The water levels or currents expected from astronomical forcing alone, computed by summing constituents with their local amplitudes, phases and the year's nodal corrections. Predictions published in tide tables give times and heights of high and low water relative to chart datum. They do not include wind, pressure or river flow, so observed water can differ by a few centimetres on calm days and by metres in storms.
Tidal prediction says 1.2 metres at 1500, but with this northeaster sitting on us I'd add half a metre.
On calm days observed water usually stays within a few centimetres of a good prediction; a persistent offset points to a datum or gauge problem before weather.
Also heard as
- tide prediction
- tide tables
- predicted tide
Related terms
Harmonic analysis
Breaking an observed water level or current record into its tidal constituents to find each one's amplitude and phase.
Astronomical tide
The part of the water level caused only by the gravitational effects of the Moon and Sun.
Residual
The observed water level minus the predicted tide, showing the effect of weather, rivers and other non-tidal causes.
Reference station
A tide or current station with its own harmonic constants, for which full daily predictions are published.