geoid


Also found in: Dictionary, Acronyms, Wikipedia.

geoid

(jee -oid) The form of the Earth obtained by taking the average sea level surface and extending it across the continents. It is an equipotential surface defined by measurements of the variation of the Earth's gravitational attraction with latitude and longitude and the acceleration produced by the Earth's rotation. The geoid differs from a sphere in that the equatorial diameter (12 756.32 km) is greater than the diameter through the poles (12 713.51 km). The flattening of the Earth corresponds to the difference between these diameters as a fraction of the equatorial diameter and has a value of 1/298.257.

Studies of the perturbations affecting the orbits of artificial Earth satellites have shown that the geoid departs from an oblate spheroid, or ellipsoid; this is a result of density anomalies within the Earth and supports the theory of dynamic convective processes in the mantle. There are more than ten elevations and depressions on the true geoid, scattered worldwide and typically of 40–60 meters. The greatest departures are a 105-meter depression to the south of India and a 75-meter elevation to the north of Australia.

Geoid

 

the figure that the surfaces of the oceans and their contiguous seas would form at a certain average level of water free from disturbances caused by tides, currents, variations in atmospheric pressure, and the like. The surface of the geoid is one of the equipotential surfaces of the force of gravity. This surface, which is theoretically extended beneath the continents, forms a closed figure that is accepted as the smoothed figure of the earth. Often the geoid is understood to mean the equipotential surface passing through a certain fixed point on the earth’s surface near the shore of the sea. The necessity of such a definition of the concept of geoid resulted from difficulties in determining the connection between the real earth and the undisturbed average sea level. The notion of a geoid formed as a result of the prolonged development of the conceptions of the earth’s shape as a planet, and the term itself was suggested by I. Listing in 1873. Leveled elevations are calculated from the geoid. According to present data, the average deviation of the geoid from the best selected spheroid of the earth is about ±50 m, and the maximum deviation does not exceed ±100 m. The sum of the geoid’s height and the orthometric height determines the height N of a corresponding point above the earth’s ellipsoid. As long as the distribution of density in the earth’s interior is not accurately known, the height N in geodetic gravimetry and geodesy is determined, according to M. S. Molodenskii’s proposal, as the sum of the normal height and the height of the quasi-geoid. (Height N is necessary for determining the coordinates of points on the earth’s surface of approximate-earth space in a single Cartesian system.) The surface of the quasi-geoid (“almost a geoid”) is determined by the values of the potential gravity on the earth’s surface, and to study the geoid it is not necessary that the results of measurements be reduced within the attracting body. The quasi-geoid deviates from the geoid by 2-3 m in high mountains and by 2-3 cm in low-lying plains. The surfaces of the geoid and quasi-geoid coincide in the seas and oceans. The shape of the quasi-geoid is determined by astronomical-gravimetric leveling or by preliminary determination of the perturbation potential on the continents by terrestrial gravimetric surveys and observations of the movements of artificial earth satellites. The latter data is necessary because of the lack of gravimetric studies of some areas on the earth.

REFERENCE

Zakatov, P. S. Kurs vysshei geodezii, 3rd ed. Moscow, 1964.

M. I. IURKINA

geoid

[′jē‚ȯid]
(geodesy)
The figure of the earth considered as a sea-level surface extended continuously over the entire earth's surface.
References in periodicals archive ?
After comparing the geoid sea level measurements with those predicted by ocean circulation simulations, the researchers at last found agreement between the two approaches.
In the determination of regional geoid models it has become customary to employ modified Stokes' formula, which combines local gravity data with a GGM.
This was the first geoid determination in Australia and the techniques were extensively used over later years in mapping the geoid over the continent and determining the geodetic datum of best fit.
243 MITROVICA JX,J GEO R-SOL,vol 0096,page 0053,1991,cites= 13,ON POSTGLACIAL GEOID SUBSIDENCE OVER THE EQUATORIAL OCEANS
For specific geological, geophysical and geodetical studies in such areas, if Bouguer anomalies are interpolated, the information deduced from the gravity interpretations (such as subsurface modelling, basin exploration, geoid computation .
This includes the coordinate system, the horizontal and vertical units of measure, the definition of the geoid, and the precision or resolution of the data.
The geoid as an equipotential surface of the Earth's gravity field coincides with the mean sea level over the oceans and extends hypothetically into the continental regions.
The researchers used three different methods to arrive at their estimate--(1) a 1998 global gravitational model produced by NASA and the National Imagery and Mapping Agency, (2) a 1999 regional, high-resolution geoid model, and (3) recent measurements made by the National Geodetic Survey of a reference marker on the NIST site.
The obtained geoid height reference surface was then compared with Latvian gravimetric geoid model LV'98 which is broadly used by land surveyors for more than 10 years.
The geoid is the shape of an imaginary global ocean dictated by gravity in the absence of tides and currents.