The National Ocean Service (NOS) requires tidal datum information such as mean high water (MHW) and mean lower low water (MLLW) to support nautical charting, navigational safety, shoreline photogrammetry, and marine boundary determination. Tidal datum information is also needed for referencing NOS' bathymetric data to any one of the other vertical elevation reference systems so the data can be incorporated into integrated bathymetric-topographic Digital Elevation Models (DEMs) for use in coastal GIS applications. NOS routinely collects water level observations at shore-based stations along U.S. coasts and analyzes them to produce tidal datums. An important need is to obtain two-dimensional tidal datum fields that cover coastal waters between the water level stations. One method of simulating tidal datum fields takes values at the tide gauges and spatially interpolates them throughout the region. The values at the gauges which are spatially interpolated are the differences between a reference elevation datum such as local Mean Sea Level (MSL), a geoid, or an ellipsoidal surface and a specific tidal datum such as MHW, MLLW, mean higher high water, mean tide level, diurnal tide level, or mean low water. The spatial interpolation at the core of this method is accomplished by the use of a set of weighting functions that quantify the local contribution from each of the shore gauges. The weighting functions are generated numerically by solving Laplace's Equation on a grid. The interpolation method was applied to Puget Sound, Washington, to create datum fields on a regular grid with cell sizes of 278 m (0.15 nautical miles). Tidal datums for 57 stations were used. Similar tidal datum fields have been produced for Tampa Bay and coastal southern Louisiana to produce DEMs, and for the New York Bight, central coastal California, and Delaware Bay for other purposes. NOS is planning on producing tidal datum fields for all U.S. coastal waters using a combination of hydrodynamic models and spatial interpolation to support hydrographic surveying and other NOS projects.