The NGVD of 1929 is defined or definable everywhere by first-order leveling. The other 

 tidal datums are defined with respect to a specific tide gage location. Thus, when a low 

 water datum, half-tide level datum, or any tidal datum other than the NGVD of 1929 is 

 used, tlie location at which the datum appUes should be specified. In areas where subsidence 

 or emergence is known to be in progress, the date of the survey used should also be given. 

 Reference datums have also been established by many States and local jurisdictions. 



The relation between the various datums discussed in this section at the reference tide 

 stations (where NOS pubUshes daily tide predictions) are presented in Table 4. Note tliat 

 older low water datums, below the MLW or MLLW, continue to be used at a few locations. 

 The locations of these tide stations are sliown in Figure 17. 



In 1977, a new regional datum called the Gulf Coast Low Water Datum (GCLWD) was 

 adopted by National Oceanic and Atmospheric Administration (1977b; 1977c). This datum 

 is defined as tlie MLLW where tlie tide is mixed and MLW in regions witli diurnal tides. Tliis 

 new datum, which wHl become the chart datum for the Gulf of Mexico, is desirable because 

 of the frequent shifts in the type of tide along tlie gulf coast (Fig. 18). This will end 

 numerous discontinuities in the chart datum along the gulf coast. Because of the generally 

 small tidal ranges in the Gulf of Mexico, and tlie sparcity of data, tlie importance of tliese 

 discontinuities was not fuUy recognized until tlie past decade. 



The adoption of MLLW and the mean diurnal tidal range as the standard everywhere wiU 

 simplify the definition of tidal datums but would require numerous associated changes. 



The Environmental Protection Agency (EPA) has recentiy asked the Corps of Engineers 

 (through court action) to propose an extension of its jurisdiction Umits beyond MHW to 

 include entire salt marshes. Provost (1976) argues tliat this will require the introduction of a 

 new datum defined as the average of the highest predicted tide for each year of the 19-year 

 cycle. 



V. SPECIAL DATUMS FOR THE GREAT LAKES 

 1. Early Great Lakes Datums. 



Special datums for the Great Lakes region, based on the assumption tliat the mean water 

 level in each lake defines a level surface, have been in use for more tlian a century. Moore 

 (1939a; 1939b) presents a history of the various datums used by the U.S. Lake Survey 

 before 1939. The earliest datum widely used was based on tlie high water of 1838. Other 

 widely used datums based on low water levels were established for the years 1877 and 1903. 

 A system of datums for the Great Lakes adopted in 1935 was defined as elevations above 

 sea level at New York, New York. Moore (1939a) reported that tlie level Unes cannot be 

 traced back to specific sets of observations in the coastal region, that the site may not have 

 been New York as often stated, and that the datum used may have been half-tide level and 

 not MSL. Moore (1939a; 1939b) also recognized tlie importance of crustal motion in 

 changing the relative height of bench marks in the Great Lakes region. The entire region is 

 rising with respect to sea level. The rate of rise is greatest north of Lake Superior and 



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