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Understanding Tide Datums

A datum is the reference level that tide heights are measured from. Different regions use different datums, so the same tide will show different height values depending on which datum you choose.

LAT Lowest Astronomical Tide Recommended

The lowest tide level that can be predicted under normal conditions. Used as the international standard by the IHO (International Hydrographic Organization). All tide heights above LAT are positive, making it intuitive to read. Widely used in the UK, Europe, Australia, and most maritime charts worldwide.

MLLW Mean Lower Low Water

The average height of the lower of the two daily low tides, computed over a 19-year tidal epoch. Primarily used in the United States, the Philippines, and some Pacific nations. Heights can occasionally go negative during especially low tides.

MSL Mean Sea Level

The average water level over time. Used in scientific and engineering contexts. Tide heights will be both positive (above average) and negative (below average), showing how the tide oscillates around the mean.

Which should I choose?

LAT is recommended for most users because all heights are positive and it matches international nautical charts. If you are in the United States, you may prefer MLLW to match NOAA tide tables. MSL is useful if you want to understand how far above or below the average water level the tide is.

The tidal range (difference between high and low tide) is the same regardless of which datum you choose — only the reference point changes.

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