95 



retrogresses through substantially 180°. The corrections are of im- 

 portance in a close study of the effect of a channel improvement upon 

 the mean tidal ranges. 



180. Precision of observations. — Observations extending over 9 years 

 are considered by the United States Coast and Geodetic Survey to 

 afford a primary determination of the mean low and high water datums 

 at a tidal station. In general observations for a year, corrected for 

 the longitude of the moon's node, determine the relation of these 

 datums to mean sea level, or half tide level, at the station, within 

 0.05 foot of the 9 year mean; and observations for a month within 0.1 

 foot (Special Publication 135, U. S. Coast and Geodetic Survey, 

 p. 107). 



MEAN LOW AND HIGH WATERS OF SPRING TIDES 



181. Dirffering definitions. — Spring low waters and high waters are 

 most accurately defined as the low waters and high waters nearest 

 the time of conjunction of the principal lunar and solar semidiurnal 

 components of the tide. Ma and S2 (par. 143) ; but muiy be more loosely 

 taken as the lowest low waters and highest high waters occurring 

 semimonthly soon after new and full moon. At English ports and in 

 other regions where the lowest low waters and highest high waters 

 follow consistently the conjunction of these components, and the 

 tides run through a regular variation from springs to neaps, with small 

 diurnal differences, spring tides are readily identified in the recorded 

 low and high waters, and their means over a number of months afford 

 fairly definite datums. Because of the small number of spring tides 

 in a half year or a year, a single abnormal tide would have a relatively 

 large effect on the mean value. Thus a storm disturbance of 3 feet 

 would change the mean low water of spring tides during a 6-month 

 period by a quarter of a foot, while it would change the mean of all 

 low waters during the same period by less than one hundredth of a 

 foot. It is therefore the English practise to reject abnormal spring 

 tides from the computations, and to designate the datums as mean 

 low and high water of ordinary spring tides. These datums depend 

 to some extent, consequently, on the judgment of the computer. 



182. In regions where the tides have a considerable variation from 

 apogee to perigee, or from equatorial to tropic tides, the spring tides 

 may not be as readily identified; and when the diurnal components of 

 the tide are large, the low and high waters occurring next before the 

 time of conjunction of the M2 and S2 components may differ widely 

 from those next following. In some countries the datums designated 

 as mean low and high waters of spring tides are in fact the means of 

 the lowest low and highest high waters which occur soon after the 

 successive full and new moons. Such determinations obviously are 

 somewhat haphazard, but afford a low water datum below which the 



