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To obtain the geopotential height (h) in (geopotential) metres (of the land and sea surface or at particular heights in the atmosphere), simply divide the geopotential by the Earth's gravitational acceleration, which has a fixed value of 9.80665 m/s2 in the IFS. This geopotential height is relative to the geoid (over ocean, mean sea level is assumed to be coincident with the geoid) - for more information see ERA5: data documentation - spatial reference systems.
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