What is chart datum for?
A nautical chart gives the charted depth (sounding): the depth of water below chart datum. An official tide table gives the height of tide above that same datum. Because both start from the same level, you add them:
Since chart datum lies below almost every low water, the heights in an official table are nearly always positive. Only in unusual weather, or at exceptional tides, can the water drop below it, and then the table shows a negative low water.
What does each country use?
| Country | Official tide source | Chart datum |
|---|---|---|
| United States | NOAA Tides & Currents | Mean Lower Low Water (MLLW): the average of the lower low water of each tidal day over the 19-year National Tidal Datum Epoch (currently 1983–2001) |
| Canada | Canadian Hydrographic Service | on most coastal charts, Lower Low Water, Large Tide (LLWLT) |
| France | SHOM | zéro hydrographique, the Lowest Astronomical Tide or a level very close to it |
| Spain | Instituto Hidrográfico de la Marina | Lowest Astronomical Tide (LAT) |
| Portugal | Instituto Hidrográfico | zero hidrográfico (ZH), set below the Lowest Astronomical Tide; on the mainland, 2.00 m below the adopted mean sea level (Cascais 1938) |
| Mexico | Servicio Mareográfico Nacional (UNAM) | check the datum stated by the official source for each station |
The International Hydrographic Organization recommends LAT as chart datum in areas with a noticeable tide. NOAA's MLLW is an average of low waters, so it sits higher than LAT: that is why negative low tides, such as −1.2 ft, are common in US tables around spring tides.
How is chart datum worked out?
Ideally a tide gauge would record for 19 years, to cover the 18.6-year cycle of the Moon's orbit (the nodal cycle). In practice, hydrographic offices compute harmonic constants from at least a year of data and predict 19 years of tides; from that prediction they take the level they need (the lowest tide for LAT, an average for MLLW). The datum of each port is a published value, revised from time to time.
And on Tides Pro?
Times do not change with the reference: only the height figure does. When you compare two tide tables, check their datum first: the same tide can differ by half a metre, or several metres, depending on the reference. Or compare the range between high and low water, which is the same whatever the datum.
What is chart datum?
The reference level from which the depths on nautical charts and the heights in official tide tables are measured. It is set near the lowest tides, so official heights are almost always positive.
What chart datum does NOAA use?
Mean Lower Low Water (MLLW): the average of the lower of the two daily low waters over the National Tidal Datum Epoch, currently 1983–2001.
What chart datum does Canada use?
On most Canadian coastal charts, the Canadian Hydrographic Service uses Lower Low Water, Large Tide (LLWLT).
How much water is there at a point on the chart?
Water available = charted depth + height of tide, both measured from chart datum.
Why does Tides Pro sometimes show negative low tides?
Because at ports without an official datum value we give heights above mean sea level, and low waters fall below it. Below MLLW, US tables also show negative low tides at spring tides.
Sources: NOAA Tides & Currents — Tidal datums · Canadian Hydrographic Service — Chart No. 1 · International Hydrographic Organization · SHOM — Marées · Instituto Hidrográfico de la Marina — Cero hidrográfico · DGT (Portugal) — bathymetric lines and chart datum