On the variation of LAT resulting from predictions computed using different tidal constituent sets

While the Lowest Astronomical Tide (LAT) has a clear definition as the lowest water level caused by astronomical attractions at a water level station, there are variations in the implementation schemes for practically deriving LAT from a time series of tides. This article explores the variation resulting from different tide constituent sets, as named by the numbers, 25, 37, or 60 constituents. Hourly observations of 34 stations around Taiwan were examined in this study. Using LAT-60, the LAT with 60 constituents, as the reference, when the nodal correction is applied, the average deviation of LAT-25 is −2.12 cm, LAT-37 is 0.70 cm, and ISLW is 22.44 cm. Based on the experiments conducted, it is found that the difference between LAT-25 and LAT-60 is 1.18 cm as measured by the third quartile without nodal correction, and 0.89 cm with nodal correction. Nodal correction will lower the water level about 2 cm on average and maximum about 6.7 cm. ISLW is higher and has less influence by nodal correction than LAT so it is difficult to be regarded as a fairly stable water level for chart datum.