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Meridional profile of geoid undulation (red) relative to the reference ellipsoid (black), greatly exaggerated; see also: Earth's pear shape. The geoid undulation (also known as geoid height or geoid anomaly), N, is the height of the geoid relative to a given ellipsoid of reference.
The Indian Ocean Geoid Low (IOGL) is a gravity anomaly in the Indian Ocean. A circular region in the Earth's geoid, situated just south of the Indian peninsula, it is the Earth's largest gravity anomaly. [1] [2] It forms a depression in the sea level covering an area of about 3 million km 2 (1.2 million sq mi), almost the size of India itself.
The free-air correction adjusts measurements of gravity to what would have been measured at mean sea level, that is, on the geoid. The gravitational attraction of Earth below the measurement point and above mean sea level is ignored and it is imagined that the observed gravity is measured in air, hence the name.
The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity measured at every point on its surface would be given precisely by a simple algebraic expression. However ...
The separation between the geoid and the reference ellipsoid is called the undulation of the geoid, symbol . The geoid, or mathematical mean sea surface, is defined not only on the seas, but also under land; it is the equilibrium water surface that would result, would sea water be allowed to move freely (e.g., through tunnels) under the land.
GeographicLib provides a utility GeoidEval (with source code) to evaluate the geoid height for the EGM84, EGM96, and EGM2008 Earth gravity models. Here is an online version of GeoidEval . The Tracker Component Library from the United States Naval Research Laboratory is a free Matlab library with a number of gravitational synthesis routines.
These geopotential coefficients may be used to compute geoid height, gravity anomalies, and changes in the distribution of mass on Earth's surface. [38] Gridded products estimating changes in mass in units of liquid water equivalent thickness are available at JPL's GRACE Tellus website.
The geoid undulation with respect to the reference ellipsoid, =, finds an analogue in the so-called height anomaly, : ζ = h − H N {\displaystyle \zeta =h-H^{N}} The geoid–quasigeoid separation (GQS), N − ζ {\displaystyle N-\zeta } , is zero over the oceans and maximum in the Himalayas , where it attains approximately 5 meters.