116 U. S. COAST AND GEODETIC SURVEY 
Secondary Determination 
Secondary determinations of mean lower low water, that is, determimations from 
series less than 19 years in length, are most conveniently derived by determining the 
mean value of the low water inequality and applying it to mean low water. The 
derivation of mean low water was discussed in Section VIII, and it therefore remains to 
discuss the derivation of the diurnal low water inequality. 
Two methods are available for the determination of mean low water diurnal 
inequality: (1) comparison of simultaneous observations; (2) correction by tabular 
values. The method of comparison of simultaneous observations will be taken up 
first. 
Comparison of Simultaneous Observations 
To exemplify the determination of mean low water inequality by this method, the 
procedure used and the accuracy attainable will be illustrated below for periods of 
various lengths. The work will be carried through to the determination of mean 
lower low water. 
In the tabulation of the high and low waters it is customary to derive the monthly — 
averages of high water, low water, half-tide level, higher high water and lower low 
water. The difference between the monthly average low water and lower low water 
then gives immediately the monthly average of the low water diurnal inequality or 
DLQ. Yearly values of the various data are derived by averaging the corresponding 
monthly values. 
Year.—To exemplify the derivation of mean lower low water from a year of 
observations we may take every other year from 1936 to 1946 at La Jolla, using Los 
Angeles as the primary station. The procedure is shown in tabular form below. 
Los Angeles La Jolla Factors for corrections La Jolla 
Year an es 
LW below LW below r ] y 
HTL | DLQ HTL. | DLQ TN DLQ  |pelow HTL DLO nelow HDL 
Feet Feet Feet Feet Feet Feet Feet 
O'S Oster eee 1. 89 0. 96 1. 83 0. 93 0. 995 0. 990 1. 82 0. 92 4. T/A 
TOS Sie ee 1. 89 0. 87 il, ll 0. 84 0. 995 1. 092 1. 80 0. 92 Die 
IGA. = cya 1. 92 0. 82 1. 86 0. 80 0. 979 oe 539) 1. 82 0. 93 2215 
OAD easier bis 1. 90 0. 84 1. 83 0. 82 0. 989 1, USI 1. 81 0. 93 2A: 
NO Areas Seat eye spe il, Ql 0. 91 1. 83 0. 88 0. 984 1. 044 1. 80 0. 92 DEP: 
QA Giga Mee oe 1. 87 0. 99 1. 81 0. 96 1. 005 0. 960 1. 82 0. 92 2. 74 
The epoch to which the data will be corrected will be taken as the 19 years 1928— 
1946. For this epoch low water below half-tide level at Los Angeles is 1.88 feet, and 
DLQ is 0.95 foot. In the sixth column the factors are derived by dividing 1.88 by 
the values in column 2, and in the seventh column they are derived by dividing 0.95 
by the values in column 3. The values in columns 4 and 5 are then multiplied by their , 
respective factors in column 6 and 7 to give columns 8 and 9, the addition of which 
gives column 10. | 
