TIDAL DATUM PLANES 89 
to give the values of mean high water above half-tide level in column 12, and the cor- 
rections of column 6 are applied to the corresponding values of column 10 to give the 
values of mean half-tide level in column 13. By adding columns 12 and 13, the values 
of mean high water in column 14 are derived. 
The observed average heights of daily high water in aclu 8 are seen to have 
varied from 10.05 feet to 12.15 feet, a difference of 2.10 feet. After correction to mean 
values the difference between the largest and smallest values in column 14 is seen to be 
0.16 foot. From a 14-year series of tide observations at Seaveys Island, the value of 
mean high water corrected to the 19 years 1930-1948 is 10.38 feet. The values of 
mean high water derived from each of the one day of observations thus differ from the 
best determined value by less than 0.1 foot. 
Boston and Seaveys Island are about 50 miles apart, have tides of the same ate 
and during the month of May 1946 the tide in the region was relatively free from large 
disturbances of wind and weather. Hence mean high water derived from one day of 
observations gave a value within 0.1 foot of the best determined value from a long series 
of observations. In general it may be taken that like sea level or half-tide level one day 
of observations will determine mean high water within a quarter of a foot, if a suitable 
primary tide station is available for comparison. 
Month.—The tide at the station used in the previous section to derive mean high 
water from one day of observations is of the semidaily type. To exemplify the deriva- © 
tion of mean high water from a month of observations we may turn to the daily type. 
In this type of tide it will be recalled mean high water is the same as higher high water. 
At Mobile, Ala., and at Pensacola, Fla., the tide is principally of the daily type and we 
may use the tide observations at the former place for every other month of the year 
1936 for deriving mean high water by comparison with Pensacola. The two places are 
about 50 miles apart but lie in different bays.. 
In the table below the data are given. The procedure is similar to that for deriving 
mean high water from one day of observations. An abridgement of the tabular form is 
made by omitting the column for low water since the half-tide level for the month can 
be taken directly from the high and low water tabulation. 
Pensacola Mobile 
Height on staff Corrections for Height on staff 
Date MEHW 
. HWw- HW- 
HTL HTL above MHTL | MHW 
HW | HTL Map| Mean | aw | arw HTL 
range 
1936 Feet Feet Feet Feet Factor Feet Feeé|} Feet Feet Feet Feet 
January ------- 9. 24 8. 61 0. 63 0.12 1. 001 3. 67 2. 91 0. 76 0. 76 3. 03 3.79 
FSC) Wepre eee 8. 88 8. 25 0. 63 0. 48 1.001 3.31 22, fail 0. 80 0. 80 2.99 3.79 
IN ee 9. 48 8. 82 0. 66 —0. 09 0. 956 3. 87 3.08 0.79 0.76 2.99 3.75 
ip Sa eee 9. 42 8. 75 0. 67 —0. 02 0. 942 3.72 2.93 0.79 _ 0.74 2.91 3. 65 
Sen cnaber wees 9. 58 9. 04 0. 54 —0. 31 1.169 3. 84 3.17 0. 67 0. 78 2. 86 3. 64 
November---_- 9. 09 8. 42 0. 67 0. 31 0. 942 3. 21 2. 43 0.78 0. 73 2.74 3.47 
For Pensacola the 19 years of observations 1930-1948 give mean high water as 
9.363 feet on staff and mean half-tide level as 8.732 feet, so that the half range is 0.631 
feet. From these values the corrections for each month are computed in columns 5 
and 6, and used for deriving the values in columns 10 and 11, from which the last 
aplusan is derived. 
949995—51—_—_% 
