Where the tides come from

Every High Water Prints poster is drawn from real tide predictions for the spot you pick, for every day of 2027. Here is how we get them and what to expect.

Predictions, not measurements

Tides follow the Moon and Sun so precisely that they can be predicted years ahead. The times and heights on your poster are predictions of that astronomical tide. Weather moves the real sea: a storm surge, a strong onshore wind or a spell of low pressure can raise or lower the water, and shift its timing, by a good deal. Expect the printed time of high tide to be close on a normal day, not exact to the minute.

Two sources, so most coasts are covered

If an official tide station or tide gauge is within 30 km of your pin, we use the tide constants measured there. The stations come from public records of national hydrographic offices, including NOAA in the United States, and from a global archive of tide-gauge analyses.

Where there is no station that near, we use a global model of the open-water tide, as long as open water is close. Where a nearby station and the model describe noticeably different water, such as a sheltered sound beside an ocean beach, the designer asks which one you mean.

We turn a pin down when it is on land, far out at sea, too far from open water, or where the tide rises and falls by less than 10 cm, because there is nothing worth drawing.

Times and heights

• Times are local time at your pin, including daylight saving where it applies. The day the clocks change shows as a one-hour shift.
• Heights are in metres above average low water, so zero is a typical low tide. A negative height is a lower-than-usual low.
• A river mouth or a harbour behind a barrier can differ from the sea outside it. Put the pin on the water you care about.

Not for navigation

These posters are made for the wall. Please don't use them to plan a crossing, a swim or any activity where the exact tide matters for safety. Use an official tide table for that.

Credits

• Tide stations: the open Neaps tide database, built from NOAA, the Norwegian Mapping Authority (CC BY 4.0), TICON-4 (Hart-Davis, Dettmering and Seitz, 2025, CC BY 4.0) and other national sources.
• Open-water tides: EOT20 ocean tide model, Hart-Davis et al. (2021), DGFI-TUM, licensed CC BY 4.0, doi.org/10.17882/79489.
• Map: © OpenStreetMap contributors, tiles from OpenFreeMap. Place search by Photon.

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