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Gravity Anomaly Maps and The Geoid

The Earth's gravity field is depicted in two principal ways: gravity anomaly maps and maps of the Earth's geoid.

Gravity anomaly maps (see globe below) show how much the Earth's actual gravity field differs from the gravity field of a uniform, featureless Earth surface. The anomalies highlight variations in the strength of the gravitational force over the surface of the Earth. Gravity anomalies are often due to unusual concentrations of mass in a region. For example, the presence of mountain ranges will usually cause the gravitational force to be more than it would be on a featureless planet - positive gravity anomaly. Conversely, the presence of ocean trenches or even the depression of the landmass that was caused by the presence of glaciers millennia ago can cause negative gravity anomalies.
Earth's gravity field as seen by GRACE

These "gravity anomaly" maps show where models of the Earth's gravity field based on GRACE data differ from a simplified mathematical model that assumes the Earth is perfectly smooth and featureless. Areas colored yellow, orange, or red are areas where the actual gravity field is larger than the featureless-Earth model predicts-such as the Himalayan Mountains in Central Asia (top left of the left-hand globe)-while the progressively darker shades of blue indicate places where the gravity field is less-such as the area around Hudson Bay in Canada (top center of right-hand globe).

The geoid is a hypothetical Earth surface that represents the mean sea level in the absence of winds, currents, and most tides. The geoid is a useful reference surface. It defines the horizontal everywhere and gravity acts perpendicular to it. A carpenter's level aligns itself along the geoid and a carpenter's plumb bob points down the vertical or perpendicular to the geoid. Water will not flow in aqueducts if the pipes are perfectly aligned along the geoid. Surveyors use knowledge of the geoid and the horizontal when they lay out highways and boundaries.


image above shows how the average variability in Earth's gravity field in August 2002


As the geoid map becomes more detailed, the accuracy of satellite altimetry, synthetic aperture radar interferometry, and digital terrain models covering large land and ice areas - all used in remote sensing applications and cartography - will improve. These techniques provide critical input to many scientific models used in oceanography, hydrology, geology, and related disciplines, and will be used for a variety of applications including:

  • measuring the changing mass of polar ice caps;
  • measuring changes in water resources on land
  • understanding shallow and deep ocean current transport;
  • understanding atmosphere-ocean mass exchange;
  • understanding the forces that generate Earth's geomagnetic field; and


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The GRACE mission is jointly implemented by NASA and DLR under
the NASA Earth System Science Pathfinder Program.

Last Modified: Wed Nov 03, 2004