Variance in the accuracy of tidal levels with increasing data length

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