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The '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. Gauss, who first described it, it is the "mathematical figure of the Earth," a smooth but highly irregular surface that corresponds not to the actual surface of the Earth's crust, but to a surface which can only be known through extensive gravitational measurements and calculations. Despite being an important concept for almost two hundred years in the history of geodesy, it has only been defined to high precision in recent decades. It is often described as the true physical figure of the Earth, in contrast to the idealized figure of a reference ellipsoid.

Contents

Description

Spherical harmonics representation

References

External links

See also

Description

The geoid surface is irregular, unlike the reference ellipsoids often used to approximate the shape of the physical Earth, but considerably smoother than Earth's physical surface. While the latter has excursions of +8,000 m (Mount Everest) and −11,000 m (Mariana Trench), the total variation in the geoid is less than 200 m (compared to a perfect mathematical ellipsoid).
Sea level, if undisturbed by tides and weather, would assume a surface equal to the geoid. If the continental land masses were criss-crossed by a series of tunnels or narrow canals, the sea level in these canals would also coincide with the geoid. 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.
Being an equipotential surface, the geoid is by definition a surface to which the force of gravity is everywhere perpendicular. This means that when travelling by ship, one does not notice the undulations of the geoid; the local vertical is always perpendicular to it, and the local horizon tangential component to it. Likewise, spirit levels will always be parallel to the geoid.
Note that a GPS receiver on a ship may, during the course of a long voyage, indicate height variations, even though the ship will always be at sea level. This is because GPS satellites, orbiting about the center of gravity of the earth, can only measure heights relative to a geocentric reference ellipsoid. To obtain one's geoidal height, a raw GPS reading must be corrected. Conversely, height determined by spirit leveling from a tidal measurement station, as in traditional land surveying, will always be geoidal height.

Spherical harmonics representation

Spherical harmonics are often used to approximate the shape of the geoid. The current best such set of spherical harmonic coefficients is EGM96 (Earth Gravity Model 1996)^[2], determined in an international collaborative project led by NIMA. The mathematical description of the non-rotating part of the potential function in this model is
:$$
V=rac{GM}{r}left(1+{sum_{n=2}^{n_{max}}}left(rac{a}{r}
ight)^n{sum_{m=0}^n}
overline{P}_{nm}(sinphi)left[overline{C}_{nm}cos mlambda+overline{S}_{nm}sin mlambda
ight]
ight),

where $phi$ and $lambda$ are ''geocentric'' (spherical) latitude and longitude respectively, $overline\{P\}\_\{nm\}$ are the fully normalized Legendre functions of degree $n$ and order $m$, and $overline\{C\}\_\{nm\}$ and $overline\{S\}\_\{nm\}$ are the coefficients of the model. Note that the above equation describes the Earth's gravitational potential $V$, not the geoid itself, at location $phi,;lambda,;r,$ the co-ordinate $r$ being the ''geocentric radius'', i.e, distance from the Earth's centre. The geoid is a particular^[3] equipotential surface, and is somewhat involved to compute. The gradient of this potential also provides a model of the gravitational acceleration. EGM96 contains a full set of coefficients to degree and order 360, describing details in the global geoid as small as 55 km (or 110 km, depending on your definition of resolution). One can show there are
:$$
sum_{k=2}^n 2k+1 = n(n+1) + n - 3 = 130,317

different coefficients (counting both $overline\{C\}\_\{nm\}$ and $overline\{S\}\_\{nm\}$, and using the EGM96 value of $n=n\_\{max\}=360$). For many applications the complete series is unnecessarily complex and is truncated after a few (perhaps several dozen) terms.
New even higher resolution models are currently under development. For example, many of the authors of EGM96 are working on an updated model^[4] that will incorporate much of the new satellite gravity data (see, e.g., GRACE), and will support up to degree and order 2160 (1/6 of a degree, requiring over 4 million coefficients).

References

1. data from http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/wgs84_180.html
2. NIMA Technical Report TR8350.2, ''Department of Defense World Geodetic System 1984, Its Definition and Relationships With Local Geodetic Systems'', Third Edition, 4 July 1997. [Note that confusingly, despite the title, versions after 1991 actually define EGM96, rather than the older WGS84 standard, and also that, despite the date on the cover page, was actually updated last in June 23 2004. Available electronically at: http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html]
3. There is no such thing as "The" EGM96 geoid
4. Pavlis, N.K., S.A. Holmes. S. Kenyon, D. Schmit, R. Trimmer, "Gravitational potential expansion to degree 2160", ''IAG International Symposium, gravity, geoid and Space Mission GGSM2004'', Porto, Portugal, 2004.

External links

★ Main NGA (was NIMA) page on Earth gravity models

★ EGM96 NASA GSFC Earth gravity model

★ NOAA Geoid webpage

★ Verify Locality Tool

★ Geoid tutorial from Li and Gotze (964KB pdf file)

★ Geoid tutorial at GRACE website

See also

★ Physical geodesy

★ Geodesy