90 COAST AND GEODETIC SURVEY 
At Mobile there are available four years of observations, from 1933 to 1937. 
From these 4 years, after correction by comparison with Pensacola to the 19 years 
1930-1948, mean high water is 3.70 feet, mean tide level 2.96 feet and half the mean 
range 0.74 foot. These latter values, while not primary determinations may be con- 
sidered as well-determined values. By comparing with the last column in the table 
above it is seen that mean high water determined from one month of observations is 
generally within 0.1 foot of a well-determined value. For November the difference is 
0.23 foot, reflecting the effect of disturbed weather conditions. 
Year. —T'o exemplify the determination of mean high water from a year of observa- 
tions, it will be instructive to use the mixed type of tide. Los Angeles Harbor and 
La Jolla, both on the coast of California, are about 75 miles apart and both have the 
mixed type of tide. We may derive mean high water at La Jolla for every other year 
from 1936 to 1946 by comparison with Los Angeles Harbor. The procedure may be 
shortened from that used in the previous examples, by using half-tide level and range. 
The data are given in tabular form below. 
Los Angeles Harbor La Jolla 
Year Height on staff Corrections for 
————— HTL Range | MHTL ee MHW 
HTL Range HTL Range 
Feet Hed Feet Factor Feet Feet Feet Feet Feet 
1936S SCs eee 6. 54 Sh de 0.00 0. 998 6. 66 3. 66 6. 66 3.65 8. 48 
TCR Tso n cee a A 6. 51 3. 78 0. 03 0. 995 6. 63 3. 62 6. 66 3. 60 8. 46 
1940F 2S METe ees 6. 59 3. 85 —0.05 0. 978 6. 77 3.72 6. 72 3. 64 8. 54 
140 Mae ra area 6. 3. 80 —0. 08 0. 990 6.75 3. 66 6. 67 3. 62 8. 48 
1OG4E we OEE 2 6. 57 3. 83 —0. 03 0. 982 6.72 3. 67 6. 69 3.60 8. 49 
O46 Hee EO 6. 57 3. 74 —0. 03 1.006 6. 78 3. 62 6.75 3. 64 8. 57 
In Los Angeles Harbor, the 19-year series 1928-46 gives half-tide level as 6.54 
feet and the range as 3.762 feet. The corrections for the different years for half-tide 
level and range are derived from these values and entered in columns 4 and 5. These 
corrections are then applied to columns 6 and 7, deriving thereby the values in columns 
8 and 9. The last column is then derived by adding one-half the mean range to the 
mean half-tide level. 
The difference between the greatest and the least of the values in the last column 
is 0.11 foot and the average of the six values is 8.50 feet. From the 19-year series 
1928-1946 available at La Jolla the primary determination of half-tide level is 6.71 
feet and of the range 3.62 feet giving the primary determination of mean high water 
as 8.52 feet. The secondary determination for each of the years in the table above 
thus are correct to within about 0.05 foot as compared with the primary determination. 
Correction by Tabular Values 
The corrections to derive mean high water from a short series of observations are 
of two kinds: (1) correction of half-tide level to mean value; (2) correction of range to 
mean value. The former correction arises primarily from the effects of wind and 
weather, but the latter correction is primarily of a periodic character depending on 
the positions of sun and moon with respect to the earth. In the method of comparison 
of simultaneous observations, both corrections are determined by comparison. But 
it is also possible to derive the correction for range from theoretical consid erations by 
