Gravity Anomaly Maps and The Geoid
From NASA's Earth Observatory:
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.
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. Producing a precise model of the geoid has proven to be a challenge. Until recently, there was no single source for producing a geoid map. Data from several dozen satellites, along with surface measurements over land and from ships at sea, had to be combined to produce a model of the gravitational field. Traditionally, the models have done a fairly good job reproducing large-scale features of the gravity field, but have fallen short when it comes to reproducing finer-scale features or accurately describing time-variable gravity effects like those associated with the hydrologic cycle. GRACE provides, for the first time, global coverage of the Earth’s gravity field every 30 days from a single source. GRACE is already able to measure the gravity field with a level of precision that is at least 100 times greater than any existing measurement, and continued improvements are expected as the mission progresses. The finer details of the geoid that have evaded scientists for so long are on the verge of being revealed. GRACE also gives us our best opportunity to date to study time-variable gravity effects. As the mission progresses and more data are added to the model, the resolution of the geoid will improve even further.
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Link to NASA's Earth Observatory page: http://earthobservatory.nasa.gov/Features/GRACE/page3.php
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