The GOCE satellite flies extraordinarily low and it thrusts constantly to compensate for air drag while making exquisite measurements of the gravitational gradient. The just-released result is a map of the geoid — the gravitational equipotential surface of the Earth — shown below as a delta from an idealized ellipsoid. GOCE can measure the geoid with an accuracy of 1 – 2 cm vertically, with 100 km spatial resolution.
This has many uses. For example, if Earth’s ocean were quiescent, its shape would track the geoid, and the discrepancies from this ideal shape are informative. Changes in the geoid over time can track the movement of magma under the Yellowstone caldera and the depletion of groundwater in India. The BBC has a story, and the European Space Agency has more information, including a slick video of the mission.
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{ 2 comments… read them below or add one }
I don’t understand why, given that water is level-seeking and land is not, there aren’t discontinuities around the margins of the continents.
Chris
@ Chris Phoenix — It’s a map of an equipotential surface (not the physical surface itself, of course — radar does that), and any mass distribution, discontinuous or not, will produce a smooth equipotential.