The Harmonic Constant Datum Method: Options for Overcoming Datum Discontinuities at Mixed–Diurnal Tidal Transitions

Abstract

The harmonic constant datum (HCD) method is a computationally efficient way of estimating tidal datums relative to mean sea level, without the need to compute long time series. However, datum discontinuities can occur between mixed and diurnal tidal regimes using this method. Solutions to this problem are investigated, with a hypothetical strait that contains a semidiurnal node, using three different procedures: algorithms specifically designed for diurnal tides (DTA), mixed tidal algorithms (MTA) throughout, and cubic polynomial interpolation (CPI) across the diurnal region. DTA creates small discontinuities (≤11% for the strait) in mean higher high water and mean lower low water but does not provide estimates of mean high water or mean low water. MTA gives continuous datums but creates artificial structures in the middle of the diurnal region. CPI provides smooth, continuous datums but does not use the tidal information within the diurnal regions. Which procedure works best depends on the size of the diurnal region and the application. The standard time series method can be used for limited transitional regions requiring high accuracy, with the efficient HCD method used elsewhere. However, the discontinuity issues still exist. Global distributions of datums computed by the HCD method are shown, based on the 0.5° ? 0.5° Oregon State University (OSU) TPXO 5.0 tide model.