Glossary

Datum

A tide height is meaningless without naming the surface it is measured from. That surface is the datum. The number 1.4 m by itself says nothing — 1.4 m above what? The answer is the datum.

The four datums you see on TideTurtle and on tide-related sources generally:

• MLLW (mean lower low water) — average of the lower of each day's two low tides over a 19-year tidal epoch. The standard datum on US nautical charts and NOAA tide tables.
• MSL (mean sea level) — long-term average sea surface, with no tidal cycle. Open-Meteo Marine uses MSL.
• ODN (Ordnance Datum Newlyn) — geodetic datum for British land surveying, anchored to long-term mean sea level at Newlyn in Cornwall. UK Environment Agency gauges report against ODN.
• LAT (lowest astronomical tide) — lowest tide predictable from astronomical forcing alone over 19 years. The datum on most modern navigation charts (UKHO, BSH, French Service Hydrographique). Lower than the others, by definition.

Converting between them needs a published offset for the specific location. At Cuxhaven the offset between MSL and LAT is roughly 1.7 m; at Honolulu it is around 0.3 m. That is why a tide height on TideTurtle and a tide height on a paper chart for the same place at the same time can differ by half a metre or more without either being wrong. Both are correct against their own datum. See harmonic prediction for how the gauge record is turned into a forecast, and methodology for the full source-by-source picture.