124. U. S. COAST AND GEODETIC SURVEY 
The height of higher high water varies from day to day, this variation being partly 
of a periodic nature, due to the change in position of the moon relative to earth and 
sun, and partly nonperiodic, due to secular variation in sea level. Referring to Figure 
39, it is seen that with regard to the moon’s declination higher high water goes through 
a fortnightly cycle, being highest about the time of maximum north or south declina- 
tion and lowest about the time when the moon is on the Equator. The height of higher 
high water varies also with the phase and parallax of the moon, but the declinational 
effect is the principal one. 
Monthly values of higher high water generally differ by several tenths of a foot 
from month to month, while within a year two such monthly values may differ by as 
much as a foot. The variation in monthly higher high water is partly in response to 
variations in sea level, but primarily in response to the change in declination of the 
sun, so that this latter variation has a period of a year. 
From year to year higher high water varies about a tenth of a foot, although at 
times two consecutive yearly values of higher high water may differ by as much as 
three-tenths of a foot. The variation of yearly higher high water is in part due to 
secular variations in sea level and in part to a regular variation with a period of 19 
years, depending on the longitude of the moon’s node. 
Definition of Mean Higher High Water 
In view of the variations to which the height of higher high water is subject, 
mean higher high water at any place may be defined simply as the average height of 
the higher high waters at that place over a 19-year period. 
Primary Determination 
A primary determination of mean higher high water is based directly on the average of 
the higher high waters over a 19-year period. This implies that if there were no change 
in sea level from one 19-year period to another, and no change in tidal regime had 
taken place, two different 19-year Jstesmiaatiene of mean higher high water at a 
given place would have the same value. 
From the 51 years of observations, 1898-1948, at San Francisco three slightly 
overlapping 19-year series may be formed, 1898-1916, 1914-1932 and 1930-1948. 
The value of mean higher high water on the staff for each of these series is, respectively, 
in feet, 11.152, 11.214 and 11.354. The difference between the first two determinations 
is 0.062 foot and between the first and third 0.202 foot. These are relatively large 
differences, but these differences reflect changes that may have taken place in sea level. 
The mean sea level for each of the three 19-year periods on the staff is, respectively, 
8.544, 8.601 and 8.720. Referred to the respective sea levels, therefore, the three 
19-year series give mean higher high water above sea level as, 2.608, 2.613 and 2.634. 
The difference between the first and third determinations is 0.005 foot and between the 
first and third 0.026 foot. As noted in connection with the discussion of the primary 
determination of mean lower low water, the location of the San Francisco tide station 
had been changed several times. Furthermore it is possible too that improvements in 
