TIDAL DATUM PLANES 125 
the harbor may have changed the range of tide somewhat. In any event 0.026 foot 
is a small quantity and the three determinations are in practical agreement. : 
From the 50 years of observations at Seattle, 1899-1948, three slightly over- 
lapping 19-year series may be formed, 1899-1917, 1915-1933, and 1930-1948. The 
value of mean higher high water on staff for each of these series is, respectively, 18.744, 
18.754 and 18.867. The difference between the first and second determinations is 
0.010 foot and between the first and third 0.123 foot. Referring these values to the 
respective mean sea levels for the three periods, the results are, respectively, 4.700, 
4.699, 4.705. The greatest difference between any two of these three determinations 
is 0.006 foot. 
Secondary Determination 
In deriving mean higher high water from a short series of observations the pro- 
cedure and accuracy attainable are exactly the same as in the like determination of 
lower low water. For a series varying in length from a month to several years two 
methods are available. One method consists in comparing with simultaneous obser- 
vations at some suitable primary tide station, while the other method makes use of 
factors derived from theoretical considerations. The procedure is given in detail in 
the section on lower low water. Here it will be sufficient to exemplify the procedures 
briefly by an example of each method, using La Jolla as the secondary station and Los 
Angeles as the primary station, taking every other year from 1936 to 1946. : 
The epoch to which the data will be corrected is the 19 years 1928-1946. For 
this epoch high water above half-tide level at Los Angeles is 1.88 feet and mean DHQ 
is 0.75 foot. The factor for each year in column 6 is derived by dividing 1.88 by the 
corresponding value in column 2, and the factors in column 7 by dividing 0.75 by the 
corresponding value in column 3. 
Mean higher high water from a year of observations, La Jolla, by comparison with Los Angeles 
SS= 
Los Angeles La Jolla Factors for correction La Jolla 
Year HW HW S MHW Me MHHAHW 
PENG Ce) GN ie ee ta Ee ica |, Cea 
E Feet Feet Feet Feet eet Feet Feet 
ES 3 Gee es 1.89 . i 83 0. 7 0.995 0. 949 il 0. 7 2. 55 
LOS S27 sss soe 1. 89 0. 72 1, 81 0. 70 0. 995 1. 042 1. 80 0. 73 2. 53 
LOA Q aie So 1.93 0. 67 1. 86 0. 65 0. 974 1.119 1. 81 0. 73 2. 54 
LOAD ee oe 1. 90 0. 65 1. 83 0. 65 0. 989 1.154: 1. 81 0. 75 2. 56 
VOAAR Sees 1. 92 0. 69 1. 84 0. 69 0. 979 1. 087 1. 80 0. 75 2. 55 
NGAGE eee eek 1. 87 0. 74 il, Sil 0. 74 1. 005 1. 014 1. 82 0. 75 2. 57 
From the 19-year series 1928-1946 at La Jolla the primary determinations are: 
MHW above HTL, 1.81 feet; mean DHQ, 0.74 foot; and MHHW above HTL, 2.55 
feet. From the last three columns of the table above it is seen that the derived mean 
values differ from the primary mean values by not more than 0.02 foot. 
To exemplify the method of correction by tabular factors, we may take La Jolla 
for every other month of the year 1946. The procedure is given in tabular form below. 
