TIDAL DATUM PLANES 73 
year to year on the Atlantic coast, while on the Gulf and Pacific coasts larger differences 
will appear. This is brought out by the tide observations. 
For tides of the daily type the above formula for the relation of half tide level to 
sea level does not apply, since the higher low waters and lower high waters which occur 
only infrequently are not taken into consideration. In determining half-tide level in 
the case of the daily type of tide, only the higher high waters and lower low waters are 
used. The relation of half-tide level to sea level at two nearby stations which differ in 
type of tide may therefore be quite different. 
Galveston, Tex., furnishes a good example. Here the ratio of K,+-O,, to M.+S, is 
1.8 and therefore according to the criterion on page 21 it should be classed with the daily 
tide. But it is only infrequently that but one high water and one low water occur in a 
day, and for some purposes it is useful to tabulate the tide at this station as if it were of 
the mixed type. This can be done easily by introducing a high and a low water having 
the same heights on any day when it is necessary to make the tidal day contain two 
high and two low waters. 
When the tide observations at Galveston are tabulated as semidaily tides, haltf- 
tide level is found to be above sea level by 0.06 foot. If the tabulations are made in 
conformity with the practice for daily tides, that is, using only the higher high and 
1932 1934 1936 1938 1940 1942 1944 1946 1948 
Mixed Tides 
weuaeE Daily Tides AS: 
Fic. 36.—Relation of half-tide level to sea level, mixed and daily types. 
lower low waters of each day, half-tide level is found to be below sea level by 0.11 foot. 
In other words, the two half-tide levels derived for the same station by treating them 
as different types of tide differ by 0.17 foot. 
Figure 36 gives the yearly values of half-tide level minus sea level at Galveston 
for the 18-year period 1931 through 1948. The upper diagram gives the values deter- 
mined by tabulating the tide record here as a mixed type, while the lower diagram gives 
the values derived by tabulating the tide record as a daily type. The horizontal line 
associated with each diagram represents the average value of HTL-SL for each of the 
respective series, in accordance with the scale at the left. 
949995—51——_6 
