TIDAL DATUM PLANES BWA 
range of the tide for the month (Mn), the half-tide or mean tide level (MTL), higher 
high water, lower low water and the height inequalities. 
The Record From the Standard Tide Gage 
Before a tabulation of the hourly heights or of the high and low waters can be made 
from the tide record furnished by the standard tide gage, it is necessary to determine 
the relation of the curve to the zero of the tide staff. This is done by means of a tabula- 
tion of comparative readings of staff and curve, using a reading scale graduated in feet 
and tenths to the same scale as that to which the tide curve is drawn by the tide gage. 
A specimen sheet of the tabulation of the comparative readings for the tide record at 
Charleston, S. C., for the month of November 1948, is shown in Figure 17. 
In the first three columns of the comparative readings tabulation the tabulator 
notes, respectively, the day, the time of staff reading, and the height of staff, which 
items are taken from the tide roll as recorded in the observer’s notes. In the fourth 
column the tabulator notes the height of the curve by his reading scale at the time of 
the staff reading. This height obviously will depend on the height assumed for the 
datum line on the curve. It is most convenient to assume for the datum line a height 
which will be somewhat less than the staff reading for that point on the curve, so that the 
differences between staff and scale will be positive and lie between zero and 2 feet. The 
scale reading for the datum line in the specimen sheet of comparative readings shown 
in Figure 17 was taken as 5 feet. It will be noted that the staff height in the third column 
and the scale reading from the tide curve in the fourth column are taken to the nearest 
half-tenth of a foot. 
In the fifth column of the comparative readings tabulation the difference between 
staff and scale is derived, and in the sixth column the phase of the tide at the time of 
staff reading is noted. The letters F’, R, H, and L are used to designate, respectively, 
the falling tide, rising tide, high water, and low water. 
For the period of the month shown in Figure 17, the mean difference between scale 
and staff is found to be 0.34 foot. With a preliminary setting of 5 feet the height of the 
datum line on the tide rolls is 5.34 feet. A constant of 0.01 foot is included to refer all 
the tabulations to a fixed datum, since in April 1948 a new tide staff was installed which 
leveling to bench marks showed was 0.01 foot higher than the old staff. Hence the 
correct height of the datum line is 5.35 feet. This height the tabulator marks on his 
reading scale which is then used for tabulating that month’s record. 
Any change in the adjustment of the gage during the month will change the rela- 
tion between staff and scale. In such a case the two parts of the record are treated 
separately. The scale settings are computed separately for the two parts of the record 
and each part tabulated in accordance with its proper scale setting. 
The differences between staff and scale in the fifth column of the comparative 
readings tabulation will vary somewhat from day to day, primarily because of the 
difficulty of reading the staff to the nearest half-tenth of a foot if any wave motion is 
present. Since the figures on the staff increase upward an error of a foot is occasionally 
made by the observer in reading the staff, and this error of 1 foot will appear in the 
column of differences. An error of this kind, however, is easily noted and should be 
corrected before the differences are summed for the derivation of the mean. 
