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http://earthobservatory.nasa.gov/IOTD/view.php?id=87189
There is another way to see the depths of the ocean: by measuring the shape and gravity field of Earth, a discipline known as geodesy. David Sandwell of the Scripps Institution of Oceanography and Walter Smith of the National Oceanic and Atmospheric Administration have spent much of the past 25 years negotiating with military agencies and satellite operators to allow them access to measurements of the Earth’s gravity field and sea surface heights. The result of their efforts is a global data set that tells where the ridges and valleys are by showing where the planet’s gravity field varies.