The surface to which depths are referred on bathymetric maps and nautical charts is called a chart datum. It is a tidal datum in tidal waters, mean low water in the Atlantic Ocean and the Gulf of Mexico, and mean lower low water in the Pacific and Arctic Oceans. Special datums sometimes are used for a body of coastal water where tidal characteristics are significantly altered by the physiography of the basin; each is generally based on a selected tidal datum. Chart datums for the Great Lakes are low lake levels, defined as elevations referred to the IGLD of 1955 as determined from water level observations at selected gaging stations. Sloping datums are necessary for some connecting waterways because of the natural gradient of the waterway bed. Soundings, or measurements of water depth, made during bathymetric and hydrographic surveys, must be referred to the appropriate vertical datum. The preferred datum, especially in hydrographic surveying, is the chart datum. However, because the tide observations for establishing the datum are made during the course of the survey and several weeks are needed to establish the various datums at a tide station, the hydrographer usually refers his observations to a temporary sounding datum that is corrected to the final datum before map or chart production. National Geodetic Vertical Datum The National Geodetic Vertical Datum (NGVD) of 1929 (formerly called the Sea Level Datum of 1929) covers too large an area to be based on a single local tidal datum. It is therefore based on, but not necessarily equal to, mean sea level at 26 tide stations in the United States and Canada, with the best fit over a broad area resulting from the 1929 general adjustment of the United States and Canadian leveling networks. The NGVD of 1929 and local mean sea level cannot be interchanged because their relationship varies from place to place. All elevations in the National Vertical Control Network are referred to the NGVD of 1929. These elevations are FIGURE 2.-Tidal datums and their relations to a generalized shore cross section. Figure 4 shows the tidal heights in relation to a tidal day. used in topographic mapping, geodesy, engineering studies, construction work, and similar activities. The elevations are not applied directly to mapping boundaries that depend on a tidal datum (in tidal waters) or on a lake level (in the Great Lakes region). Both the NGVD of 1929 and the National Vertical Control Network are used in the conterminous United States and part of Alaska. Other geodetic vertical datums are used in Alaska, Puerto Rico, the Virgin Islands, Hawaii, Guam, other oceanic islands, and islands fairly near the mainland that cannot be connected to the NGVD by leveling. They are based on tidal datums (usually local mean sea level) where practical. TIDAL DATUMS AND LOCAL MARINE The mean high water line forms the boundary between sovereign and private property in 14 of the 24 States bordering tidal waters. It is shown as the shoreline on USGS topographic maps and NOS nautical charts. However, publication scales of those products are generally too small for them to be of practical benefit to boundary affairs in the coastal zone. Furthermore, tidal datums at many places were established for hydrographic surveys and may not be adequate for precise boundary applications. In seven of the ten exceptions noted above, the boundary between sovereign and private property is the mean low water line. That line is compiled on NOS nautical charts when scale permits. Except on the 1:10,000-scale NOS boundary series (app. 7, fig. 22) it is not compiled on most small-scale charts or on any topographic maps published by USGS. Anyone concerned with local boundaries based on tidal datums should assure himself that the maps and tidal datums he is using, or intends to use, are adeqate for the purpose. Advice, assistance, and products can be obtained from: Oceanographic Division, C-33 National Ocean Survey National Oceanic and Atmospheric Administration 6001 Executive Boulevard Rockville, Maryland 20852 Telephone: 301-443-8274 Coast FIGURE 3.—Interrelations of tidal datums. Mean sea level may coincide with half tide level, but seldom does; the variation (represented by the dot pattern) is generally about 3 cm (0.1 ft) and rarely exceeds 6 cm (0.2 ft). LAKE LEVELS AND RELATED LOCAL BOUNDARIES The Great Lakes States have selected several ways to define the boundary between public trust waters. and private property. Some States use the ordinary (mean) high water mark, others use a line which fluctuates with the lake level, and still others use a specific lake level. Except on Lake Michigan, the States have proprietary interest in the waters of the Great Lakes lying between the appropriate Stateprivate line and the boundary between the United States and Canada, subject to the interest of the general public (including Canada and other foreign nations) in navigation and access. The boundary between Michigan and Illinois and Wisconsin follows a large part of the approximate centerline of Lake Michigan. The States retain the right to control or limit access to natural resources in and under the water outside the line chosen to represent the Stateprivate boundary. Since there is no measurable rise and fall of tide in the Great