99 



The mean annual values of the correction, given in the last column, 

 are the reduction factors to be applied the diurnal inequalities derived 

 from observations extending through the year. 



The approximations introduced by these corrections are practically 

 eliminated in a determination of these datums from observations 

 extending over 9 years. 



191. Example. — The application of the reduction factors to obtain 

 the corrected mean lower low and mean higher liigh waters is illus- 

 trated by the determination of these corrections to the observed 

 annual mean tidal heights above an arbitrary datum plane at Ketch- 

 ikan, Alaska, in 1922, given in Special Publication, No. 127, United 

 States Coast and Geodetic Survey (Tides and Currents in Southeast 

 Alaska). 



The observed heights are: 



Mean liigher high water (HHW) = 21.55. 



Mean high water (MHW) = 20.75. 



Mean lower low water (LLW)=6.06. 



Mean low water (MLW) = 7.43. 



Annual half tide level (HTL) = K (20.75 + 7.43) = 14.09. 



Mean high water above HTL=6.66. 



Mean low water below HTL =6.66. 



Mean range (Mn) = 20.75-7.43 = 13.32. 



Annual high water inequality (DHQ) = 21. 55-20. 75 = 0.80. 



Annual low water inequality (DLQ) = 7.43 — 6.06 = 1.37. 



