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geoid The geoid is an imaginary surface near the surface of the Earth, along which the force of gravity is the same and equivalent to that at sea level. This so-called equipotential surface can be thought of as equivalent with sea level and extending through the continents on the Earth, and is often referred to as the figure of the Earth. Theoretically it exists 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 equi-potential surfaces would consist of a series of concentric shells with increasing potential energy extending away from the Earth, much like raising a ball to a higher level increases its potential energy. Since the Earth is not perfectly spherical (it is a flattened oblate spheroid) and it is spinning, however, 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, phenomena 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 by a
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