# sea level

Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. 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, the flight level is one-eight-zero.

## Measurement

Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 20 centimeters (8 inches) during the 20th century (2 millimeters/year).

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, 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.

## 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. 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. 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. 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.

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. That problem can compound when landing on an aircraft carrier in a gravitational anomaly. In aviation mean sea level is increasingly being defined according to the reference ellipsoid defined by the World Geodetic System. Compared to a geoid, an ellipsoid is simpler to model mathematically and therefore lends itself to use with the Global Positioning System.

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.g. melting of ice sheets). The term "steric" refers to global changes in sea level due to thermal expansion and salinity variations. 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. The melting of glaciers at the end of ice ages is an 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", serves equivalently as a reference for the height of planetary features.

## Changes through geologic time

Comparison of two sea level reconstructions during the last 500 Myr. The scale of change during the last glacial/interglacial transition is indicated with a black bar. Note that over most of geologic history long-term average sea level has been significantly higher than today.
Sea level change since the end of the last glacial episode. Changes displayed in meters.

Sea level has changed over geologic time. 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). 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.

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. 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. 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. 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. This results in sedimentary cycles which in some cases can be correlated around the world with great confidence. This relatively new branch of geological science linking eustatic sea level to sedimentary deposits is called sequence stratigraphy.

Global warming refers to the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.
The three-letter acronym SEA may refer to:
• Scientists and Engineers for America, a pro-science political advocacy group.
• Schoof-Elkies-Atkin algorithm
• Seattle-Tacoma International Airport (IATA: SEA, ICAO: KSEA)
• Sea Education Association

A tide gauge is a device for measuring sea level and detecting tsunamis.

Sensors continuously record the height of the water level with respect to a height reference surface close to the geoid.
wave is a mode of energy transfer from one place to another, often with little or no permanent displacement of the particles of the medium (i.e. little or no associated mass transport); instead there are oscillations around almost fixed positions.
Tides are the cyclic rising and falling of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. More generally, tidal phenomena can occur in any object that is subjected to a gravitational field that varies in time and space, such as the
Motto
"Dieu et mon droit" [2]   (French)
"God and my right"
Anthem
"God Save the Queen" [3]
Newlyn
Cornish - Lulynn

Newlyn ()
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Cornwall
Kernow

(Flag)
Motto: Onen hag oll
(Cornish: One and all)

Geography
Status Ceremonial & (smaller) Non-metropolitan county
Region South West England
Area
- Total
City of Liverpool
Liverpool skyline, as seen from across the River Mersey

Coat of Arms
Location within England
Coordinates:
Sovereign state United Kingdom
Merseyside

Shown within England
Geography
Status Metropolitan county &
Ceremonial county
Origin 1974 (Local Government Act 1972)
Region North West England
Area
- Total Ranked 43rd
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. Usually mean sea level (MSL) is used.
This article describes a concept from surveying and geodesy. Datum is also the last known position of a submarine in ASW, the singular form of data, and a city in ancient Macedonia, probably on the site of the modern Kavala.

geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were to be extended through the continents (such as with very narrow canals). According to C.F.
Gravitation is a natural phenomenon by which all objects with mass attract each other. In everyday life, gravitation is most familiar as the agency that endows objects with weight.
The Metonic cycle or Enneadecaeteris in astronomy and calendar studies is a particular approximate common multiple of the year (specifically, the seasonal i.e. tropical year) and the synodic month.
eclipse cycles. The series of eclipses separated by a repeat of one of these intervals is called an eclipse series.

## Eclipse conditions

Eclipses may occur when the Earth and Moon are aligned with the Sun, and the shadow of one body cast by the Sun falls on the other.
Earth's oceans
(World Ocean)
• Arctic Ocean
• Atlantic Ocean
• Indian Ocean
• Pacific Ocean
• Southern Ocean

The Pacific Ocean (from the Latin name Mare Pacificum
Panama Canal (Spanish: Canal de Panamá) is a major ship canal that traverses the Isthmus of Panama in Central America, connecting the Atlantic and Pacific Oceans.
Atlantic Ocean is the second-largest of the world's oceanic divisions; with a total area of about 106.4 million square kilometres (41.1 million square miles), it covers approximately one-fifth of the Earth's surface.
Aviator (common usage term: pilot, regulatory usage term: airman) is a person qualified in the operation of aircraft, whether for pleasure or as a profession.
Instrument Flight Rules (IFR) are a set of regulations and procedures for flying aircraft whereby navigation and obstacle clearance is maintained with reference to aircraft instruments only and separation from other aircraft is provided by Air Traffic Control.
Schiphol (IATA: AMS, ICAO: EHAM) (municipality Haarlemmermeer) is the Netherlands' main airport. Located 20 minutes (17.5 km) south-west of Amsterdam, Schiphol is a European mainport, competing in passenger and cargo throughput with Heathrow International Airport

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 average sea level datum.
AirPort is a local area wireless networking brand from Apple Inc. based on the IEEE 802.11b standard (also known as Wi-Fi) and certified as compatible with other 802.11b devices. A later family of products based on the IEEE 802.11g specification is known as AirPort Extreme.
aircraft carrier is a warship designed to deploy and in most cases recover aircraft, acting as a sea-going airbase. Aircraft carriers thus allow a naval force to project air power great distances without having to depend on local bases for staging aircraft operations.
In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body. Because of their relative simplicity, reference ellipsoids are used as a preferred surface on which geodetic network
The World Geodetic System defines a reference frame for the earth, for use in geodesy and navigation. The latest revision is WGS 84 dating from 1984 (last revised in 2004), which will be valid up to about 2010.
Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its
Water is a common chemical substance that is essential to all known forms of life.[1] In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor.