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Tidal Datums and Tide Heights

What a tide height is measured from: chart datum, Lowest Astronomical Tide (LAT), Mean Lower Low Water (MLLW), and why datums differ between countries.

Every tide height is a measurement above a reference level called a datum. When a tide table says the water will reach 1.8 metres, it means 1.8 metres above the local tidal datum — not above the seabed and not above sea level in the everyday sense. Knowing the datum is what makes a height meaningful.

What a tidal datum is

A tidal datum is a reference elevation defined by a particular stage of the tide, averaged over a long period — conventionally about 19 years, which smooths out the 18.6-year cycle in the Moon’s orbit. Because it is an average of real observations, a datum is specific to a location and to the authority that computed it.

Chart datum and common references

Nautical charts use a low-water datum so that the actual depth is almost always at least the charted depth. This chart datum is the zero from which both charted depths and tide heights are reckoned. Different countries adopt different datums:

  • LAT (Lowest Astronomical Tide) — the lowest level predictable under average weather; used by the United Kingdom and many other nations.
  • MLLW (Mean Lower Low Water) — the average of the lower of each day's two low waters; the standard chart datum in the United States.
  • MSL (Mean Sea Level) — the long-term average water level, used mainly as a reference for land elevations rather than for charts.

High-water datums such as Mean High Water (MHW) and Mean Higher High Water (MHHW) are used for the opposite purpose, for example to define a shoreline or a bridge clearance.

Why heights are not directly comparable

Because datums differ, a height at one station cannot be compared directly with a height at another unless both use the same reference. A 0.5 metre low water measured from LAT is not the same water level as a 0.5 metre low water measured from MLLW. This is also why the same physical tide can be described by different numbers in different countries' tables.