Chapter IV 

 TIDAL DATUM PLANES 



Paragraphs 



Principal tidal datums and ranges 157-158 



Mean sea level 159-163 



Half-tide level 164 



Low and high water datums in general 165-168 



Mean low and high water 169-180 



Correction for longitude of moon's node 171-180 



Mean low and high water of spring tides 181-185 



Mean low and high water of neap, perigean, apogean, and tropic tides. 186 



Mean lower low and higher high waters 187-192 



Other datum planes ^ 193-194 



Typical relation between datums 195 



Determination of datums by comparison 1 96-207 



Fixation of datums 208-211 



Tidal observations 21 2-215 



157. Princiiml tidal datums. — The mean height of sea level, and the 

 mean heights of low or high waters of various descriptions, afford the 

 datums to which the elevations of upland areas, and of the bottom of 

 the sea and of tidal waterways, ordinarily are referred. The datums 

 which need be especially considered, and the abbreviations by which 

 they are designated, are as follows: 



Mean sea level, MSL. 



Halj tide level, HTL. 



Mean low water and mean high water, MLW and MHW. 



Mean lower low water and mean higher high water, LLW and 



HHW. 

 Mean low and high water oj spring tides. In England, these 

 datums are taken as mean low and high water of ordinary 

 spring tides, after rejecting any spring tides which differ 

 substantially from the usual, and are designated as LWOST 

 and HWOST respectively. 

 In some cases channel depths at foreign ports are referred to mean 

 high or low water of neap tides. Mean low and high waters of peri- 

 gean, apogean, and tropic tides are rarely if ever used as a reference. 



158. Tidal ranges. — The symbols conventionally assigned to the 

 tidal ranges determined by the datums listed in the preceding para- 

 graph, are as follows: 



Mean range, Mn=MHW— MLW. 



Diinnal or great diurnal range, Gt = HHW— LLW. 



Spring range, Sg, mean low water to mean high water of spring 



tides. 



(87) 



