A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch. (See http://tidesandcurrents.noaa.gov/publications/glossary2.pdf or http://tidesandcurrents.noaa.gov/datum_options.html)