TIDAL DATUM PLANES TH 
rections for January 1940 and September 1948 are, respectively, 0.91 and —0.97, 
which give mean values for these months at Eugene Island of 2.16 feet and 2.09 feet 
respectively. The mean value of daily half-tide level at HKugene Island from the 9 
years of observations when corrected by comparison with Galveston is 2.22 feet. 
Hence the derived mean values for the two months differ by 0.06 and 0.13 foot from a 
primary determination. 
In general it may be said that the accuracy with which the datum of half-tide 
level can be derived from short series of observations is-nearly the same as for sea level. 
By comparison with a suitable primary tide station, a day of observations will give 
mean half-tide level correct to within a quarter of a foot; a month of observations to 
within a tenth of a foot and a year of observations to within 0.05 foot. 
Since half-tide level will be affected by slow changes in the relation of land to 
sea exactly as is sea level, it is necessary to specify the 19-year period which is used 
to derive mean values. 
