104 



high water and of mean low water depends principally on the semi- 

 diurnal components of the tides, while the diurnal inequalities depend 

 wholly on the diurnal components. Since the ratio of the amplitudes 

 of the diurnal components at two stations tends to differ from the 

 ratio of the amplitudes of the semidiurnal components, even when 

 the two stations are in the same region, the diurnal inequalities are 

 separately compared. The ratios of the observed mean inequalities 

 at the base station, during a period of concurrent observations, to the 

 established long term mean values of these inequalities at the base 

 station, applied to the observed mean inequalities at the secondary 

 station, give the corrected values of the inequalities at the secondary 

 station. These, added to and subtracted from the corrected mean 

 high and low waters on the staff, give the corrected mean higher 

 high and lower low waters on the staff at the secondary station. 



204. Example. — The computation of the mean lower low water 

 datum at Anacortes, Wash., from concurrent observations extending 

 over 7 days at this station and at a base station at Seattle is shown 

 below. The "accepted datums" in the second column are the estab- 

 lished long-term means at Seattle. 



Mean lower low water at Anacortes, corrected=HTL-i'^Mn-DLQ=20.10-2.38-2.54=15.18 



205. In the usual form of computation, the difference in the height 

 and time of each tide at the two stations is computed, and any tide 

 showing an abnormal difference in height or time is rejected. When 

 a record of the actual high and low waters at a suitable base station is 

 not available, a comparison based on the predicted tides at a suitable 

 station, as given in the tide tables, affords a better determination than 

 a short-term record at the secondary station alone, although clearly 

 not as reliable as a comparison with the actual tides at the base station, 



206. Precision of a determination by comparison. — The precision of a 

 determination of tidal datums by comparison depends on the water 

 distance between the stations, the freedom of the movement of the 

 tide between them, and the length of the observations. When the 

 secondary station is only a few miles from the base station, on an open 

 and unrestricted waterway, observations extending over a short time 



