MANUAL OF TIDE OBSERVATIONS 

 Tabic for reducing Greenwich intervals to local intervals 



73 



HEIGHT REDUCTIONS 



234. Form 138, on which are tabulated the high and low waters, 

 provides for the regular computation each month of certain tidal 

 planes and ranges. Mean high water (HW) and mean low water 

 (LW) for each month are obtained by summing all the high waters 

 and all the low waters and dividing by the number of observations, 

 the latter being indicated by small figures just above the sum. The 

 means, written below the sums, should be carried to two decimal 

 jjlaces. The mean range (Mn) is obtained by subtracting the mean 

 of the low waters from the mean of the high waters. The mean tide 

 level (MTL), which is also known as half -tide level, is obtained by 

 taking the half sum of the mean of the high waters and the mean of 

 the low waters. 



235. For stations on the Pacific coast, the means of the higher high 

 waters and of the lower low waters and the diurnal inequalities 

 should also be obtained. The higher of the two high waters and 

 the lower of the two low waters of each day of the month are first 

 indicated by a check mark. If the two high or two low waters on 

 the same day are equal, either may be selected as the higher high 

 or lower low water. When only one high or one low water occurs 

 on a calendar day, by reason of one of the tides having occurred 

 after midnight and therefore on the next calendar day, the single 

 tide is to be checked if the tide just above it is unchecked ", otherwise 



