The earth is not actually round, but rather somewhat wider at the equator than at the poles, and with many surface irregularities including bulges and depressions. This irregular shape is referred to as a geoid and is used to describe the actual shape of the earth.

A hypothetical, global, and continuous sea-level surface perpendicular to the direction of gravity at all points.

Level (or equipotential) surface at mean sea level. Surface of constant gravitational potential that is chosen to define the earth's shape. At sea the geoid corresponds to the time-averaged surface of the ocean (to an accuracy of 1-2 m).

The geoid is the surface within or around the earth that is everywhere perpendicular to the direction of gravity and coincides with mean sea level in the oceans at rest. Sea level of a moving ocean has topography, the topography that is measured by altimeter satellites to observe ocean currents.

an equipotential surface that coincides with mean sea level in the open ocean. On land it is the level surface that would be assumed by water in an imaginary network of frictionless channels connected to the ocean.

The surface that corresponds to "mean sea level". It is an equipotential surface, including gravitational and centrigugal potentials. The geoid is approximated by a bi-axial ellipsoid with equatorial radius 6378.137 km, and polar flattening of 0.00335281. Locally, the geoid deviates from this "reference ellipsoid" by up to 100 meters due to local gravitational anomolies. The geoid defines the reference for geodetic corrdinates.

Imaginary surface representing the average sea level over the ocean and under the land masses.

a an equipotential surface - that is, it is a field of equal value derived from the earth's gravity field and the outward centrifugal force of the earth's rotation (Thanks to Anthony Bruno of Trimble Navigation)

a close representation of the shape of the Earth

an equipotential surface In fluid mechanics, equipotentials are lines or surfaces of equal head that are in direct relation to pressure

an equipotential surface with respect to the gravitational acceleration of the Earth

a surface of constant gravitational potential lying close to the mean sea level

An imaginary shape for the earth defined by mean sea level and its imagined continuation under the continents at the same level of gravitational potential. A more complete explanation is available in the Standards Section.

True shape of the Earth, which deviates from a perfect sphere because of a slight bulge at the equator.

The shape of the earth as a three-dimensional spheroid that coincides with the surface of the earth at sea level and extends in an imaginary surface through the continents with a direction of gravity that is perpendicular at every point.

A surface of constant gravitational potential that would represent the sea surface if the oceans were not in motion.

An equipotential surface, defined by a surface of equal gravitational force, that approximates where sea level would be in the absence of land.

the equipotential surface (i.e. the surface on which the gravity potential is constant) which best approximates mean sea level

Figure of the Earth visualized as a mean sea level surface extended continuously through the continents. It is a theoretically continuous surface that is perpendicular at every point to the direction of gravity (the plumbline).

The figure of the earth as defined by the level surface that over the oceans coincides with mean sea level. See pp. 189-96.

The particular equipotential surface that coincides with mean sea level and that may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of gravity.

The spheroid (surface formed by rotating an ellipse about the polar or Z-axis of the terrestrial coordinate system) that most closely approximates the Earth's surface.

The figure of the earth considered as a sea-level surface extended continuously over the entire earth's surface.

The equipotential surface of the Earth's gravity field which best fits, in the least squares sense, MEAN SEA LEVEL.

An equipotential (level) surface ( i.e., one to which, at every point, the plumb line is perpendicular). Specifically, the figure of the earth considered as the level surface of a motionless 35,0,p ocean, where 35,0,p is the specific volume of a 35 psu, O°C , ocean, at a particular time.

An equipotential surface (a surface of equal gravity) coincident with the National Geodetic Vertical Datum of 1929 or the North American Vertical Datum of 1988.

the hypothetical surface of the Earth that coincides everywhere with the mean sea level

That equipotential surface (a surface of equal gravity potential) which most closely matches mean sea level. An equipotential surface is normal to the gravity vector at every point

The surface of the Earth at mean sea level, idealized to pass through the continents. The geoid is not an ellipsoid, but rather an irregularly shaped surface approximating a pear. The geoid may be used as a vertical datum.

A complex earth model used more in geodesy than cartography or GIS that accounts for discrepancies over the earth from the reference ellipsoid and other variations due to gravity, and so on.

a geometric figure of the earth which coincides twith the mean sea level over the oceans and extends continuously through the continents. A geoid has the surface at every point perpendicular to the direction of gravity. [AHDOS

The particular geopotential surface that most nearly coincides with the mean level of the oceans of the earth. For mapping purposes it is customary to use an ellipsoid of revolution as an adequate and convenient approximation to the geoid. The dimensions and orientation of the assumed ellipsoid may represent an attempt to find the ellipsoid that most nearly fits the geoid as a whole, or they may represent an attempt to fit only a particular part of the geoid without regard to the remainder of it. When mention is made of the dimensions of the earth, reference is usually made to the dimensions of the ellipsoid most nearly representing the geoid as a whole. The actual geoid can depart from a best-fitting sphere in places by as much as 100 m.

A geoid is an equipotential surface which (approximately) coincides with the mean ocean surface. It is often referred to as a close representation or physical model of the figure of the Earth. According to C.F.