I see that many Admiralty Charts are now referring vertical clearance heights above Highest Astronomical Tide (HAT). Where can I find values of HAT?
Values of HAT can only be accurately obtained at specific locations via the investigation of daily predictions at those places over a significantly long period of time (ideally the full Metonic cycle of 18.6 years). For all Standard Ports, values of HAT are published in Table V of the relevant volume of Admiralty Tide Tables (ATT) – for the UK this is Volume 1 (NP201). However, at Secondary Ports we do not, as a matter of course, compute daily predictions for them, and are unable to provide accurate values of HAT. You may, however, utilise the information provided in the relevant volume of ATT to obtain inferred levels of HAT at Secondary Ports, using the linear relationships in High Water Height Differences between the Secondary Port, and its reference Standard Port. Information on how to do this is given in the preliminary pages of the relevant volume of ATT. In addition, our commercial tidal predictions software TotalTide displays values of HAT at all port locations for which the ne
