A tide is the sum of a few dozen astronomical cycles. Each one has an amplitude and a phase that are fixed for a given harbour, so once those constants are known the water level can be computed for any moment, years ahead, with no network:
h(t) = Z₀ + Σ f(t) · A · cos( V(t) + u(t) − κ )
A and κ are the station's harmonic constants, V is the astronomical argument from the positions of the moon and sun, f and u are the corrections for the 18.6-year cycle of the moon's orbit, and Z₀ is mean sea level above the chart datum. Tidewise carries the constants for more than six thousand stations and evaluates this on your phone.
Predictions are checked against the official published tables on every release. Recent figures: San Francisco within 3.5 minutes and 2 cm; Kealakekua Bay, Hawaii within 4 minutes and 1 cm; Anchorage, Alaska — a nine-metre range needing 120 constituents — within 7.5 minutes and 11 cm.
Harmonic constants, subordinate-station corrections and datums for the United States, Hawaii, Alaska and its territories. Published by the National Oceanic and Atmospheric Administration, Center for Operational Oceanographic Products and Services. Public domain. tidesandcurrents.noaa.gov
Tidal constants derived from tide-gauge records worldwide, which give Tidewise its coverage outside the United States. Hart-Davis, M., Dettmering, D. and Seitz, F., Deutsches Geodätisches Forschungsinstitut, Technical University of Munich. Licensed under CC BY 4.0. doi.org/10.17882/109129
Weather, marine and sea-level forecasts. Licensed under CC BY 4.0. open-meteo.com
The closest gauge is not always the right one. Half Moon Bay sits on the open Pacific, but the nearest gauge is twenty kilometres away inside San Francisco Bay, where the tide arrives one hour and forty-four minutes later with half again the range. Tidewise compares the phase of each nearby station against the conditions at your spot and chooses the one that matches, and lets you override it by hand.
Everything the App shows is a prediction. Weather, storm surge and local conditions regularly push the real water level and sea state beyond any forecast. Use it to plan, never as your only source of judgement on rocks, jetties or a boat.
Feedback, or a station you would like added: tt94211@gmail.com