Tide datums explained

Every tide height is measured from a reference level called a datum, and the datum is the part of the chart most people never think about. It is why a “0.0 ft” tide still has water in it, why a “-1.2 ft” tide is not a typo, and why the same physical water level can carry different numbers at different stations. Ten minutes with datums makes every tide table you read afterward make more sense.

MLLW: the zero of US tide charts

In the United States, tide predictions and nautical chart depths are both referenced to Mean Lower Low Water, or MLLW. Most US coasts get two low tides a day of unequal size; MLLW is the average of the lower one, computed over a 19-year period called a tidal epoch. Setting zero at the average lowest water is deliberately conservative: the chart shows roughly the least depth you can expect on an ordinary day, so surprises tend to be extra water rather than missing water.

MSL and the other datums

Mean Sea Level, MSL, is the average of all water levels, and it sits well above MLLW anywhere with real tidal range. Between them live a family of relatives: MLW, MHW, and MHHW, averaging the lows, highs, and higher highs respectively. MHW matters legally in many states as the boundary of public shoreline, and MHHW is the base for coastal flood warnings. Elevation on land maps uses yet another reference, so a “5 foot tide” and a “5 foot elevation” are not measured from the same zero.

The chart depth math

Because US charts and tide tables share the MLLW datum, real depth is simple addition: charted depth plus predicted tide. A bar charted at 4 feet with a 2.5 foot tide has about 6.5 feet over it. On a -1 foot tide the same bar has about 3 feet. This one habit, adding the tide to the chart, is most of what “local knowledge” of depth actually is. For reading the rest of the table, see how to read a tide chart.

Negative tides

Since zero is an average of low tides, roughly half of all lower lows dip below it. Spring tides around the new and full moon push well into negative territory. These minus tides expose flats, rocks, and tidepools that stay covered the rest of the month, which is why clammers and tidepoolers watch for them. We cover planning those days in minus tides, clamming, and tidepooling.

Why stations differ

Each station’s datum is computed from its own 19-year record, so MLLW at one harbor is a different physical elevation than MLLW at the next. Ranges differ too, so the same moon produces a 2 foot swing at one station and 10 feet at another. Always read heights against the station they came from, not a neighbor.

Datums, handled by Slackwater

The Slackwater app computes heights on your device from NOAA harmonic data, already referenced to each station’s MLLW, so the number you see matches the local chart with no conversion needed. Browse tide times by location to see your station’s curve, negative tides and all.