What the controls do, and how to read the chart and the numbers underneath it. For where the underlying data comes from, see about & data sources.
The date field is a UK local calendar day (midnight to midnight in Europe/London time, so it automatically accounts for GMT/BST) — not UTC. Any date from 1970 to 2037 is accepted; what you actually get back depends on which date you pick, covered below. Resolution sets how many points are plotted per day, from every 5 to every 30 minutes; it doesn't change the calculation itself, just how finely it's sampled. Show sets how many days are plotted at once, from a single day up to two weeks — on a multi-day chart, both the hover readout and the high/low table show the date alongside the time, since the same time of day repeats once per day.
Pure astronomical prediction (the dashed blue line) only accounts for the Sun and Moon — it can't know about the weather, so it misses storm surge: the extra rise in sea level a deep low-pressure system and strong onshore wind can pile up on top of the normal tide. See it on the chart: 13 December 2011, over a few days — a real storm surge added roughly 1.5 m on top of what the tide alone predicts that day (where this deployment also shows the real gauge reading, that shows up directly as the gap between the red and blue lines). The solid teal weather-corrected line, trained on real pressure and wind readings from that same event, captures most of this; the dashed blue harmonic-only line has no way to, since it never sees any weather data at all. Across the full held-out test period this measurably cuts typical prediction error, most noticeably on days like this one — see about & data sources for the full accuracy figures and how the weather-correction model itself was built.
A dashed, slightly faded blue line: the pure astronomical prediction, no weather involved. Always present, for any date in range.
A solid teal line: the astronomical prediction plus the machine-learned weather correction. Only appears when weather data is available for that date (see "when there's no line" below).
A solid red line: what the gauge actually recorded, for dates between 2002-01-01 and 2012-04-10 (the span of the real measured record this whole site is built from) — shown only on deployments that have permission to publicly redistribute the underlying gauge data itself. The astronomical and weather-corrected predictions above don't depend on that and are always shown regardless.
The small high and low dots mark the astronomical high and low tide times and heights for the day (these come from the harmonic-only line specifically, not the weather-corrected one).
historic weather means the weather-corrected line used real, measured pressure and wind readings for that date. live forecast means it used a live weather forecast instead, because the date is in the future — that line then carries the forecast's own uncertainty on top of the model's, and obviously can't be checked against an observed line yet.
When a real observed line is available, the panel below the chart compares both predictions against it for that day, using two numbers:
The teal line and the validation panel's second column only appear when weather data is actually available for the date you picked:
The astronomical-only line itself has no such gap — it's available for the whole 1970–2037 range regardless.