D. Charting 



The soundings and depth curves on nautical charts and bathymetric maps must 

 have numerical values below some reference base. Likewise, the heights of struc- 

 tures, bridge clearances, et cetera, must have a value above some reference base. 

 The logical choice for the base is the ocean surface. But, since the ocean (and 

 land) surface moves up and down in random, periodic, and secular motions, as 

 described before, it must be stopped in its vertical movement to be useful; that is, 

 it must be mathematically fixed by man in order to become a suitable reference base. 



The result of mathematically fixing the ocean surface in terms of an observed 

 tidal phenomenon is called a tidal datum. Chart datum, the reference base for 

 soundings and depth contours, is the tidal datum of mean lower low water. The 

 reference base for structure heights, bridge clearances, et cetera, is the tidal 

 datum of mean high water. Mean lower low water is the average of the lowest low- 

 water height of each tidal day observed over the National Tidal Datum Epoch. Mean 

 high water is the average of all the high-water heights observed over the National 

 Tidal Datum Epoch. 



The National Tidal Datum Epoch is the particular 19-year series over which the 

 phases (such as lower low water) are averaged. It is used because: (1) it encom- 

 passes all significant tidal periods (including the 18.6-year period for the regres- 

 sion of the Moon's nodes); (2) it is a long enough series to average out practically 

 all of the meteorological, hydrological , and oceanographic variability; and (3) by 

 specifying which 19 years are to be used, comparability among stations and stability 

 over decades are achieved. 



Although these definitions almost fix the ocean surface, the insidious relative 

 apparent secular sea-level changes remain. The specific 19-year average dampens the 

 changes, fixes an elevation at middate, and takes care of the nonlinearity for 

 uniformity throughout and among all charts and maps. 



As the years progress, however, any new 19-year average will have a different 

 value. Eventually, relative apparent secular sea-level changes will make the value 

 for the particular specified years of the Epoch unrealistic for navigational pur- 

 poses. When this occurs (usually every two or three decades), a new Epoch must be 



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