Lowest Astronomical Tide in the North Sea derived from a vertically referenced shallow water model, and an assessment of its suggested sense of safety

Water level reduction with GNSS [FIG Commission 4 Working Group 4.2 2006, Dodd and Mills 2011] requires the availability of the ellipsoidal heights of CD, i.e. the Lowest Astronomical Tide (LAT) as adopted by the International Hydrographic Organization [International Hydrographic Organization, 2011, Technical Resolution 3/1919]. LAT is defined as the lowest tide level to occur under average meteorological conditions and under any combination of astronomical conditions. Ideally, the ellipsoidal heights of LAT are derived from the analysis of time series of observed water levels relative to a reference ellipsoid at tide gauges, GNSS buoys, or satellite radar altimeter crosstracks. Here, radar altimeter data are crucial in case one aims to derive an area covering surface, since only these data have a homogeneous spatial coverage. In shallow water, however, the limited temporal resolution of radar altimeter data does not allow to resolve all relevant tidal constituents. Hence, in order to obtain an accurate, area covering realization of the ellipsoidal heights of LAT in these waters, LAT values determined at tide gauge stations are complemented by LAT values derived from a shallow water tide model after adding the modeled LAT values to an altimetry-derived MSL model [e.g., Simon, 2001, Turner et al., 2010] including some interpolation procedure to bridge the gap between tide gauge stations and MSL data from satellite radar altimetry. Here, the MSL represents the water level under average meteorological conditions and is identified as the model’s reference surface. This procedure, however, cannot be applied in coastal regions and estuaries, since these regions lack reliable radar altimeter data (i.e. information about MSL). Here we develop an alternative procedure for LAT realization in coastal waters that does not rely upon MSL data from satellite radar altimetry. The procedure will be used in the future to realize LAT in Dutch coastal waters, the Wadden Sea, and the Western and Eastern Scheldt estuaries.