Geoid

  • the geoid (d/) is the shape that the ocean surface would take under the influence of the gravity and rotation of earth alone, if other influences such as winds and tides were absent. this surface is extended through the continents (such as with very narrow hypothetical canals). according to gauss, who first described it, it is the "mathematical figure of the earth", a smooth but irregular surface whose shape results from the uneven distribution of mass within and on the surface of earth. it can be known only through extensive gravitational measurements and calculations. despite being an important concept for almost 200 years in the history of geodesy and geophysics, it has been defined to high precision only since advances in satellite geodesy in the late 20th century.

    all points on a geoid surface have the same effective potential (the sum of gravitational potential energy and centrifugal potential energy). the force of gravity acts everywhere perpendicular to the geoid, meaning that plumb lines point perpendicular and water levels parallel to the geoid if only gravity and rotational acceleration were at work. the surface of the geoid is higher than the reference ellipsoid wherever there is a positive gravity anomaly (mass excess) and lower than the reference ellipsoid wherever there is a negative gravity anomaly (mass deficit).[1]

    geoid undulation in false color, shaded relief and vertical exaggeration (10000 scale factor).
    geoid undulation in false color, to scale.
  • description
  • undulation
  • spherical harmonics representation
  • determination
  • anomalies
  • time-variability
  • other celestial bodies
  • see also
  • references
  • further reading
  • external links

The geoid (d/) is the shape that the ocean surface would take under the influence of the gravity and rotation of Earth alone, if other influences such as winds and tides were absent. This surface is extended through the continents (such as with very narrow hypothetical canals). According to Gauss, who first described it, it is the "mathematical figure of the Earth", a smooth but irregular surface whose shape results from the uneven distribution of mass within and on the surface of Earth. It can be known only through extensive gravitational measurements and calculations. Despite being an important concept for almost 200 years in the history of geodesy and geophysics, it has been defined to high precision only since advances in satellite geodesy in the late 20th century.

All points on a geoid surface have the same effective potential (the sum of gravitational potential energy and centrifugal potential energy). The force of gravity acts everywhere perpendicular to the geoid, meaning that plumb lines point perpendicular and water levels parallel to the geoid if only gravity and rotational acceleration were at work. The surface of the geoid is higher than the reference ellipsoid wherever there is a positive gravity anomaly (mass excess) and lower than the reference ellipsoid wherever there is a negative gravity anomaly (mass deficit).[1]

Geoid undulation in false color, shaded relief and vertical exaggeration (10000 scale factor).
Geoid undulation in false color, to scale.