101 



established by reference to such a bench mark, are arbitrarily adopted 

 for these purposes, without particular relation to one of the character- 

 istic tidal planes. 



TYPICAL RELATIONS BETWEEN DATUM PLANES 



195. The relations between these planes at Fort Hamilton, New 

 York Harbor, where the tide is of the semidiurnal type, and at the 

 Presidio, San Francisco Harbor, where the tide is of the mixed type, 

 are as follows: 



Elevations below mean sea level {feet) 

 New York San Francisco 

 {1912-30) {1898-1923) 



Mean low water 2.^2 1.87 



Mean lower low water 2. 64 3. 02 



Low water of spring tides 2. 88 2.26 



Harmonic tide plane 3.20 4.14 



DETERMINATION OF TIDAL DATUMS BY COMPARISON 



196. Because of the variation from day to day, from month to 

 month, and from year to year in the elevation of mean sea level, and 

 the periodic variations in the height of the successive high and low 

 waters with respect to mean sea level, long-continued observations are 

 necessary to establish, with good precision, the several tidal datums 

 at a station; but after these datums have been established at one 

 primary or base station they may be determined at other stations in 

 the same region, where the tidal variations are due to like causes, by 

 comparing, during a relatively short period, the high and low water 

 elevations at the secondary station with those at the base station. 

 This method is applicable only when the tides at the base and second- 

 ary stations are similar; i. e. when the ratio 2 (DHQ+DLQ)/Mn at 

 the two stations is substantially the same, and the higher high and 

 lower low waters are in the same sequence (par. 151). Such condi- 

 tions are to be anticipated at stations on the same general embayment 

 of the coast line, and with free connections with the sea. They may 

 be fulfilled at stations several hundred or even a thousand miles apart. 

 The method of comparison is not applicable to stations on tidal rivers 

 and estuaries in which the water levels are sensibly affected by the 

 inflow from large rivers. 



197. Establishment of half-tide level by comparison. — While the violent 

 disturbances produced by storms may vary considerably even at 

 stations in the same bay, the ordinary fluctuations of mean sea level, 

 and of half-tide level, when averaged over a sufficient period of days, 

 generally affect the elevations of these datums by substantially the 

 same amount over quite extensive areas. To determine the half- tide 

 level at a secondary station from the established datum at a base 

 station, concurrent observations are therefore made for a suitable 



