British Admiralty Tide Tables

The Admiralty Tide Tables (ATT) are the annual UKHO publication, NP201 to NP208, giving predicted times and heights of high and low water for standard ports worldwide, plus secondary-port differences and tidal-stream data. Predicted heights are referenced to chart datum, which the UKHO sets at approximately Lowest Astronomical Tide so that charted depths add to the tabulated height. The tables use harmonic prediction from the full constituent set and are issued in eight regional volumes. They are the primary tidal source for under-keel and tide-window planning on Admiralty charts, alongside the EasyTide digital service.