America. Areas like volcanic islands are experiencing relative sea level rise as a result of isostatic cooling of the rock which causes the land to sink.
On other planets that lack a liquid ocean,
planetologists 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.
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 (such as a month or a year) long enough that fluctuations caused by
waves and
tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as
sea level changes. Some land movements occur because of
isostatic adjustment of the
mantle to the melting of
ice sheet
In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
s 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
The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
and
true polar wander.
Atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
,
ocean currents and local ocean temperature changes can affect LMSL as well.
Eustatic sea level change (as opposed to local change) results in an alteration to the global sea levels due to changes in either the volume of water in the world's oceans or net changes in the volume of the
oceanic basin
In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large geologic basins that are below sea level.
Most commonly the ocean is divided into basins fol ...
s.
Short-term and periodic changes
There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level. Two major mechanisms are causing sea level to 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.
Recent changes
Aviation
Pilots can estimate height above sea level with an
altimeter
An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
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
Automatic terminal information service, or ATIS, is a continuous broadcast of recorded aeronautical information in busier terminal areas, i.e. airports and their immediate surroundings. ATIS broadcasts contain essential information, such as current ...
(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.
Aviation charts 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
The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a ...
(ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.
[US Federal Aviation Administration, Code of Federal Regulation]
Sec. 91.121
/ref>
See also
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References
External links
Sea Level Rise:Understanding the past – Improving projections for the future
Permanent Service for Mean Sea Level
Measuring Sea Level from Space
Rising Tide Video: Scripps Institution of Oceanography
Sea Levels Online: National Ocean Service (CO-OPS)
Système d'Observation du Niveau des Eaux Littorales (SONEL)
Sea level rise – How much and how fast will sea level rise over the coming centuries?
{{Authority control
Geodesy
Physical oceanography
Oceanographical terminology
Vertical datums