This is because the density of the earth is not uniform; the earth is not a perfect sphere and therefore the height of the surface above the core varies from ocean deeps to high mountains. The gravitational pull of the earth is inversely proportional to the height one is above it - if you are far enough from the surface you experience weightlessness, after all.
The European Space Agency launched the Goce satellite into a low earth orbit in March 2009 to map the variations of the earth's gravitational field. Goce, or the Gravity field and steady-state Ocean Circulation Explorer was launched with the mission of mapping the earth's gravitational field in greater detail than ever before and producing a geoid, or accurate gravity map of the globe.The geoid will produce a sea surface height reference model which will be key to accurate surveys of the ocean water circulation patterns which play a large influence on climate.
It will also assist in monitoring sea-level changes which may be triggered by global warming and help to refine climate models through knowledge of how gravity affects ocean circulation patterns and sea level.
In combination with other tools, Goce will also help to refine estimates of the thickness of polar ice sheets which is again critical in monitoring climate change.
The mission should also shed light on the earth's interior and magma distribution patterns under volcanoes. This should lead to improved understanding of tectonic movements and seismic hazards (i.e. earthquakes).
ESA has used data from the mission to develop an exaggerated rendering of the globe which shows gravitational variability through colour.
The variation in the earth's gravitational field is measured with the aid of three precision engineered platinum blocks within the satellite's gradiometer instrument which are used to determine the tiny accelerations that variability in the earth's gravitational field imparts to them.
If additional funding can be provided, the spacecraft can continue to gather data well into 2014.