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The planet is far from a smooth sphere; the radius of the globe at
the equator is about 20km longer than at the poles. This ellipsoid is
then marked by tall mountain ranges and cut by deep ocean trenches.
The Earth's interior layers are also not composed of perfect shells
of homogenous rock - some regions are thicker or denser.
Such factors will cause the gravitational force at the surface to
deviate from place to place by very small but significant amounts.
The Gravity Field and Steady-State Ocean Circulation Explorer (Goce)
will map these differences. This information will then be used to
fashion what is, in essence, an idealised globe. Scientists call it
the geoid.
The geoid is of paramount interest to oceanographers who study the
causes of the "hills" and "valleys" on the sea surface.
If local gravity differences are not creating these features, then
other factors such as currents, winds and tides must be responsible.