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No. 27 (2022)

Variance in the Accuracy of Tidal Levels with Increasing Data Length

October 26, 2022


The Highest Astronomical Tide level (HAT) is derived from sea level datasets of sufficient length to encapsulate the lunar nodal cycle. HAT based on datasets of 35, 90, 180 and 400 days was determined for 14 sites. The difference between HAT based on short datasets and on 20-year datasets indicate that short datasets gave an average difference of 0.22 to 0.24 m. Increasing the length to 400 days decreased the difference to 0.06 m. Sites with diurnal form gave a larger difference of 0.4 to 0.9 metres. Inferring shallow water tidal constituents from the nearest longterm site reduced the difference to 0.15 to 0.19 m.