Many sources of water level and current data are available. The National 

 Ocean Survey of the National Oceanic and Atmospheric Administration is 

 responsible for monitoring sea level variations at 115 station locations nation- 

 wide (Hicks 1972). Coastal Corps of Engineers District offices collect tidal 

 elevation data at additional locations. Daily readings are published in reports 

 that are titled "Stages and Discharges of the (location of District office) 

 District." Predicted water levels and tidal current information for each day 

 can be obtained from the annual "Tide Tables: High and Low Water 

 Predictions" and "Tidal Current Tables" published by the National Ocean 

 Service (NOS). A convenient way to obtain daily tides is a personal computer 

 (PC) program called TIDEMASTER. ' Background information concerning 

 tidal datums and tide stations can be found in NOS publications titled "Index 

 of Tide Stations: United States of America and Miscellaneous Other Stations," 

 and "National Ocean Service Products and Services Handbook." 



An important consideration for evaluating water-level information or for 

 constructing and examining shoreline change maps is the level and type of 

 datum used. Because water levels are not constant over space and over time, 

 datums must be established from which depth and elevation changes can be 

 referenced. Common water level datums include mean lower low water 

 (mllw), mean low water (mlw), mean sea level (msl), mean tide level (mtl), 

 mean high water (mhw), and mean higher high water (mhhw) (Figure 1 1 and 

 Table 1). Of these, msl is most often used and is computed as the arithmetic 

 means of hourly water elevations observed over a specific 19-year cycle. 

 Some areas of the United States have established regional datums, based on 

 combinations of other datums, or based on local measurements of water level 

 over different periods. Often these water level datums are cited in reference 

 to fixed surfaces for land surveys; namely, the National Geodetic Vertical 

 Datum (NGVD) developed in 1929, and the North American Datum of 1983 

 (NAD 83). Specific definitions of the various datums and the relationship 

 between major water-level datums and geodetic datums are listed in references 

 from the NOS and HQUSACE (1989). Note that the land benchmarks, which 

 represent the various datums, can move because of the factors described 

 earlier. Therefore, datums must be corrected and updated periodically. 



Low water reference datums used on the Great Lakes and their connecting 

 waterways are currently based on the International Great Lakes Datum (IGLD) 

 1985. This datum, which was established and revised under the auspices of 

 the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic 

 Data, was implemented in January 1992, and replaces IGLD 1955. The main 

 difference between IGLD 1955 and IGLD 1985 is corrections in the elevations 

 assigned to water levels (Table 2). This is a result of benchmark elevation 

 changes due to adjustments for crustal movements, more accurate measure- 

 ment of elevation differences, a new reference zero point location, and an 



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Commercially available from Zephyr Services, 1900 Murray Ave., Pittsburgh, PA 15217. 

 Other similar programs exist, some of which are updated quarterly or yearly. 



Chapter 2 Secondary Sources of Coastal Information 



