Tide glossary
Quick definitions of the terms that show up on TideTurtle pages. Each entry links to a longer explanation. For the underlying method behind every number on the site, see the methodology page.
- Datum
- The reference surface that a tide height is measured from. Different sources use different datums; the same physical water level reads as different numbers under each.
- Spring tide
- A higher-than-average tide, with both larger highs and lower lows, that happens around new and full moons when sun and moon align.
- Neap tide
- A lower-than-average tide, with smaller highs and shallower lows, that happens around the quarter moons when sun and moon pull at right angles.
- Solunar
- An angler tradition that rates each day for fish-bite likelihood from moon transits and rise/set. Tradition, not science — TideTurtle treats it that way.
- Tidal coefficient
- A 20-to-120 number used in French and Iberian tide tables to label how big a tide is. Common on European pages, less so in the US and UK.
- Harmonic prediction
- The method NOAA and most national tide authorities use: decompose a long gauge record into sine waves, then sum them forward. The gold standard for tide prediction.
Common questions
What's the difference between spring and neap tides?
Spring and neap describe the size of the daily swing, not the season. Spring tides happen around new and full moons, when the sun and moon line up and their tide-generating pulls add together — the highs go higher, the lows go lower, the daily range is biggest. Neap tides happen around the quarter moons, when sun and moon are pulling at right angles to each other — the highs are lower than average, the lows are higher than average, the daily range is smallest. The difference between a spring and a neap on the same coast is typically 30 percent of the mean range.
Why does my tide page show a different datum than my paper chart?
TideTurtle pages name the datum each source uses. NOAA harmonic uses MLLW (mean lower low water). Open-Meteo Marine uses MSL (mean sea level). UK Environment Agency uses ODN (Ordnance Datum Newlyn). A paper navigation chart usually uses LAT (lowest astronomical tide), which sits a bit below all three of those. The same physical water level looks like a different number against each datum. The methodology page has a worked example.
What does 'tidal coefficient' on European pages mean?
Tidal coefficient is a French and Iberian convention. It's a single 20-to-120 number that summarises how big today's tide is on a given coast: 20 means the smallest neap of the year, 70 is an average tide, 120 is the largest spring. Tide tables in France, Portugal, and parts of Spain show the coefficient prominently. North-American and British tide tables usually don't — they show heights and ranges directly. Both approaches encode the same physics; the coefficient is just a quick label.
Not for navigation.