Definition
Mean sea level is the average height of the sea over a long period, typically 19 years, at a specified place, and it is the basis of national height systems. In Great Britain, Ordnance Datum Newlyn is defined from mean sea level measured at Newlyn between 1915 and 1921 and is the zero for land contours and spot heights. Land map heights therefore differ from chart elevations, which use MHWS, and from soundings, which use chart datum.
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