Tidal Datum Modeling in Support of NOAA's Vertical Datum Transformation Tool

Many coastal modeling applications rely on accurate bathymetry and/or topography data. The best data sets are often synthesized products with data collected from several sources which may be referenced to different vertical datums including Mean Low Water (MLW), Mean Lower Low Water (MLLW), North American Verical Datum of 1988 (NAVD 88), and others, depending on the age and source of the data. Transforming all of the data to a common vertical datum is a non-trivial problem, and so the National Ocean Service (NOS) of the National Oceanic and Atmospheric Administration (NOAA) has created a software tool called VDatum, which is designed to transform among approximately 30 vertical reference datums (including ellipsoidal, orthometric, and tidal datums). This program transforms among ellipsoidal datums using Helmert transformations, between the North American Datum of 1983 (NAD 83) (an ellipsoidal datum) and NAVD 88 (an orthometric datum) using a geoid model; between NAVD 88 and Mean Sea Level (MSL) using a topography of the sea surface model; and among tidal datums using tidal datum models. Regions where tidal datum models have been implemented include: Puget Sound and the Strait of Juan de Fuca, central California, southern coastal Louisiana, Lake Charles, Louisiana, Tampa Bay, the Outer Banks area in North Carolina, Delaware Bay, the New York Bight and Long Island Sound. A large collaboration of partners within NOS has participated to create this tool. The focus of this paper is on the work done in the Coast Survey Development Laboratory (CSDL) on the tidal datum modeling. Three types of models that have been employed to determine the tidal datum fields for the regional scale models are discussed: spatial interpolation of the datums, computation of the datums from a harmonic constant database, and computation of the datums from a regional hydrodynamic model. The results from these models are then interpolated onto the structured grid used with the VDatum software.