Every tide height is measured from a zero line. That line is called a datum. When a table says high tide is 5.4 ft, it means 5.4 feet above that zero, not 5.4 feet of water.
The US and Canada use different zero lines. If you use tide tables on both sides of the border, it helps to know how they differ.
Why tides need a datum
Water depth changes all day. Charts and tide tables need a fixed reference so that the numbers work together.
Nautical charts show depths below the datum. Tide tables show how far the water is above it. Add the two and you get the actual depth at that moment. That is the basic idea, although for navigation you should use official charts and tools, not this site.
MLLW: the US zero line
NOAA uses Mean Lower Low Water, or MLLW, for tide predictions. NOAA defines it as "the average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch."
In plain words:
- Most US coasts have two low tides a day.
- Take the lower of the two, every day.
- Average them over a 19-year period.
The epoch in use on NOAA's datum page is 1983 through 2001. NOAA is moving to a newer 2002 to 2020 epoch (NOAA datum updates). A 19-year period is used because it covers an 18.6-year cycle in the moon's orbit that affects tides.
So on a US tide table, 0 ft is roughly the level of an average lower low tide. Since it is an average, plenty of daily lows fall below it. That is why minus tides, such as -0.3 ft, are fairly common in US tables.
You can see this on any US station page. The datum line appears under "Station details", and the chart says "Height above MLLW". Try Charleston, SC or Fort Pulaski, GA.
Other US datums you may see
NOAA publishes several other levels for each station. You may see them on NOAA's site or on flood maps.
| Datum | NOAA definition, in short |
|---|---|
| MHHW | Average of the higher high tide each day |
| MHW | Average of all high tides |
| MSL | Mean sea level, the average of hourly heights |
| MLW | Average of all low tides |
| MLLW | Average of the lower low tide each day |
All of these use the same 19-year epoch. MHHW tells you how high a normal day's top tide reaches, so a high tide well above it is worth a closer look.
Chart datum: the Canadian zero line
The Canadian Hydrographic Service (CHS) uses chart datum. CHS says chart datum is "the reference plane to which depths on a published chart, all tide height predictions, and most water level measurements are referred."
CHS sets it low on purpose. In its words, Canadian charts use datums that are "deliberately set low so that the water level will seldom fall below it."
On most Canadian coastal charts, that level is Lower Low Water, Large Tide, or LLWLT. Charts still label it "lowest normal tide" (LNT) because some older charts used other choices.
In plain words, Canadian zero is set near the lowest tides of a normal year, not the average daily low.
What this means for you
Because the Canadian zero sits lower than the US one, the same sea at the same moment would read differently on each system. A few practical effects:
- Minus tides are rare in Canada. Canadian lows seldom go below 0 m. In US tables, minus tides are a regular sight, especially around the new and full moon.
- Canadian heights look bigger. Heights are measured from a lower line, so a normal Canadian high tide shows a larger number.
- Don't compare numbers across the border. A 0.5 m low in Canada and a 1.6 ft low in the US don't describe the same level. Compare each station with its own normal range instead.
- Units differ too. US pages show feet. Canadian pages show metres.
Canadian tide tables use metres above chart datum.
Datum and depth are not the same
A datum is only a reference line. The ground below it can be anywhere. A sandbar can sit above datum and dry out at every low tide. A channel can be many feet below it.
This is why "the tide is 2 ft" never tells you how deep the water is where you stand. You would need to know the height of the ground too. Our boat launch window uses a default threshold for this reason, and you can change it to suit your ramp.
Weather moves the real level
Datums and predictions describe the tide from the moon and sun. They don't include wind or pressure. NOAA notes it cannot predict how wind, rain and river flow will affect the water.
So on a windy day, the real level can sit well above or below the table. Where a live gauge exists, our pages show the gap as "Gauge vs forecast". Measured levels use the same datum as the prediction, so the two lines on our chart are directly comparable.
Summary
- A datum is the zero line for tide heights.
- The US uses MLLW, the average daily lower low over a 19-year epoch.
- Canada uses chart datum, usually LLWLT, set near the lowest normal tides.
- Minus tides are common in US tables and rare in Canadian ones.
- Heights are not depths.
Nothing on this site is for navigation. For boating, use official NOAA or CHS charts and predictions.