Tidal datums and their applications.

The United Nations declared 1998 to be the International Year of the Ocean. This declaration provides an opportunity to rais e public awareness of a fundamental boundary defined by the intersection of the ocean with th e land. This intersection is not as simple as it may seem. It is determined by a plane called a tidal datum, and refers to an average height of the water level at particular phases of the tidal cycle. This vertical reference surface is derived from water level measurements record ed along coastlines, estuaries, and tidal rivers of the United States. Tidal datum planes, refe renced to a system of bench marks, are fundamental to the determination of the spatial coordinates of latitude, longitude, and elevation relative to mean sea level. Tidal datums are chiefly used to determine horizontal boundaries, and for estimating heights or depths. The legal determinations of private and public lands, state owned tide lands, state submerged lands, U.S. Navigable wa ters, U.S. Territorial Sea, Contiguous Zone, and Exclusive Economic Zone, and the High Seas, or international waters, depend on the determination of tidal datums and their survey ed intersection with the coast. Navigation in U.S. Harbors, shipping channels, and intraco astal waterways requires an accurate knowledge of the depth of the ocean and submerged hazards at the low-water phase of the tidal cycle. Passage underneath bridges requires knowledge of the clearance at the high water phase of the tide. In addition, coastal construction and engineering requires knowledge of the tidal cycle; significant wave heights, periods, and directions; the heights of storm surges, or tsunami waves; and, the frequency and horizont al extent of flooding in the coastal zone. Organizing these environmental data into mean ingful, decision-making contexts requires the establishment of tidal datums, and their reference to the geodetic control network. Other countries publish tidal datums that ma y differ significantly from those of the U.S. In fact, there are hundreds of local datums used throughout the world. This has led to efforts to define a global vertical datum. The ellipso id serves as a suitable candidate because of its horizontal and vertical accuracy, its relative ease of calculation, and its global accessibility via GPS. A set of vertical transformation f unctions are required to translate the vertical coordinate provided by GPS into a coordinate referenced to a tidal datum plane. Preliminary research suggests promising results in the c onstruction of a seamless vertical reference system. This document has been prepared by NOAA’ s Center for Operational Oceanographic Products and Services Division to provide background information about tidal datum planes. The chapters present overviews of the histor y of tidal datums in the U.S., domestic and international legal regimes, water level measurement system and bench marks, derived products available from NOAA, and examples of the practical and legal applications of tidal datums.