program, the definitions used for HHW, HW, LW, LLW, and the mean and diurnal ranges are 

 identical on aU stations. Each identifiable high or low water used in defining MHW and MLW 

 provided the difference between adjacent high and low waters is 0.1 foot or greater. The 

 difference between MHHW and MLLW is defined as the diurnal range. The definition of 

 MHW and MLW and mean range agrees with NOS practice at locations with semidiurnal- or 

 mixed-type tides. In 1981, NOS will phase in a classification change: MLLW wUl be used as 

 the primary datum for aU nautical charts; the diurnal range wiU be used as the principal 

 measure of tidal range. A complete changeover will require several years. The author 

 believes this change wiU be an improvement over past practice. The classification in the 

 1979 NOS Tide Tables (NOAA, 1979a; 1979b) is used in this report. Where the type of 

 tide is classified as diurnal, NOS neglects all secondary high and low waters, and the highest 

 and lowest values are considered high and low water. Thus, for stations with diurnal tides, 

 the MHW and MLW given by NOS correspond to the MHHW and MLLW in this report; 

 the mean range given by NOS for diurnal-type tides corresponds to the diurnal range in this 

 report. When determining the HHW or LLW for days with a single high or low water, NOS 

 accepts the single tide as a HHW only if it is larger than the preceding or following high 

 tides, and as a LLW only if it is lower than the preceding or low tides. 



Table 7 shows that the net differences are generally much less than 0.1 foot for mean 

 ranges and sUghtly less than 0.1 foot for diurnal ranges. Differences up to 0.4 foot which 

 occur at a few locations result from the necessity of basing the tide calculations on a shorter 

 or earlier period of record than that currently used by NOS in defining the tidal datums on 

 bench-mark sheets. The differences between the working definition of HHW and LLW used 

 in this report (discribed only in this section) and the definition used by NOS account for a 

 part of this difference. It is recommended that range determinations based on the latest 

 bench-mark sheets be used in obtaining dimensional values from the tabulated data. 



4. Tide Probability Graphs. 



ProbabUity density distribution tables have been developed for seven tide parameters: 



(a) The highest predicted tide for each calendar month. 



(b) The predicted HHW of each solar day. 



(c) AU predicted high waters of the 19-year period. 



(d) Predicted hourly tidal heights. 



(e) All predicted low waters of the 19-year period. 



(f) The predicted LLW of each solar day. 



(g) The lowest predicted tide level of each calendar month. 



The predicted astronomical tide is a bounded function. The maximum range cannot 

 exceed double the sum of tlie amplitudes of all constituents as multipUed by tlie node 

 factors. The actual range wUl always be less than the maximum just cited because the larger 

 values of some node factors occur in the same years as tlie smaller value of other node 

 factors. 



66 



