<P> The ocean surface has highs and lows, similar to the hills and valleys of Earth's land surface depicted on a topographic map . These variations, called ocean surface topography (or sea surface topography), also dynamic topography, are mapped using direct (usually satellite - based) or indirect measurements of sea surface height relative to Earth's geoid . Earth's geoid is a calculated surface of equal gravitational potential energy and represents the shape the sea surface would be if the ocean were not in motion . The main purpose of measuring ocean surface topography is to understand the large - scale circulation of the ocean . </P> <P> On a daily basis, sea surface height (SSH) is most obviously affected by the tidal forces of the Moon and the Sun acting on the Earth . Over longer timescales, SSH is influenced by ocean circulation . Typically, SSH anomalies resulting from these forces differ from the mean by less than ± 1 m (3 ft) at the global scale . Other influences include temperature, salinity, tides, waves, and the loading of atmospheric pressure . The slowest and largest variations are due to changes in the Earth's gravitational field (geoid) due to the rearrangement of continents, formation of sea mounts and other redistribution of rock . </P>

What is dynamic topography used to determine in the oceans
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