74 U. S. COAST AND GEODETIC SURVEY 
The average difference between the two tide level values is 0.17 foot, with a maxi- 
mum difference of 0.21 foot in 1932 and a minimum difference of 0.13 foot in 1940 and 
1942. In part these differences arise from the periodic variation in the relation of half- 
tide level to sea level which becomes appreciable where the ratio of (K,+-O,)? to M2 
is large. For Galveston this ratio is 1.3. ForSeattle, Washington, it is 5.1 and hence 
for this latter station the periodic variation in the relation of yearly half-tide level to 
sea level shows up much more clearly, as is illustrated in Figure 37. The ratio of K,+Oh,, 
to M.+S, at Seattle is 0.97, and the tide here is definitely of the mixed type. 
Figure 37 gives the yearly values of HTL-SL for the 38 years 1911 through 1948. 
The horizontal line shows the average value of this difference for the 38 years to be 0.02 
1910 1915 1920 1925 1930 1935 1940 1945 
Fic. 37.—Relation of half-tide level to sea level, Seattle. 
foot, and the yearly values range themselves very closely along the sine-lke curve 
which has its maxima in the years of highest declination of the moon, 1913 and 1932, 
and its minima in the years of the moon’s lowest declination, 1922 and 1941. The range 
of this variation, although well defined at Seattle, is relatively small, being only about 
0.1 foot in a 19-year period. 
Table 3 gives the relation of half-tide level to sea level as determined directly 
from the tide observations at a number of tide stations along the coasts of the United 
States. The table gives the values of half-tide level minus sea level. Negative values 
therefore indicate that sea level lies above half-tide level, while positive values indicate 
that sea level lies below half-tide level. Values in the table that are marked with an 
asterisk indicate that the half-tide level used was derived from the higher high and 
lower low waters. 
In general it appears that along the Atlantic coast sea level is above half-tide 
level by about 0.1 foot, while along the Pacific coast sea level is below half-tide level 
by about 0.05 foot. The two sets of values for Eugene Island and Galveston show 
that half-tide level derived from the higher high and lower low waters is nearly 0.2 
foot lower than that derived from all four tides, at those places. 
