TIDAL DATUM PLANES Vi 
results, the direct determination of the tidal datum planes and other tidal character- 
istics should be based on a series of observations that covers a number of years. Values 
derived from shorter series must be corrected to mean values. 
As a rule, it is impracticable to secure long series of tide observations at all places 
where tidal datum planes are desired. To correct to mean values the results derived 
from short series of observations, two different methods may be employed.’ One 
method makes use of tabular values, determined both from theory and from observa- 
tions, for correcting for the different variations. The other method makes use of 
direct comparisons with simultaneous observations at some nearby place for which 
mean values have been determined from a series of considerable length. The latter 
method is the more satisfactory one in connection with the determination of tidal 
datum planes. 
