114 U. S. COAST AND GEODETIC SURVEY 
Primary Determination 
A primary determination of mean lower low water is based directly on the average 
of the lower low waters over a 19-year period. And if there were no change in sea level 
from one 19-year period to another, we would expect two different 19-year determina- 
tions of mean lower low water at any place to agree, unless some change in tidal regime 
had taken place. 
At San Francisco, continuous tide observations are available for the 51-year period 
1898-1948. Three slightly overlapping 19 year series may be formed from these observa- 
tions; 1898-1916, 1914-1932 and 1930-1948. The values of mean lower low water on 
the staff for each of these series is 8.544 feet, 8.601 feet and 8.720 feet, respectively. 
Referred to the respective 19-year sea level, mean lower low water below sea level is 
respectively, 3.052 feet; 3.066 feet and 3.072 feet. 
The greatest difference between the three primary determinations of mean lower 
low water below sea level is 0.02 foot. It must be noted that during the 51-year period 
the location of the tide station had been changed several times. It is possible too, that 
because of improvements in the harbor the range of tide has increased slightly. In any 
event, 0.02 foot is a small quantity, so that the three determinations are in practical 
agreement. 
At Seattle, the 50 years of observations permit three slightly overlapping series; 
1899-1917, 1915-1933 and 1930-1948. For these three series the value of mean lower 
low water on the staff is 14.044 feet, 14.055 feet and 14.162 feet, respectively. Re- 
ferred to the respective 19 year sea level, mean lower low water below sea level is, 
respectively, 6.637 feet, 6.622 feet and 6.645 feet. The greatest difference between any 
two: of these three primary determinations is again 0.02 foot. And as in the previous 
case this difference is so smali as to be scarcely significant. 
Primary determinations of mean lower low water below sea level are therefore in 
practical agreement. However, since sea level at many places appears to be subject to 
a slow change, for precise purposes the datum of mean lower low water must be specified 
with regard to the 19-year series used. 
Relation to Low Water 
, It is necessary to distinguish not only between higher low water and lower low 
water, but also between the latter and low water. In general the term ‘low water’ 
embraces both higher low water and lower low water; but, when used in contradistinc- 
tion to lower low water, it refers to the average of the two low waters. For any day 
the difference between low water and lower low water is known as the diurnal low-water 
inequality, which in abbreviated form is written DLQ. 
The diurnal low-water inequality varies from day to day throughout a fortnight, 
being greatest a little after the time of the moon’s maximum north or south declination 
and least a little after the time when the moon is over the Equator. The mean value of 
the diurnal low-water inequality gives the difference between the planes of mean low 
water and lower low water. If, therefore, the plane of mean low water at any place is 
determined, the plane of lower low water becomes determined as soon as the mean 
value of the diurnal inequality is derived. 
