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The geoid is 19. 356 meters above the theoretical ellipsoid shape of the earth at Tharwa.
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geoid and is
International Atomic Time ( TAI, from the French name Temps atomique international ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.
Thus for clocks on or near the geoid, T < sub > eph </ sub > ( within 2 milliseconds ), but not so closely TCB, can be used as approximations to Terrestrial Time, and via the standard ephemerides T < sub > eph </ sub > is in widespread use.
The geoid is essentially the figure of the Earth abstracted from its topographical features.
The geoid, unlike ellipsoid, is irregular and too complicated to serve as the computational surface on which to solve geometrical problems like point positioning.
The geometrical separation between the geoid and the reference ellipsoid is called the geoidal undulation.
A reference ellipsoid, customarily chosen to be the same size ( volume ) as the geoid, is described by its semi-major axis ( equatorial
The reference surface for orthometric heights is the geoid, an equipotential surface approximating mean sea level.
This is approximately the same as the direction of the plumbline, i. e., local gravity, which is also the normal to the geoid surface.
In the first step the physical surface is modelled by the geoid, a surface which approximates the mean sea level over the oceans and its continuation under the land masses.
The second step is to approximate the geoid by a mathematically simpler reference surface.
The simplest choice for the reference surface is a sphere, but the geoid is more accurately modelled by an ellipsoid.
In relativistic terms, the SI second is defined as the proper time on the rotating geoid.
This terminology is also used for astronomical bodies such as the planet Earth, even though it is not spherical and only approximately spheroidal ( see geoid ).
TT was defined to be a linear scaling of TCG, such that the unit of TT is the SI second on the geoid ( Earth surface at mean sea level ).
Experimental determination of the gravitational potential at the geoid surface is a task in physical geodesy.
( As measured on the geoid surface, the rate of TCG is very slightly faster than that of TT, see below, Relativistic relationships of TT.
In relativistic terms, TT is described as the proper time of a clock located on the geoid ( essentially mean sea level ).
The geopotential surface called the geoid is one definition of the shape of the Earth.
In the case of the geoid and ellipsoids, the fixed distance from any point on the model to the specified center is called " a radius of the Earth " or " the radius of the Earth at that point ".
This difference is the geoid height, positive above or outside the ellipsoid, negative below or inside.
The geoid height variation is under 110 m on Earth.
A sphere being a gross approximation of the spheroid, which itself is an approximation of the geoid, units are given here in kilometers rather than the millimeter resolution appropriate for geodesy.
geoid and .
Starting from Julian Date 2443144. 5 ( 1 January 1977 00: 00: 00 ), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at mean sea level ( the geoid ).
* First, it had been shown that floating masses on a rotating geoid would collect at the equator, and stay there.
Satellite positioning receivers typically provide ellipsoidal heights, unless they are fitted with special conversion software based on a model of the geoid.
A regional, ellipsoidal datum like ED50 can be fixed by prescribing the undulation of the geoid and the deflection of the vertical in one datum point, in this case the Helmert Tower in Potsdam.
The traditional spirit level produces these practically most useful heights above sea level directly ; the more economical use of GPS instruments for height determination requires precise knowledge of the figure of the geoid, as GPS only gives heights above the GRS80 reference ellipsoid.
As geoid knowledge accumulates, one may expect use of GPS heighting to spread.
They are used in gravity surveys over large areas for establishing the figure of the geoid over these areas.
Maps that depict the surface of the Earth also use a projection, a way of translating the three-dimensional real surface of the geoid to a two-dimensional picture.
A uniform second was produced by correcting the output of each atomic clock to mean sea level ( the rotating geoid ), lengthening the second by about 1.
The TAI service, running since 1958, attempts to match the rate of proper time on the geoid, using an ensemble of atomic clocks spread over the surface and low orbital space of the Earth.
In effect it defined the geoid ( mean sea level ) in terms of a particular level of gravitational time dilation relative to a notional observer located at infinitely high altitude.
geoid and meters
Map of the undulation of the geoid, in meters ( based on the EGM96 gravity model and the WGS84 reference ellipsoid ).
Undulation of the geoid is the mathematical process of determining the height in meters above the geoid ( relative to the mean sea level ) from the height provided by the GPS system which uses the ( WGS84 ) ellipsoid as reference.
geoid and above
In reality the geoid does not have a physical meaning under the continents, but geodesists are able to derive the heights of continental points above this imaginary, yet physically defined, surface by a technique called spirit leveling.
Then they are able to correct the height above WGS ellipsoid to the height above WGS84 geoid.
Note that the above equation describes the Earth's gravitational potential, not the geoid itself, at location the co-ordinate being the geocentric radius, i. e., distance from the Earth's centre.
In some places, like west of Ireland, the geoid — mathematical mean sea level — sticks out as much as 100 m above the regular, rotationally symmetric reference ellipsoid of GRS80 ; in other places, like close to Ceylon, it dives under the ellipsoid by nearly the same amount.
Topographic elevation is the vertical distance above the reference geoid, a precise mathematical model of the Earth's sea level as an equipotential gravitational surface.
From this distance or height, the local surface effects such as tides, winds and currents are removed to obtain the satellite height above the geoid.
The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ( see Geodetic system, vertical datum ).
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