Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. This article is about the body of water For other uses see SEA and Seas. Defining the reference level [1], however, involves complex measurement, and accurately determining MSL can prove difficult.
Mean Sea Level as used by military Flight Surgeons and Aerospace Units: Using pressure to measure altitude results in two other types of altitude. True or MSL (mean sea level) is the next best measurement to absolute - and in some ways better. MSL tells you how far you are above an imaginary line at sea level. If you then know the elevation of terrain, the next step is to determine how far you are above ground. It also tells you how thin the air is, which determines your physiological response to that altitude. True Altitude (MSL) has been adjusted for local high or low pressure conditions. FL or Flight Level is another related term that is measured in hundreds of feet. At a standard pressure that correlates to 18,000 feet (5,500 m), the flight level is one-eight-zero.
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Measurement
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. A tide gauge is a device for measuring sea level and detecting tsunamis A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. g. , due to the tides, also get averaged out. Characteristics A tide is a repeated cycle of sea level changes in the following stages Over several hours the water rises or advances up a beach in the flood 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, mean sea level has been measured at Newlyn in Cornwall and Liverpool on Merseyside for decades, by tide gauges to provide Ordnance Datum for the zero metres height on UK maps. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located Newlyn (Lulynn is a town in southwest Cornwall, England, UK. The town forms a small Conurbation with neighbouring Penzance, Cornwall ( Kernow ˈkɛɹnɔʊ is the most southwesterly county of England, on the Peninsula that lies to the west of the River Tamar Liverpool ( is a City and Metropolitan borough of Merseyside, England along the eastern side of the Mersey Estuary Merseyside is a Metropolitan county in North West England, with a population of 1365900 In the British Isles, an Ordnance Datum or OD is a vertical datum used by an ordnance survey as the basis for deriving Altitudes on Maps
Difficulties in utilization
To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or datum, called the geoid. This article describes a concept from Surveying and Geodesy. For other meanings see Datum (disambiguation. The geoid is that Equipotential surface which would coincide exactly with the mean ocean surface of the Earth if the oceans were in equilibrium at rest and extended through 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. Gravitation is a natural Phenomenon by which objects with Mass attract one another In reality, due to currents, air pressure variations, temperature and salinity variations, etc. , this does not occur, 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 (stationary) sea surface topography. It varies globally in a range of ± 2 m.
Traditionally, one had to process sea-level measurements to take into account the effect of the 228-month Metonic cycle and the 223-month eclipse cycle on the tides. Metonic cycle or Enneadecaeteris in Astronomy and Calendar studies is a particular approximate common multiple of the Tropical year may occur repeatedly separated by certain intervals of time these intervals are called eclipse cycles. Mean sea level does not remain constant over the surface of the entire earth. For instance, mean sea level at the Pacific end of the Panama Canal stands 20 cm higher than at the Atlantic end. The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Panama Canal is a man-made Canal in Panama which joins the
Despite the difficulties, aviators flying under instrument flight rules (IFR) must have accurate and reliable measurements of their altitudes above (or below - see Schiphol Airport) mean sea level, and the altitude of the airports where they intend to land. Amsterdam Airport Schiphol is the Netherlands ' main Airport, located 20 minutes (17 The term above mean sea level ( AMSL) refers to the Elevation (on the ground or Altitude (in the Air) of any object relative to the An airport is a location where Aircraft such as airplanes, Helicopters and blimps take off and land That problem can compound when landing on an aircraft carrier in a gravity anomaly. An aircraft carrier is a Warship designed with A gravity anomaly is the difference between the observed Gravity and a value predicted from a model In aviation mean sea level is increasingly being defined according to the reference ellipsoid defined by the World Geodetic System. In Geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the Geoid, the truer Figure of the Earth, or other planetary body The World Geodetic System defines a reference frame for the earth for use in Geodesy and Navigation. Compared to a geoid, an ellipsoid is simpler to model mathematically and therefore lends itself to use with the Global Positioning System. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth
