TIDAL DATUM PLANES 85 
same, the rise of high water above sea level at any station would, from year to year, 
be constant. This, however, is found to be not the case, for investigation shows that 
the rise of high water above sea level varies from year to year. 
In part such differences must be ascribed to difficulties inherent in the operation 
of a tide station over considerable periods of time, and also to the disturbing effects of 
wind and weather. Changes in hydrographic features, whether natural or artificial, 
likewise may change the relation of high water to sea level, for such changes generally 
do not affect sea level but do affect the range of tide. 7 
The causes enumerated above for the variation in the relation of yearly high water 
to yearly sea level are not of a periodic character. If the height of high water above 
sea level from year to year is plotted for a number of years, a distinct periodic variation 
comes to light, the period of variation being approximately 19 years. In Figure 43 
ISZS 2 NSO 1935 1940 1945 
Boston 
Pensacola 
FIG. 43.—Variation of yearly high water in relation to yearly sea level. 
