considered. The National Ocean Service is presently updating the National Tidal 

 Datum Epoch from 1941 through 1959, to 1960 through 1978. 



E. Coastal and Marine Boundaries 



Coastal and marine boundaries are those associated with the line where the 

 ocean surface meets the land. Since the ocean (and land) moves up and down in both 

 periodic and secular motions, the location of this line moves up the beach landward 

 and down the beach seaward as a function of time. Furthermore, the beach at the 

 ocean surface moves horizontally both landward and seaward by the processes of 

 erosion and accretion, respectively. If the line is to be used as a boundary (or 

 the source of a boundary), it should be stopped from these horizontal movements; 

 that is, it should be mathematically fixed by man. It follows that to fix the line 

 in its landward-seaward movement, the up-down motion of the water and the 

 erosion-accretion movement must likewise be mathematically fixed. 



The up-down motion is fixed by the use of tidal da turns, as described in the 

 previous section. Unfortunately, the erosion-accretion part of the movement cannot 

 be stopped and, therefore, the line cannot be permanently fixed. The line, 

 logically, takes on the name of its associated datum, such as mean lower low water 

 line or mean high water line. 



There are several coastal and marine boundaries. The mean high water line is a 

 coastal boundary between state and private lands in most coastal states, for ex- 

 ample. The apparent mean high water line is also the shoreline on nautical charts 

 of the National Ocean Service. Points on the mean lower low water line are used to 

 construct segments to form a baseline from which marine boundaries are constructed 

 offshore. 



As in the case of soundings and elevations on nautical charts and bathymetric 

 maps, relative apparent secular sea-level changes eventually make the value for the 

 particular specified years of the National Tidal Datum Epoch unrealistic for coastal 

 and marine boundary accuracy. Continuous monitoring of relative apparent secular 

 sea level is thus required to evaluate the necessity of possible Epoch revision. 



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