Figure 12. Relationsliip between NGVD and several 

 tidal datums between Montauk and The 

 Battery (Swanson, 1974). 



1. The National Geodetic Vertical Datum (NGVD). 



A reference for comparing land elevations is often needed for locations near the coast 

 where no tide observations are available and at interior locations where tide observations are 

 impossible. By the mid-1 920's, several first-order leveling lines connecting the Atlantic and 

 Pacific coasts, and many tide gages on botli coasts, had been surveyed. In first-order leveling, 

 each Une is surveyed at least twice, once in each direction, and the maximum allowable 

 height difference in surveys is 4 miUimeters per \/k or 0.017 feet per \/k, where k is 

 distance measured in kilometers or statute miles (National Oceanic and Atmospheric 

 Administration, 1974; 1977b). These surveys consistently show sea level to be higher on the 

 Pacific coast than on the Atlantic coast and higher in the north than in the south on both 

 coasts. It seemed desirable to have the zero of the geodetic leveUng net coincide with local 

 MSL wherever both quantities were known. Thus, a general adjustment was made in 1929 in 

 which it was assumed that the geodetic and local sea levels were equal to zero at 26 selected 

 tide gages in the United States and Canada (Rappleye, 1932). The differences previously 

 computed were treated as errors and were distributed over the network of observation 

 points. The locations of the tide gages used are shown in Figure 13. The period of the 

 observations from U.S. tide stations used in defining the reference datum and the height of 



38 



