100 



From table V, paragraph 134: 



Ki = 1.648 Oi=1.014 M2=:6.138 



Whence (Ki+Oi)/M2=-0.43. 



From table VII, paragraph 174, /=18°.3. 



From table VI, paragraph 173, i^(Mn) = 0.971. 



Corrected Mn=13. 32X0.971 = 12. 93. 



Correction to MHW and MLW=K(13.32- 12.93) =0.20. 



Corrected MHW above HTL=6.66-0.20=6.46. 



Corrected MLW below HTL=6.46. 



1.02 F, (from table VIII) = 1.177. 



Corrected DHQ = 0.80 XI. 177 = 0.94. 



Corrected DLQ=1.38X1. 177=1. 62. 



Corrected HHW above HTL= 6.46 + 0.94 = 7.40. 



Corrected LLW below HTL=6.46 + 1.61=8.08. 



Corrected HHW on staff =14.09+7.40=21.49. 



Corrected LLW= 14.09-8.08 = 6.01. 



It may be noted that in this case the corrections to DHQ and 

 DLQ nearly counterbalance the corrections to Mn. The correction 

 factor 1.02 J^i to the mean annual diurnal inequalities decreases with 

 /, whUe the correction factor i^(Mn) to the mean range increases 

 with that angle. A glance at table VIII shows, however, that the 

 plane of lower low water goes through marked variations from month 

 to month. 



192. Precision oj determinations. — As with the other datum planes, 

 a determination of mean lower low or higher high water from corrected 

 observations extending over a period of 9 years is considered by the 

 Coast and Geodetic Survey as a primary determination. In general, 

 observations for a year, similarly corrected, determine the relation of 

 these datums to mean sea level, or half tide level, within 0.1 foot of 

 the 9 year determination; and observations over a month with a 

 quarter of a foot. At least 3 days observations should be used to 

 determine this datum within a foot of the long term value. (Special 

 Publication 135, U. S, Coast and Geodetic Survey, p. 124.) 



OTHER DATUM PLANES 



193. Harmonic tide plane. — A tidal plane often referred to, and used 

 at some ports in India, is that at an elevation of M2 + S2+K1+O1 

 below mean sea level. It nearly coincides with what might be called 

 tropic lower low water of spring tides. It has the advantage of being 

 so low that normal tides rarely fall below it. 



194. Arbitrary datum planes. — As will later be shown, the tidal 

 datums herein before listed, after being determined from a more or 

 less extended set of observations, are referred to standard bench marks 

 which thereafter become the controlling reference for charts, tide 

 tables, and channel depths. In some countries local datum planes, 