Several terms are used to describe the changing relationships between sea level and dry land. When the term "relative" is used, it connotes change that is not attributed to any specific cause. The term "eustatic" refers to global changes in the sea level due to water mass added (or removed from) the oceans (e. Water is a common Chemical substance that is essential for the survival of all known forms of Life. An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. g. melting of ice sheets). An ice sheet is a mass of Glacier Ice that covers surrounding terrain and is greater than 50000 km² (20000 mile²) The term "steric" refers to global changes in sea level due to thermal expansion and salinity variations. Thermal Expansion is the tendency of matter to change in Volume in response to a change in temperature Salinity is the Saltiness or dissolved salt content of a body of Water. The term "isostatic" refers to changes in the level of the land masses due to thermal buoyancy or tectonic effects and implies no real change in the volume of water in the oceans. This article discusses the geologic usage for the philosophical or architectural usage see Architectonics ' Or see Plate tectonics. The melting of glaciers at the end of ice ages is an example of eustatic sea level rise. "Glacial" and "Glaciation" redirect here For the geological periods see Glacial period. An ice age is a period of long-term reduction in the Temperature of the Earth 's surface and atmosphere resulting in an expansion of continental Ice sheets The subsidence of land due to the withdrawal of groundwater is an isostatic cause of relative sea level rise. Groundwater is Water located beneath the Ground surface in Soil pore spaces and in the Fractures of lithologic formations Paleoclimatologists can track sea level by examining the rocks deposited along coasts that are very tectonically stable, like the east coast of North America. Paleoclimatology (also Palaeoclimatology) is the study of Climate change taken on the scale of the entire History of Earth. 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. Planetary science, also known as planetology and closely related to planetary astronomy, is the Science of Planets or Planetary systems This altitude, sometimes referred to as a "sea level", serves equivalently as a reference for the height of planetary features.
Changes through geologic time
Sea level has changed over geologic time. The geologic time scale is a chronologic schema (or idealized Model) relating Stratigraphy to time that is used by Geologists and other As the graph shows, sea level today is very near the lowest level ever attained (the lowest level occurred at the Permian-Triassic boundary about 250 million years ago). The Permian is a geologic period and system that extends from 299 The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago For this reason, sea level is more prone to rise than fall today, and small changes in climate can have noticeable effects during human lifetimes. Climate encompasses the temperatures humidity rainfall atmospheric particle count and numerous other meteorogical factors in a given region over long periods of
During the most recent ice age (at its maximum about 20,000 years ago) the world's sea level was about 130 m lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice in northern hemisphere glaciers. Seawater is Water from a Sea or Ocean. On average seawater in the world's oceans has a Salinity of about 3 "Snowfall" redirects here For other uses see Snow (disambiguation or Snowfall (disambiguation. Ice is a Solid phase, usually crystalline, of a Non-metalic substance that is liquid or gas at Room temperature, such as Ammonia The majority of the glaciers had melted by about 10,000 years ago, but minor glacial melting has continued (with occasional reversals) throughout recorded human history. Most "recently" (within last 10,000 years) a "normal" interglacial increase of Earth temperature was interrupted at a level of a few degrees below peak, and this probably correlates with slowdown in the sea level increase at present. There is no clear data, was this Earth temperature decrease a result of human activities or just a new step in continuing "Earth freezing". More detail about the changes in sea level for the past 140,000 years can be seen by accessing this chart.
Hundreds of similar glacial cycles have occurred throughout the Earth's history. The history of Earth covers approximately 46 billion years (4567000000 years from Earth ’s formation out of the Solar nebula to the present Geologists who study the positions of coastal sediment deposits through time have noted dozens of similar basinward shifts of shorelines associated with a later recovery. A geologist is a contributor to the Science of Geology, studying the physical structure and processes of the Earth and planets of the solar system This results in sedimentary cycles which in some cases can be correlated around the world with great confidence. Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of This relatively new branch of geological science linking eustatic sea level to sedimentary deposits is called sequence stratigraphy. Sequence stratigraphy is a relatively new branch of Geology that attempts to link relative Sea-level changes to Sedimentary deposits
See also
- Above mean sea level
- List of places on land with elevations below sea level
- North West Shelf Operational Oceanographic System
- Sea level rise
- World Geodetic System
External links
- Sea Level Rise:Understanding the past - Improving projections for the future
- Permanent Service for Mean Sea Level
- Global sea level change: Determination and interpretation
- Environment Protection Agency Sea level rise reports
- Properties of isostasy and eustasy
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