A specific 19-year span of observations chosen so that tidal datums average out the 18.6-year lunar nodal cycle and the yearly weather cycle. All official datums are expressed as if observed over the same epoch, which keeps stations comparable. Because sea level keeps changing, the epoch is replaced every 20 to 25 years, and stations with rapid land motion use a modified 5-year epoch built from their latest observations.
Those datums are on the old tidal datum epoch, so expect MLLW to jump when the new one is adopted.
When the epoch is updated, every datum at a station shifts at once, so drawings and permits should say which epoch their datum belongs to.
Also heard as
- NTDE
- National Tidal Datum Epoch
- datum epoch
Related terms
Nodal cycle
The 18.6-year cycle in the tilt of the Moon's orbit that slowly raises and lowers tidal ranges.
Tidal datum
A local zero for heights, computed by averaging one stage of the tide, such as high or low water, across many years.
Simultaneous comparison
Computing datums at a short-term station by comparing its record with a long-term control station over the same days.
Relative sea level trend
The long-term rate of change in mean sea level at a gauge, including both ocean rise and land motion.