TIDAL DATUM PLANES 93 
An examination of Table 5 shows that the factor for correcting yearly high water 
to mean high water decreases with increasing values of the ratio K;+0O, to My. Thus, 
for a ratio of K;+O, to M2 of 0.0 to 0.2 it appears that the rise of high water determined 
from one year of observations may require a correction as great as 3 percent to derive 
a mean value, while, when the ratio is 1.5, this correction is, at most, barely 1 percent. 
In other words, this correction is relatively large for tides of the semidaily type and 
small for tides of the mixed type. 
In the use of Table 5 for the determination of mean high water it is clear that only 
the range of tide is corrected to mean value by the use of that table. If no suitable pri- 
mary tide station is available for correcting half-tide level to mean value, it will be 
sufficient to use the half-tide level determined from the particular series of observations 
as the best-determined value of half-tide level. 
To exemplify the use of Table 5 it will be sufficient to employ it for correcting the 
range of tide to mean value. Ketchikan, Alaska, has a range of nearly 13 feet, and in the 
table below the yearly values of the range of tide for the 18-year period 1931-1948 are 
corrected to mean value. 
Correction of yearly range of tide to mean value 
KETCHIKAN, ALASKA 
Year Range Factor Mean range : Year Range Factor Mean range 
Feet Feet Feet Feet 
NO SNee ae 2 22S 12. 61 OZ 12. 95 LOA Oi es eee 13. 37 0. 973 13. 01 
LOR i 12. 61 1. 027 12. 95 PO AS ess Aa ESS chai tpn 13. 20 0. 973 12. 84 
OS) 8 eee 12. 55 1. 025 12. 86 LOAD Rice es es 13. 21 0. 974 12. 87 
TRO 40 ere ean 12. 65 1. 019 12. 89 118 6s Si epi 13! 25 0. 978 12. 96 
QBS) Se ee 12. 83 1. O11 12. 97 pA Ya 2 Ea eam 13. 07 0. 985 12. 87 
HOS OM EE Nie Loe 12. 96 1. 002 12. 99 OAD a See a 12. 89 0. 995 12. 83 
ORY a2 = ee 12. 95 0. 991 12. 83 VOAG 52 eco 2 eee 12. 75 1. 005 12. 81 
OS Sees Ss 13, 0. 982 12. 87 IO) by epee a ee a 12. 68 1. 014 12. 86 
OS OMe eS 13. 32 0. 976 13. 00 LOA BE ered gee 12. 59 1. 022 12. 87 
Average_—_-_-_-- 1D, BAS ose ae 12. 923 13. 001 12. 880 
In column 2 of the table above, the range of the tide for the year in question is 
given. From Table 4 the ratio of K,+O, to Mz; is 0.44. If the range of the tide were 
5 feet or less it would be sufficient to take the factors from column 3 of Table 5. But 
since the range at Ketchikan is nearly 13 feet, it is better to take the factors by inter- 
polating between columns 3 and 4, these factors being entered into the third column of 
the table above. The mean range is then determined by multiplying the values of 
column 2 by the factors in column 3. 
From the continuous series of observations at Ketchikan for the 19-year period 
1930-1948 the mean range of tide is 12.90 feet. The least yearly range for the 18-year 
period in the above table is 12.55 feet for 1933 and the greatest is 13.37 feet for 1940, a 
difference of 0.82 foot, and differing from the 19-year value by 0.35 foot and 0.47 foot, 
respectively. After correcting to mean values by the factors, the difference between 
them is 0.15 foot, and the differences from the 19-year mean are 0.04 foot and 0.11 foot 
respectively. The yearly values are thus corrected to within one percent of the mean 
range. 
Table 5 may also be used for correcting the range from a month of observations to 
mean value. The values in that table are computed for July 1 of each year, so that 
values for each month can be determined by interpolation. As an example of the ac- 
