TIDAL DATUM PLANES 65 
feet. On the 2d, therefore, sea level was 0.40 foot below its mean value while on the 
31st it was 1.92 feet above. Applying these corrections to the corresponding daily sea 
levels at New York, we derive a mean sea level value of 4.94+0.40=5.34 feet for the 
2d, and 7.28—1.92=—5.36 feet for the 31st. 
From the New York observations for 1930-1948, the primary determination of 
mean sea level on staff reads 5.34 feet. Thus the two daily sea levels which differed 
from each other by 2.34 feet, give mean sea level values which differ from each other by 
0.02 foot, and which in the one case agrees with the primary determination and in the 
other differs by 0.02 foot. 
If we take each day of the month of October 1947 at New York and derive mean 
sea level by comparison with Atlantic City, it is found that on the average these daily 
mean sea level determinations differ from the primary determination by 0.07 foot, while 
the greatest difference is 0.29 foot. 
If Boston and Portland, which likewise are about 100 miles apart, are used for the 
month of October 1947 shown in Figure 23, using Portland as the primary station it 
will be found that the mean sea level value for the 2d will differ by 0.06 foot from 
the primary determination, while for the 31st it will differ by 0.51 foot. But a glance 
at Figure 23 makes clear that on the 30th and 31st Boston sea level was responding to 
meteorological conditions prevailing southward which were reflected in much lesser 
degree at Portland. In fact, for the entire month of October 1947, mean sea level at 
Boston, when derived for each day by comparison with Portland, differs, on the average 
by 0.11 foot from the primary determination, the greatest difference being 0.51 foot 
for the 31st. 
In general it may be taken that mean sea level determined from one day of ob- 
servations when compared with simultaneous observations at some suitable primary 
tide station will give a value correct to within a quarter of a foot 
Month.—As an example of the determination of mean sea level from a month of 
observations we may again take New York and Atlantic City. Figure 24 shows that 
for the two-year period 1946-1947, monthly sea level at New York was lowest in 
February 1947 and highest in November 1947, reading on the staff, respectively, 4.84 
and 5.85 feet, a difference of 1.01 feet. 
Determining mean sea level for each of these months by comparison with Atlantic 
City, we find that for February 1947 sea level at Atlantic City was 0.36 foot below the 
1930-1948 value of mean sea level at that place while for November it was 0.55 foot 
above its mean value. Applying these corrections to the corresponding monthly values 
at New York we get mean sea level values of 5.20 and 5.30 feet respectively. At New 
York the 1930-1948 value of mean sea level on staff is 5.34 feet. Hence the values 
determined from each of the month’s observations differ by 0.14 foot and 0.04 foot, 
respectively, from the primary determination. 
If we derive mean sea level at New York for each month of the two years shown in 
Figure 24 by comparison with Atlantic City, it is found that the greatest difference from 
the nineteen year value is 0.17 foot, while the average difference is 0.08 foot. In general 
it may be taken that mean sea level determined from one month of observations, when 
compared with simultaneous observations at a suitable primary tide station, will give a 
value correct to within 0.1 foot. 
Year.—To exemplify the secondary determination of mean sea level from one year 
of observations, it will be instructive to take stations farther apart than those used in 
connection with 1 month of observations. Boston and Baltimore are 360 miles apart 
