122 . COAST AND GEODETIC SURVEY 
The derivation of MLLW below HTL for Neah Bay from monthly observations 
is shown below. 
Date LW below Factor from DLQ Factor from | MLW below Mean MLLW below 
HTL Table 5 Table 6 HTL DLQ HTL 
1946 Feet Feet Feet Feet Feet 
January aee= =e 2. 74 1. 000 1. 98 0. 87 2. 74 1. 72 6 
IWiarc neers WA, 192 1. 001 iy BD ib lG 2.72 Ils n7/ 4, 29 
May =oi2 eee 2. 84 1. 003 It, 7/al 0. 90 2. 85 i, 4,39 
alee wee 2, 84 1. 004 1. 82 0. 84 2. 85 1. 53 4. 38 
September - - - --- 2. 72 1. 005 1. 33 1.14 Danlles iL, ay 4, 25 
November--_---- 2. 70 1. 006 1. 84 0. 88 4, 172 1. 62 4, 34 
For Neah Bay the best deterimined value of DLQ is 1.58 feet and of MLLW below 
HTL, 4.40 feet. From column 7 it is seen that the correction of DLQ by tabular 
values may differ from the best determined value by as much as 0.14 foot. From 
column 8 the derived value of MLLW below HTL is seen to differ from the best de- 
termined value by as much as 0.15 foot. Comparing with the results derived by 
simultaneous observations (page 118) mean DLQ is about equally well determined by 
either method. 
Summary 
Lower low water is distinguished from higher low water and also from low water, 
the latter term in this connection referring to the average of the two low waters of the 
day. Lower low water, like low water, varies from day to day, from month to month, 
and from year to year, these variations being in part due to variations in sea level and 
in part to astronomic causes. 
A direct primary determination of mean lower low water requires 19 years of 
observations. Shorter series may be corrected to mean value either by comparison 
with simultaneous observations at a primary station or by tabular values. In either 
case mean lower low water is derived with reference to half-tide level. In general it 
may be taken that from a year of observations mean lower low water can be determined, 
with reference to half-tide level, correct to within 0.1 foot; from a month, correct to 
within a quarter of a foot; and from a day’s observations, correct to within about 1 
foot. It is to be noted, however, that occasionally the value derived from one day of 
observations may be considerably in error, so that at least three days of observations 
should be used if it is desired to determine mean lower low water correct within a foot. 
The secondary determination of mean lower low water involves the determination 
of half-tide level, mean low water, and mean diurnal low-water inequality; the distance 
of mean lower low water below half-tide level being the distance of mean low water 
below half-tide level plus mean low-water diurnal inequality. 
