Tidal datums: what tide heights actually measure

A tide prediction of 0.0 ft does not mean the water disappears. Tide heights are measured from a reference line called a tidal datum, and the one on every US tide chart is MLLW — Mean Lower Low Water: the average, over a 19-year cycle, of the lowerof each day's two low tides. Zero is simply “an ordinary bad low tide.” The water routinely dips below it — that's a minus tide — and there may still be plenty of depth under a boat at 0.0 ft, depending on the spot.

The family of datums

NOAA computes a ladder of averages for every station, all published on its datums table (Perigee shows the full set on each station page). The ones worth knowing: MLLW (chart zero), MLW (average of all low tides), MSL (mean sea level, the middle of the range), MHW (average high — the legal shoreline boundary in many states), and MHHW(average of the higher daily highs — the flood-warning reference). Two derived numbers summarize a station's character: the mean range (MHW − MLW) and the great diurnal range (MHHW − MLLW), the honest answer to “how big are the tides here?”

Why zero sits at the bottom

Nautical charts choose MLLW as zero for a defensive reason: the charted depth should be the depth you can count on at almost any tide. If the chart says 12 ft and the tide table says +3.5 ft, you have roughly 15.5 ft of water; on a −1.0 ft minus tide you have 11. Add the predicted tide to the charted depth — that arithmetic only works because both numbers share the same zero.

Flooding is measured from the other end

Coastal flood thresholds key off MHHW, not MLLW. A water level of “2 ft above MHHW” means two feet above the average of the highest daily tides — real streets-and-parking- lots territory in low-lying towns. This is why king tides matter: they push water a foot or more above MHHW with no storm at all, marking exactly where chronic flooding arrives first as sea level rises.

One more zero to not confuse

Land elevations (like those on FEMA flood maps) are measured from a geodeticdatum, NAVD88, which is fixed to the continent rather than the tides. At most stations MLLW and NAVD88 differ by several feet, and NOAA publishes the local conversion on each station's datum table. Comparing a tide height to a ground elevation without converting is the classic way to get coastal flood risk wrong. When in doubt: tide numbers live on the MLLW ladder, land numbers on NAVD88, and the datums table on your station's page is the bridge between them.