Tides
Zora works out the tide on the boat, for any point on Earth and any date, with no internet. This is where its numbers come from and how far to trust them. To read the tide, see Sun & Moon.
How Zora predicts the tide
A tide is the sum of many regular waves, each set by the moon and the sun. Their timing is pure astronomy and the same everywhere. How big each wave is, and when it peaks, depends on the place. Give Zora those numbers for a place and it can work out the tide there for any moment.
Zora gets the tide from three sources, best first:
- A national tide service, when the boat is online, in a country's waters, and that country publishes its gauges. Today that is NOAA in the United States and the Canadian Hydrographic Service in Canada. For the gauge that answers at the boat's position, Zora keeps about four weeks of its tide on board, so the reading still works once you are offline.
- A tide gauge nearby, from TICON-4, a catalogue of about 3,000 gauges around the world that Zora carries on board. Each is worked out from that port's own measurements, and answers for the water close around it. Show them on the chart with the Tide Gauges overlay, and tap one for that port's tide.
- FES2022, a global tide model built from years of satellite measurements, with a value about every 4 km up to the coast.
A real gauge always wins over the model.
How far to trust it
The icon on the tide says how good the reading is:
| Icon | Title | What it means |
|---|---|---|
| Green check | Real-time station data or Tide source | A gauge in good standing, or the model out at sea. Trust it. |
| Grey check | Station data — flagged at source | A real gauge whose owner has doubts about its record. Better than the model, but check it. |
| Amber warning | Coastal waters — model estimate | The model, within 12 nautical miles of the coast, with no gauge to go on. Near the coast the global model is least sure; use the local tide tables. |
| Red warning | Tide quality unavailable | Zora cannot say how good the reading is. |
In the open ocean the model is the accepted reference. Close to land, where bays and channels change the tide, a gauge is what counts, and that is why the model is marked amber there.
Which zero
A tide height is measured from a zero, the datum, and countries use different ones. Zora does not convert between them. It shows the height from the zero its source uses, so the number matches that source's tide table. The source's details name the datum.
Zora's own predictions, from a nearby gauge or the model, are measured from the lowest astronomical tide, the zero most charts use for their soundings. A height of 1.2 m is 1.2 m more water than the chart shows.
If the height does not match your paper tide table, check which zero the table uses. The times of high and low water should still agree.
How accurate it is
At the open coast the model is good to about a minute and 10 cm. A gauge is as good as that port's own record. In bays, estuaries and channels that amplify the tide, the model can be well off; that is what the amber icon warns of.
The prediction is astronomy only. A gale on shore, a deep low or a storm surge can pile the water well above the predicted height, and a strong offshore wind can leave less water than predicted. Read the tide as the baseline, and the weather on top of it.
Sources
- FES2022 Tide was funded by CNES, produced by LEGOS, NOVELTIS and CLS, and is made freely available by AVISO.
- TICON-4: tidal constants based on GESLA-4 sea-level records, by Michael Hart-Davis, Denise Dettmering and Florian Seitz (2025), DGFI-TUM, published by SEANOE under CC BY 4.0 (doi.org/10.17882/109129). Derived in part from tide-gauge records generated by the E.U. Copernicus Marine Service.
- Station data: NOAA CO-OPS (United States) and the Canadian Hydrographic Service.