MSL at all stations relative to Galveston, Texas, is given in Table 3. The reference datum 

 defined in this manner vv^as originally called the "Sea Level Datum of 1929." The term 

 "National Geodetic Vertical Datum" (NGVD) of 1929 was adopted in 1963, and is 

 identified as MSL on many U.S. Geological Survey (USGS) charts and Corps of Engineers 

 documents. It has been well established since 1929 that the elevation of the mean water 

 level with respect to the land varies vnth time as a consequence of land subsidence and 

 emergence and a slow redistribution of the waters of tlie Earth. Braaten and McCombs 

 (1963) published a new determination of a geodetic datum of national scope based only on 

 stations for which a series of 19 years of tide data was available. The variations in sea level 

 revealed by this survey are shown in Figure 14. Some leveling lines in regions of known 

 subsidence and suspected subsidence have been resurveyed many times since 1929. An 

 example of a study of this type is given in National Oceanic and Atmospheric 

 Administration (1973). New surveys of this type are based on elevations assigned to tlie 

 more stable parts of tlie continent by the 1929 adjustment, and they lead to new 

 determination of the elevations of bench marks in the subsiding or emerging areas. Thus, the 

 elevation assigned to a specified bench mark may vary over a period of many years because 

 of changes in tlie elevation of the solid surface of the Earth or changes in the mean elevation 

 of the nearby surface of the sea. 



Shalowitz (1964), Dracup (1974), Berry (1976), and Plasker (1976) present histories of 

 the development of geodetic networks in the United States. Dracup and Plasker describe 

 plans for a new readjustment of the National Vertical Control Network scheduled for 

 completion about 1982 or 1983. 

 2. Tidal Datum Bench Marks. 



Index maps of tidal datum bench marks and lists of the estabUshed references between 

 the NGVD of 1929 and the local MSL are available for each State from NOS. Several maps 

 are required for States with long coastlines. A sample index map and related index map 

 numbers are shown in Figure 15 (a and b). Individual bench-mark sheets, describing two or 

 more bench marks estabUshed near each tide observation point and the relation between the 

 various tidal datums, are reported for each tide observation station. A sample of an NOS 

 tidal bench-mark sheet is shown in Figure 16. 



A general trend toward rising sea levels, relative to the land (see Fig. 7), is evident along 

 all of the U.S. coastline south of Alaska and in soutliern Alaska. As a result of this trend, the 

 MSL datum changes with each new epoch used for defining tidal datums. The NGVD, 

 however, is a fixed surface whose elevation does not vary with time, although elevations of 

 fixed points (as referred to NGVD) may change. 



It is well established that the elevation of the Earth's surface may rise or fall as a result of 

 earthquakes or volcanic activity. The land surface in many areas has been sinking during this 

 century as a result of the removal of petroleum, water, or other underground resources. 

 Slow rises of the land surface also occur. The extent of these changes are measured by new 

 determinations of the elevation of tlie land surface relative to the NGVD. Tliis is 

 accomplished through new surveys from regions which are beUeved to be more stable. 



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