Tides · Methodology

How we calculate tide times

Marinalo calculates tides itself, for 2025 checked tide stations in 122 countries, covering 5877 places (521 with their own page). Every number comes from open data whose licence allows reuse, and every page names its source.

Sources

• NOAA CO-OPS (United States and territories, public domain): official harmonic constants of 1221 stations; heights above Mean Lower Low Water (MLLW), as on US charts.
• LINZ (New Zealand, CC BY 4.0): official harmonic constants of 56 ports; heights above chart datum. This work is based on LINZ data.
• TICON-4 (worldwide, Hart-Davis et al., DGFI-TUM, CC BY 4.0, doi:10.17882/109129): constants derived from tide gauge records, 748 stations kept after our cross-check; heights above Lowest Astronomical Tide, which we calculate over a full 19-year lunar cycle.
• EOT20 (DGFI-TUM, CC BY 4.0): an independent global tide model, used only as a second opinion.
• GeoNames (CC BY 4.0): names and positions of places.

The cross-check

A TICON-4 station is published only if an independent model, EOT20, agrees with it at the same place (model cell within 15 km): over 14 days, 90% of heights within 10 cm or 4% of the tidal range, and high and low tide times within 30 minutes (tidal ranges above 1.2 m). Stations where the two disagree, where no second opinion exists, with doubtful records, or where the tide is negligible (range under 0.3 m) are never shown. Official NOAA and LINZ stations are published directly.

Accuracy checked against official tables

Against official tide tables, our high and low tides were 5 to 10 minutes and 1 to 7 cm apart in Sydney (Bureau of Meteorology), Auckland (LINZ), Aberdeen and Bamfield; on 63 ports tested in Australia, Canada and the UK, no station that passes the rule gave wrong times.

Checked against NOAA, station by station

For 1145 US tide stations, we compare every high and low tide we calculate with the official NOAA prediction for the same station and the same datum (MLLW), over a month (20261014-20261113). Typical station: high and low tides 2.0 minutes apart and 0.9 cm apart (90% of tides within 4.0 min and 2.0 cm). 85% of stations stay within 10 minutes for 90% of their tides.

Limits

These are astronomical tides: wind, air pressure and river flow can raise or lower the water by tens of centimetres. A place uses the nearest checked station within 15 km, which the page names with its distance. Not for navigation: always check the official tide tables of your country before going out.