What is a tidal epoch?
Tidal variations can be considered as comprised of periodic and apparent secular trends. A specific 19-year period based upon the Metonic cycle is selected so that all tidal datum determinations will have a common reference period. This period is termed a tidal epoch. For tide stations located in the United States and its possessions, this period is called the National Tidal Datum Epoch (NTDE). The present NTDE is the period 1983 through 2001. Back to top 7. Why is it important to use a particular tidal epoch? Tidal records typically contain secular trends in addition to the periodic variations. Over time, the secular trend can alter the relative elevation between tidal and related terrestrial datums. When comparing elevations which are based upon tidal datums, one should insure that common epochs are used in order to account for the effects of secular trends. Moreover, the presence of a secular trend can cause changes in related tidal datums, such as MLLW and MHHW. Back to top 8. How