Lakes, it is not relevant to attempt to determine the ordinary high water mark by analysis of water level gage records for diurnal or semidiurnal effects. The height of water levels in the Great Lakes varies in an irregular and apparently noncyclical pattern due to meterological conditions (rainfall and snowfall) in the upper watershed areas. There is no predictable cycle, but the variation is predominantly annual. Thus to determine the ordinary high water mark requires resolution and interpretation of water level gage recordings over many years. Advice, assistance, and products relative to the heights of the water surfaces of the Great Lakes can be obtained from: Tides and Water Levels Branch National Ocean Survey National Oceanic and Atmospheric Administration CONTROL SURVEYS Control, in general, consists of coordinated and correlated elevation or position data that form a framework to which detailed surveys are adjusted. Basic control is horizontal, vertical, or both, and is usually established with greater precision and accuracy than subsequent dependent surveys. Both horizontal and vertical control are fundamental to mapping and charting. Except for operations directly referenced to tidal datums, both horizontal and vertical control for coastal mapping must be established by geodetic methods. The two basic geodetic control networks in the United States are the National Horizontal Control Network and the National Vertical Control Network established and maintained by the National Geodetic Survey, NOS. Other Federal agencies such as USGS, Bureau of Reclamation, and U.S. Army Corps of Engineers (USCE), as well as many State agencies, establish geodetic control. For surveys of second or higher order accuracy, the instruments, methods, and techniques must meet the criteria set by NGS, and the records and observations acquired by the establishing agency are accepted by NGS and adjusted to the national network. The National Horizontal Control Network covers the conterminous United States and Alaska; independent networks cover Hawaii and the oceanic islands. The National Vertical Control Network covers the conterminous United States and part of Alaska; independent networks cover the rest of Alaska, Hawaii, and the oceanic islands. HORIZONTAL CONTROL Horizontal control makes it possible for cartographers to orient and scale their maps accurately, to position them properly on the Earth, and to compile details in correct positions and relations. The network maintained by NGS usually provides control sufficient for mapping and charting. The few gaps in the basic schemes can normally be bridged with modern photogrammetric techniques to avoid the expense of field geodetic surveys. VERTICAL CONTROL The first operational use of vertical control for coastal mapping is in photogrammetric aerotriangulation-a phase that does not necessarily require a vertical geodetic datum. Planimetric mapping can proceed with reference only to the horizontal geodetic datum. However, if relief is to be shown on the maps (that is, topographic maps) then a common vertical datum must be used. In the conterminous United States, the NGVD of 1929 is the vertical datum most suitable for showing relief. Local datums are used in Hawaii, the Pacific islands, Puerto Rico, the Virgin Islands, and parts of Alaska because the NGVD of 1929 does not extend to them. Each local datum is based on tidal datums established at selected tide stations and is specified in the geodetic control data provided for each of these areas. OTHER CONTROL The vertical control of greatest significance to coastal mapping is provided by tidal observations. The datum computed from the observation depends on tidal characteristics (fig. 4). In general, the tide along the Atlantic Coast is classified as semidiurnal (two nearly equal high waters and two nearly equal low waters each day). The elevations of tidal bench marks refer to mean low water. In addition, data are provided for mean high water, half tide level, mean sea level, and mean low water. In the Gulf of Mexico and the Pacific Ocean the tide is classified either as diurnal (one high water and one low water each day) or as mixed. The mixed tide is similar to the semidiurnal tide except that there is a marked difference in the height of the two high waters and (or) in the height of the two low waters each day (called diurnal inequality). Tidal bench marks along the Gulf Coast refer to mean low water and those along the Pacific Coast refer to mean lower low water. For many tide stations in the Gulf of Mexico, along the Pacific Coast, Alaska, Hawaii, and the Pacific islands, data are also provided for mean higher high water, mean high water, half tide level, mean sea level, mean low water, and mean lower low water. Mean sea level at any point is simply the mean level of the sea at that point. Technically, it is the arithmetic mean. of hourly water elevations observed over a specific 19-year Metonic cycle (the National Tidal Datum Epoch). Mean sea level is the primary tidal datum. All other tidal datums are derived from various tide stages, but are referenced to mean sea level. Local mean sea level is not generally valid elsewhere and should not be confused with the NGVD of 1929 which is an average of numerous local stations. |