98 



ally but one low water occurs (about noon) during the calendar day 

 even when the tide is wholly semidiurnal. It is included in, or ex- 

 cluded from, the summation according to its relation to the preceding 

 low water. If two low waters of the same height occur on a calendar 

 day, but one is included. When, however, the tide becomes tempo- 

 rarily dirunal, each low water is included in the summation. Mean 

 higher high water is similarly computed. 



188. Use. — Where the tides are of the mixed type, mean lower low 

 water affords a more suitable reference plane than mean low water, 

 and is the official reference plane for navigation charts and channel 

 improvements on the Pacific coast of the United States. While this 

 datum is below mean low water (by as much as 1.8 feet at Seattle) yet 

 one of the two daily tides is as like as not to fall below it, sometimes 

 considerably. Thus at Seattle normal tides occasionally fall as much 

 as 3 feet below mean lower low water. 



At localities having a tide which is wholly diurnal, mean low water 

 and mean lower low water become synonomous. On the Gulf of 

 Mexico, where the tides are generally of the diurnal type, but small and 

 irregular, mean low water affords a more satisfactory reference plane 

 than mean lower low water, and is the officially adopted plane in the 

 United States. 



189. Corrections to short term determinations. — An independent 

 determination of mean lower low or higher liigh water at a station, 

 like that of mean low or high water, must extend over a minimum 

 period of 29 days to eliminate the monthly variations in the tidal 

 range. Furthermore, the elevations of mean lower low and higher 

 high waters vary with the changing declination of the sun from month 

 to month during the year, as well as varying with the changing incli- 

 nation of the moon's orbit during a period of 19 years. The correc- 

 tions to reduce to their true mean values, determinations based on 

 observations during a month or a year, are derived by applying a 

 reduction factor, conventionally designated 1.02 Fi, to the diurnal 

 low and high water inequalities, DLQ and DHQ (par. 152). The 

 corrected diurnal low water inequality is then subtracted from the 

 corrected mean low water datum, derived as explained in paragraphs 

 171 to 177; and the corrected diurnal high water inequality added to 

 the corrected mean high water datum. 



190. The derivation of the reduction factors, 1.02 Fj, is explained 

 in appendix II. The computed values for each month of the year 

 from 1891 to 1950, are given in table 7, Special Publication No. 135, 

 United States Coast and Geodetic Survey (Tidal Datum Planes), 

 pages 114-115. The values from 1921 to 1950 are extracted there- 

 from in the following table: 



