The half-tide level is a tidal datum midway between MHW and MLW. The MTL may be 

 above or below MSL by an amount which depends on the relative importance of the diurnal 

 components of the tide. 



Several other datums are defined with respect to the tides (Fig. 11). Formal definitions 

 are presented in Appendix A. Each datum is more suitable than MSL for a restricted class of 

 problems, and all depend on the tidal range and the characteristic shape of the tidal curve. 

 Corrections may be necessary to the observed data when these datums are determined from 

 less than 19 years of record. 



Figure 11. Illustration of tidal datums (Los Angeles, California (Outer Harbor), 

 Janurary 1973; mean range = 3.78 feet or 1.15 meters). 



The most important of these datums for most navigation-related activities are MLW for 

 the Atlantic coast and mean lower low water (MLLW) for the Pacific coast, defined as the 

 average height of the tide at low water or lower low water when all tides lor a 19-year 

 period are considered. MLLW is being adopted as the standard datum for all locations as 

 NOS charts are revised (Swanson and Thurlow, 1979). 



When planning the development of land above MSL, the datums MHW and mean higher 

 high water (MHHW) may be more useful than the low water datums. They are defined in a 

 manner analogous to that used for MLW and MLLW and require similar corrections when 

 based on short series of observations. The MHW or MHHW datums are often used to define 

 the seaward limit of private property. 



The difference between MHW and MLW is called the mean range of tlie tide where the 

 tides are semidiurnal. The difference between MHHW and MLLW is called the diurnal range 

 or the great diurnal range of the tide. This is identical witli the mean range for diurnal tides. 

 The range of the tide may change drastically within short distances as shown in Figure 12. 

 This is not an extreme example. Because the range of tide, and, consequently, the high and 

 low water datums may vary greatly with short distances, measurements referred to tliese 

 datums are not suitable for comparing elevations at different locations unless both 

 comparisons are based on the same bench mark. 



37 



