Tides and currents in boston harbor 41 



TIDAL DATUMS 



Height datums. — A height datum is a horizontal plane or level 

 surface from which elevations are reckoned. Height datums may 

 be adopted either for local or general use. For local use in a limited 

 area the datum may be taken as any horizontal plane, either selected 

 arbitrarily or derived from some tide plane. For general use over 

 large areas a datum must follow the curvature of the earth, and the 

 best and most universally accepted datum is one based on the mean 

 level of the sea. 



Mean sea level. — Mean sea level has been defined as the plane 

 about which the tide oscillates or. as the surface which the sea would 

 assume when midisturbed by the rise and fall of the tide. The plane 

 thus defined is the ideal datum which is sought, but the mean sea 

 level obtained from a limited series of tide observations must be 

 considered only as an approximation to the ideal datum. 



For practical purposes mean sea level is determined independently 

 at a number of primary tide stations, and in the adjustment of a level 

 net over a large area it is assumed that these various local determi- 

 nations are in the same level surface. Whether this assumption is 

 strictly true is beyond the scope of this work, but it serves for all 

 practical engmeering purposes. 



The mean sea level in any locality is obtained by taking the average 

 of the hourly heights of the sea over a considerable period of time and, 

 in some cases, by correcting the result by a comparison with simulta- 

 neous observations at another station where the datum has been 

 determined from a longer series of observations. Mean sea level 

 thus obtained will depend to some extent upon the particular obser- 

 vations used in the determinations, and small changes may be 

 expected as additional observations become available and are included 

 in the calculations. 



Sea-level datum. — For engineering work the fixedness of a datum is 

 of more importance than a precise agreement with a theoretical 

 definition. For this reason there should be a distinction between 

 the sea level which may have been adopted as a datum for engineering 

 work and the best-laiown mean sea level which may be determined 

 from time to time from all observations available. The sea-level 

 datum adopted for Boston in connection with the precise levehng of 

 1923 was derived from the hourly heights of the sea at Common- 

 wealth Pier No. 5, covering a period of two years beginning August 1, 

 1921, the results being corrected by a comparison with those obtained 

 from simultaneous observations at Portland, Me., the corresponding 

 datum at the latter place being based upon 11 years of observations. 



This sea-level datum is defined as being 13.304 feet below bench- 

 mark 7 in the vicinity of the tide station. Through the first-order 

 leveling of 1923 this datum was referred to many other bench marks 

 in Boston and vicinity. 



Boston low-water datum. — For some purposes a low-water datum is 

 more convenient than the sea-level datum. Soundings on charts 

 for mariners and the predicted heights of the tide in published tide 

 tables are usually referred to a low-water datum. Likewise much 

 engineering work is referred to low-water datums. 



On the Atlantic coast of the United States the mean of all low 

 waters, called mean low water, is usually sought as the datum. 



