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Revision of the Tidal Prediction Method at Falmouth
Correspondence with the Honorary Port Officer for Falmouth for The Little Ship Club has drawn attention to the revision of the tide prediction method and data for Falmouth in recent months.
The 2012 Admiralty Tide Tables and Reeds Almanac Tables for Falmouth have, for the first time, been produced using the ‘Harmonic Method’ of tide prediction. This is scientifically and statistically more accurate that the ‘Non-Harmonic’ method, which has been in use for many years. The refinement of data is based on 3 years of electronic tidal gauge readings collected by the Harbour Master’s team at Falmouth.
Heights of tide predicted by the new method are numerically lower than those calculated in previous years. The highest HW in 2011 predicted by the old method was 5.7m; the highest HW in 2012, which will reach a similar point on a post or wall, is predicted at 5.5m. In light of this, those sailors familiar with the port, and residents, who have memorable, ‘come in handy’, figures for key tidal heights should check them closely.
Why has this happened? The old predictions were based on a set of readings taken manually over a 3 month period many years ago. The new much longer period of measurement has allowed the re-calculation of the accepted value of ‘mean sea level’ at Falmouth from 3.2m to 3.0m above chart datum. This fact combined with the new ‘harmonic’ constituents has resulted in the levels predicted for tides being lower than before. The new data was first used in Admiralty tide prediction software ‘EasyTide’ and ‘Total Tide’ and for a time in 2011 these products produced different predictions to those made using the tables. The tide tables and software now give consistent results.