geoid

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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 ?
A method of evaluating the truncation error coefficients for geoidal heights, Bulletin Geod 110: 413-425.
In principle, the geoidal heights can be determined from the global distribution of gravity observations (Stokes 1849).
g] on the geoidal height can roughly be estimated by a simple approximation (cf Heiskanen & Moritz 1967, eq.
N] is the resulting uncertainty of the geoidal height, g is the gravity value at the computation point, and s is polar distance.
Geopotential models are particularly useful for computing gravity anomaly and geoidal heights, which can be represented by a sum of a selected number of degree variances of different wavelengths in the spectral harmonic representation.
Caused by irregularities in mass distributions inside the Earth the geoidal heights undulate with respect to the geocentric reference ellipsoid, e.
The signal-to-noise ratio of the geoidal heights is greater at long wavelengths than for gravity anomalies, the reverse being generally true at short wavelengths.
Geoidal heights derived from EGM96 are claimed to be accurate to 1 m worldwide.
The GRACE group assessed the accuracy of the geoidal heights, implied from GGM01.
The satellite-derived geoidal undulations provide new tools required for studying the Earth's interior and the dynamic processes that take place within the Earth.
iv) The NKG computations yield height anomalies, whereas the outcomes of the Ellmann (2004) study are geoidal heights.
The geoidal heights are decreasing towards the northeast, whereas the extremes of 21 and 16 m are located the southwest and northeast corners, respectively (the length of this diagonal is ~400 km).