A vertical reference computed from observed water levels: mean higher high water, mean lower low water, mean sea level and so on, each an average of one tidal phase over a fixed span of years. Tidal datums are local, because tidal range and mean level change from bay to bay, so each station has its own set tied to nearby bench marks. They underpin chart depths, shoreline boundaries, bridge clearances and flood thresholds.
Client wants the pier deck referenced to a tidal datum, so give them MHHW and MLLW off the station sheet.
Always state which datum a height is on. A number without a datum is the most common and most expensive mistake in coastal work.
Also heard as
- tide datum
- water level datum
Do not mix it up with
- NAVD: A geodetic datum is a fixed surface defined by levelling and gravity; tidal datums are averages of local water levels and vary along the coast.
Related terms
MLLW
Mean Lower Low Water: the average height of the lower of each day's two low waters, used as chart datum in the United States.
MHHW
Mean Higher High Water: the average height of the higher of each day's two high waters over the datum epoch.
MSL
Mean Sea Level: the plain average of a station's hourly water levels across the tidal datum epoch.
Tidal datum epoch
The fixed 19-year period over which water levels are averaged to compute official tidal datums.
Bench mark
A permanent, stable mark of known height near a tide station that preserves the station's datums.