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.

Nodal tides, which follow an 18.6-year cycle, influence tidal variations at any given location in the ocean. Conventional nodal tide theory neglects land effects and topological change. Due to the complex seabed topography around Taiwan waters, the purpose of this paper is to use the long-term tidal data of six stations to discuss the effects of perigean and nodal tides on 20 constituents and to compare the results with previous theories. A modulation method is employed to fit the annual amplitude estimated by harmonic analysis (HA). The top four constituents of the fitted and theoretical values of nodal amplitude factor (AF) and phase factor (PF) are O₁, K₁, K₂, and Q₁. We find that perigean tides or second-order nodal tides considered in the fitting contribute to almost identical performance. The linear time change considered in the AF fitting has better fitting than the mean water level involved.