Figure 14-3.–Sample format showing hourly heights of tide required for computing average mean sea level (MSL).
shown in figure 14-3. The heights on this form
are added both horizontally and vertically. The
total sum covering 7 days of record is entered in
the lower right-hand corner of the page. The mean
for each calendar month is found by combining
all daily sums for the month and dividing by the
total number of hours in the month. The monthly
mean, to two decimal places, is entered on the
sheet that includes the record for the last day of
the month. Yearly means are determined from the
monthly means, and a mean is taken of all yearly
means for the period of record. Three or more
years of record should be used for a good deter-
mination of sea level. The actual value varies
somewhat from place to place, but this variation
is small. The station used for MSL determinations
should be on the open coast or on the shore of
bays or harbors having free access to the sea.
Stations on tidal rivers at some distance from the
open sea will have a MEAN RIVER LEVEL that
is higher than mean sea level because of the
river slope. It should be noted that mean sea
level is NOT identical with mean tide level (MTL).
The latter is derived from the mean of all high
and low points on the tidal curve. But MSL is
derived from the mean of a much larger number
of points taken at hourly intervals along the tidal
curve.
The datum universally used in leveling is mean
sea level (MSL), and it is considered to be the zero
unit. The vertical distance of a given point above
or below this datum then becomes the elevation
of that point.
Other Datums
Along the Atlantic coast of the United
States, the mean low water (MLW) datum
has been generally adopted as the datum
used for hydrographic surveys. It is the mean
of all low water tides observed over a long
period (usually a 19-yr period). Mean lower
low water (MLLW) has been generalIy adopted
for hydrographic surveys along the Pacific
coast of the United States, Hawaii, Alaska,
and the Philippine Islands. It is the mean
of the lower of the two low water tides for
each day observed over a long period. Mean
low water spring (MLWS) is used on the Pacific
coast of the Panama Canal Zone. It is defined
as the mean of the low waters of the spring
tides occurring a day or two after a full
moon and is obtained by subtracting one-half
of the range of the spring tides from the mean
sea tide level.
14-4
