Tidal datums and tide coordination
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The National Ocean Service (NOS) determines and charts the U.S. coastline, which is defined as the land-water interface when the water level coincides with either of two standard tidal datums: mean high water (MHW) or mean lower low water (MLLW). Delineation is accomplished by aerial photogrammetric surveys of the coast that are tide coordinated, i.e., timed to coincide closely with the time of MHW or MLLW. These times are called the tide windows. In NOS standard practice, the timing is determined from the predicted astronomical tide at one or two nearby locations in the survey region, As part of the modernization of tide coordination and to avoid installation of additional tide stations, NOS began developing a new way of utilizing the existing operational tide stations, the database of historical tidal data, and a tidal interpolation model to supply the necessary water level information throughout the entire region, The tidal interpolation model accesses the historical tidal database to predict water levels and datums everywhere along the shore and is applied to determine the times when the tide is within a given height above or below a datum (MHW or MLLW). For the post-flight analysis of shoreline photography, the tidal model generates a unique value of the water level for the precise time and location of the photographic image using the astronomical tide plus real-time observations, where available. By automating this data/model system, Web graphics can be generated, displayed, and archived for reference by NOS and other users of coastal photogrammetry.