Ocean surface topography Colors depict deviation from mean ocean surface caused by local differences in the Earths gravitational field Purple is up to 280 feet 85 m above mean through red orange yellow green to blue up to 345 feet [105 m below mean GFZPho

variety of techniques, including direct measurements of the gravity field on the surface, tracking of satellite positions (and deflections due to gravity), and satellite-based laser altimetry that can measure the height of the sea surface a fraction of an inch (the subcen-timeter level). Variations in the height of the geoid are typically tens to 50 feet, but range up to 450 feet (tens to even more than 100 meters).

The geoid is considered the baseline figure of the Earth, which can be considered a sea-level surface including local gravitational effects without taking into account topographic features. The geoid surface is continuous over the entire surface of the Earth. Calculating the surface of equal potential energy would yield a map close to that of the sea-surface topography as measured by satellite without the effect of motion of the water, but the true sea-surface topography differs from the geoid from the effects of ocean currents. oceanographers use this relationship to map ocean circulation patterns, including complex eddies, by removing the height of the sea surface caused by gravity and examining the remaining anomalies caused by motion of the water.

See also geophysics; supercontinent cycles.

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