Accurate surveying relies on knowledge of the Earth's gravity field, which defines the horizontal. However, as a result of a number of factors, such as the fact that mountains and ocean trenches make the surface of the planet uneven and materials within the Earth's interior are not uniformly distributed, the force of gravity actually varies slightly from place to place on the surface of the Earth.
Due for launch in early 2008, ESA's gravity mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) will provide an extremely accurate and unique picture of the Earth's gravity field and geoid. The geoid – which is defined by the Earth's gravity field – is a surface of equal gravitational potential and serves as a reference point from which to map all topographical features on the planet. GOCE will provide a model of the geoid with unprecedented accuracy and will not only be of importance in the field of geodesy, but will also lead to new insights into ocean circulation, climate change, sea-level rise, earthquakes and volcanism.
Over the last 10 years an improved knowledge of the geoid has revolutionised height determination on land by GPS satellites. However, there remain discrepancies - as in the example of Mount Everest where the official height in 1999 was stated as 8850 m, but the Chinese official survey in 2005 stated it as being 8844.43 m. Although this discrepancy may not seem huge when referring to the highest point on Earth, a global unified height system would iron out the main errors so that mountain ranges on one continent could be accurately measured against those on the other side of the world. Given the harsh and remote environments of many of the Earth's large mountain ranges such as the Himalayas or the Andes, an accurate map of the gravity field acquired from space will contribute significantly to geodesy and surveying.