90 



mean sea level, while on the Pacific coast, except in Alaska, it usually 

 is above. Thus at Fort Hamilton, New York Harbor, it is 0.05 feet 

 below mean sea level, at Philadelphia 0.16 feet below, and at San 

 Francisco, 0.06 feet above that datum. Quite obviously, half tide 

 level varies from month to month and from year to year by amounts 

 that closely approximate the variations in mean sea level. Half tide 

 level is rarely if ever used as a datum plane for land elevations and 

 soundings, but affords a convenient reference for the correction of 

 mean high and mean low waters. 



LOW AND HIGH WATER DATUMS IN GENERAL 



165. Since it clearly is desirable that the soundings on navigation 

 charts, and the designated depths of improved channels, show the 

 depths that generally can be counted on by navigators, they ordinarily 

 are referred to one of the low water tidal datums, and not to mean sea 

 level. Different low water datums are used for this purpose in dif- 

 ferent countries. The datums adopted in the United States are the 

 most definitely determinable, but are not as low as those generally 

 used in other countries. When comparing the channel depths in 

 foreign ports with those in this country, the respective datums must 

 be taken into consideration. Thus a channel 28 feet in depth at the 

 adopted datum in a Canadian port might be 30 feet or more in depth 

 if referred to the low water datum officially adopted in the United 

 States for the region in which the harbor lies. 



High water datums, while not suitable for charting, establish the 

 tidal ranges, which are usually noted on charts to indicate the depths 

 available at high water. In regions where the range between spring 

 and neap tides is considerable, the elevation of neap high water is of 

 especial importance, since it indicates the least depths at high water 

 which can be counted on throughout the month. 



166. Low and high water datums do not establish a level surface. — 

 Obviously, as any of the several low and high water datums may be at 

 a different height below or above mean sea level at different stations, 

 these datums do not establish the same level surface from station to 

 station, and are applicable only to the area in the vicinity of each 

 station. Thus mean low water at the head of the Bay of Fundy is 

 some 15 feet below the level of mean low water at the entrance to the 

 bay. The change in the elevation of each of these datums, from 

 station to station is, however, generally so gradual as to present no 

 practical complications. 



167. Meteorological variations in high and low water datums.- — The 

 variations in mean sea level from month to month, and from year to 

 year, produce nearly identical variations in the several low and high 

 water datums. They do not, however, produce any substantial 



