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The shape of the sea surface assuming a complete absence of perturbing forces (tides, wind, currents, etc.). The geoid reflects the Earth’s gravitational field (it is an equipotential surface) and varies in height by as much as 100 metres over distances of several thousand kilometres due to uneven mass distribution within the planet’s crust, mantle and core. Other, less pronounced, irregularities are also visible over smaller distances. These reflect the ocean’s bottom topography.