Chart datum and MLLW: where tide heights are measured from

What chart datum is

To say the water “rises 6 feet,” you need to know where you’re measuring from. At sea there’s no floor or dock that works for everyone, so hydrographers pick a fixed reference level, the same for tide tables and nautical charts. That level is the chart datum.

Two things are measured from it:

• charted depths, the numbers printed on a nautical chart, which give the depth of the bottom below chart datum;
• tide heights, the numbers in a tide table, which give how far the water stands above chart datum.

So when a tide chart says high tide is 5.8 ft, the water surface will be 5.8 feet above chart datum. It’s not the depth of the harbor and it’s not the height of the waves. It’s where the sea surface sits relative to that zero.

Why the US uses mean lower low water

On most US coasts, there are two low tides a day, and one is usually lower than the other. NOAA takes the lower of the two lows for each day and averages it over a 19-year period called the National Tidal Datum Epoch (the current one runs from 1983 through 2001). The result is mean lower low water (MLLW), the zero for NOAA tide predictions and for the soundings on NOAA nautical charts.

Why 19 years? The moon’s orbit is tilted, and that tilt swings around over an 18.6-year cycle, making tides slightly bigger in some years and smaller in others. Nineteen years of data captures the whole cycle, so the average doesn’t depend on which years happened to be measured.

Picking a low reference is about safety. With the zero near the bottom of the normal tide range, the depth on the chart is close to the least water you’ll usually find, and on most days the tide adds water on top of it.

Minus tides: why US tide charts go below zero

MLLW is an average, not a floor. Roughly half of the daily lower lows fall below it, and on spring tides around the new and full moon, some fall well below it. Tide charts show those lows with a minus sign, like −0.2 ft or −1.5 ft. They’re called minus tides.

Minus tides are a big deal in some places. In the Pacific Northwest, where the range is large, the best summer minus tides can drop below −3 feet and expose acres of beach. Clam diggers, tide poolers and beachcombers plan whole trips around them, and Washington’s razor clam digs are scheduled on low tides. On the East Coast, a −0.5 ft low in Charleston or Savannah uncovers oyster rakes and flats that normally stay wet.

Minus tides also mean less water than the chart shows. A channel charted at 3 feet holds only about 1.5 feet on a −1.5 ft tide.

How much water do you have? Depth plus tide

The math is simple:

water depth = charted depth + tide height

Here’s a worked example using a spring tide day like the one in our guide on how to read a tide chart:

Tide	Time	Height
High	6:42 a.m.	5.8 ft
Low	12:55 p.m.	−0.2 ft
High	7:10 p.m.	6.1 ft

Say the mouth of a tidal creek is charted at 1 foot, your skiff draws 1 foot, and you want at least a foot of water under it, so you need 2 feet of water:

• At the 6:42 a.m. high: 1 + 5.8 = 6.8 feet. Plenty.
• At the 12:55 p.m. low: 1 + (−0.2) = 0.8 feet. Not enough; you’d be aground.
• You need the tide at 1 foot or more. Using the rule of twelfths on the afternoon rise, the tide passes 1 foot at around 2:30 p.m.

Two warnings. First, sandbars at inlets and creek mouths move with every storm, and a chart can be years out of date in those spots. Second, weather changes the water level: strong high pressure or a stiff offshore wind can leave the water several inches lower than predicted.

Other tidal datums you’ll see

NOAA publishes several reference levels for each tide station. The most common:

• MHHW (mean higher high water): the average of the higher of the two daily high tides. Coastal flood thresholds are often described relative to it, so a forecast of “2 feet above MHHW” means water well above a normal high tide.
• MHW and MLW (mean high water and mean low water): the average of all high tides and all low tides.
• MSL (mean sea level): the average of hourly water levels over the epoch. It sits roughly halfway between average high and low tide, so on a coast with a 6-foot range it’s around 3 feet above MLLW.
• LAT (lowest astronomical tide): the lowest tide predicted from the moon and sun alone. It’s lower than MLLW.

Other countries use a lower zero

Most countries don’t use MLLW. The International Hydrographic Organization recommends the lowest astronomical tide, or a level close to it, as chart datum where tides matter. The UK, Spain and many others follow it.

With a zero that low, negative tide heights are rare, and a low of 0.0 on a Spanish or British tide table is an exceptionally low tide. On a US chart, a 0.0 low is an ordinary one. If you ever compare tide tables from different countries, check the datum first.

Land elevations use a different zero

Elevations on topographic maps, flood maps and survey benchmarks in the US are referenced to a land datum, usually NAVD 88, not to MLLW. The difference between the two varies from place to place, often by a few feet.

That matters if you’re trying to figure out whether a king tide will reach a street or a yard: a tide prediction in feet above MLLW and a street elevation in feet above NAVD 88 can’t be compared directly. For many tide stations, NOAA publishes the offset between the two on the station’s datums page.

What to remember

• Tide heights are water above chart datum, not water depth.
• For the depth at a spot, add the tide height to the charted depth, then leave a margin.
• In the US, chart datum is MLLW, so minus tides are normal around the new and full moon.
• Check which zero a number uses before comparing tide tables, flood forecasts or maps.

If you head out on a big minus tide to dig clams or explore the flats, read our guide to tide safety first: the lowest lows uncover the most ground, and the water comes back fast.