Epoch, National Tidal Datum—A period of 19 years adopted by the National Ocean Service as the period over which observation of tides are to be taken and reduced to average values for tidal datums. The epoch is designated by giving the year the period began and the year it ended, e.g. National Tidal Datum Epoch of 1941 through 1959.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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