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Well, you’ve got me looking at charts more carefully now; so what the heck does Mean Lower Low Water mean? J.H., via e-mail
What you’re asking about is the expression printed on most U.S. paper charts right under the Title Block: “Soundings in Feet at Mean Lower Low Water.” It’s abbreviated as MLLW and often referenced as the Soundings Datum. Datum means reference point, which in the case of soundings literally means what a chart maker marks as zero depth—salt water lapping mud flat. It’s always some version of low tide so that the chart doesn’t show, say, anchoring depths where they sometimes aren’t. But there are several definitions of low. Before we get to the more difficult MLLW, let’s look at Mean Low Water (MLW), which NOAA used to favor. It is derived by calculating the middle low among all the predicted low tides during a Tidal Epoch, the 19 years it takes for the sun, moon, and other tidal factors to go through every possible permutation. Hence, when using a MLW chart, you will see the low go below the charted depths during half the tide cycles.

Most of the United States has semidiurnal (or twice daily) tides, and one drains lower than the other, a phenomenon splendidly labeled Diurnal Inequality. At some point NOAA decided to be more conservative and chucked each day’s higher low (watch the twist) out of their datum calculations, leaving Mean Lower Low Water. The greenish intertidal area on every U.S. chart got a little larger! Still—and here’s the bottom line, so to speak—you can be in a place one day where the chart says six feet deep and it really is six feet at low tide. But a few days later, probably near a full or new moon, it might be three feet deep. A tide table, also based on MLLW, should show a minus-three-feet low that day.

Think about datums around bridges, too. NOAA uses Mean High Water as its Height Datum, but, whereas half the high tides exceed the mean, there are still many times when there’s less height than charted. Thankfully most bridges have physical tide/clearance gauges to help you deal with this twist. Note that if you voyage outside the United States you’ll find a variety of datums, almost all more conservative than ours. Canada, for instance, bases soundings on Low Astronomical Tide, the absolute Epoch minimum, which is why you’ll never see a minus sign on its tide tables. There are also Horizontal Datum issues, which can twist you up if you’re plotting GPS coordinates on certain foreign charts (discussed in last month’s column). Finally, while much of this detail is noted on paper charts but not on electronic ones, a tool like graphic tide software can more than make up for the deficiency, especially if you’re aware of the twists.

Got a marine electronics question? Write to Electronics Q&A, Power & Motoryacht, 260 Madison Ave., 8th Fl., New York, NY 10016. Fax: (917) 256-2282. e-mail: PMYElectronics@primedia.com. For fastest response, visit the Electronics forum at www.powerandmotoryacht.com. No phone calls, please.

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