DEFINITION OF GEOID

The surface of equipotential gravity equivalent with

sea level and extending through the continents on the Earth is

known as the geoid, and it is often referred to as the figure of

the Earth. It is theoretically everywhere perpendicular to the

direction of gravity (the plumb line) and is used as a reference

surface for geodetic measurements. If the Earth were spherically

symmetric and not spinning, the gravitational equipotential

surfaces would consist of a series of concentric shells

with increasing potential energy extending away from the

Earth, much like a ball having greater potential energy the

higher it is raised. However, since the Earth is not perfectly

spherical (it is a flattened oblate spheroid) and it is spinning,

the gravitational potential is modified so that it is an oblate

spheroid with its major axis 0.3 percent longer than the

minor axis. A best-fit surface to this spheroid is used by

geodeticists, cartographers, and surveyors, but in many places

the actual geoid departs from this simple model shape.

Nonuniform distributions of topography and mass with

depth cause variations in the gravitational attraction, known

as geoid anomalies. Areas of extra mass, such as mountains

or dense rocks at depth, cause positive geoid anomalies

known as geoid highs, whereas mass deficits cause geoid

lows. The geoid is measured using a variety of techniques,

including direct measurements of the gravity field on the surface,

tracking of satellite positions (and deflections due to

gravity), and satellite-based laser altimetry that can measure

the height of the sea surface to the sub-centimeter level. Variations

in the height of the geoid are from up to tens of meters

to even more than 100 meters.

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