76 U. §. COAST AND GEODETIC SURVEY 
two monthly values which differed by 1.20 feet give mean values which differ by 0.09 
foot. 
For Annapolis there is available a direct determination of half-tide level for the 
19-year period 1930-1948 from observations at that place. This value is 4.65 feet. 
Hence the mean values derived from one month of observations, after correction by 
comparison, differ by 0.03 foot and 0.06 foot, respectively, from the primary determina- 
tion. 
Annapolis and Baltimore are relatively near to each other, bemg about 20 miles 
apart. They are located, however, on different rivers. To exemplify the correction 
of a year’s series we may take stations farther apart, Los Angeles and La Jolla, in 
California, which are nearly 100 miles apart, using Los Angeles as the primary station. 
- For the 19-year period 1928-1946, the lowest yearly half-tide level at La Jolla 
was for 1933 when it read 6.53 feet, and the highest was for 1941 when it read 6.90 
feet. At Los Angeles the primary determination for the 19-year period 1928-1946 
is 6.54 feet, while the yearly values for 1933 and 1941 are, respectively 6.30 and 6.75. 
Hence the corrections to these years are 0.24 foot and —0.21 foot, and therefore the 
mean value at La Jolla for 1933 is 6.77 feet and for 1941 is 6.69 feet. The direct 
primary determination of half-tide level at La Jolla is 6.71 feet. The two yearly 
values at La Jolla, which differed from each other by 0.37 foot, when corrected to mean 
values by comparison with Los Angeles differ from each other by only 0.08 foot, and 
from the primary determination by 0.06 foot and 0.02 foot. 
To exemplify the determination of mean half-tide level for daily types of tide 
we may take Eugene Island, La., and Galveston, Tex., which are about 200 miles apart. 
This example will be instructive, furthermore, because at both stations, as Table 3 
shows, two half-tide level datums can be derived, the one for the tide as mixed, and the 
other as daily. 
For the 9 years of observations available at Eugene Island, 1940-1948, the lowest 
monthly value of half-tide level was for January 1940 when for the mixed tides it had 
a value of 1.42 feet and for the daily tide a value of 1.25 feet. The highest monthly 
value was for September 1948 with a value of 3.32 for the mixed tide and 3.06 for the 
daily tide. At Galveston the corresponding figures for January 1940 are 3.03 feet 
and 2.92 feet, and for September 1948, 4.97 feet and 4.80 feet. 
The primary determination of half-tide level at Galveston for the 19-year period 
1930-1948 is 4.00 feet for the mixed tides and 3.83 feet for the daily tide. Taking 
half-tide level for the mixed tides first, the corrections for January 1940 and September 
1948 are respectively 0.97 foot and —0.97 foot which give for mean values at Eugene 
Island for these months 2.39 feet and 2.35 feet respectively. Thus two monthly 
values of half-tide level at Eugene Island which differed by 1.9 feet, when corrected 
to mean values by comparison with Galveston differ by but 0.04 foot. 
Since only 9 years of observations are available at Eugene Island no direct primary 
determination of half-tide level can be derived. But it is clear that the value for the 
9 years, after correction by comparison with Galveston will give a very close approxi- 
mation to a primary determination. From these 9 years the value of half-tide level 
for the mixed tide at Eugene Island after correction by comparison with Galveston is 
2.43 feet. The two derived mean values for the two months above therefore differ 
by 0.04 foot and 0.08 foot from a primary determination. ; 
Taking up now the half-tide level for the daily tide, the primary determination of 
half-tide level at Galveston for the 19-year period 1930-1948 being 3.83 feet, the cor- 
