TIDAL DATUM PLANES 103 
ence may be as much as 0.2 or 0.3 foot. Within the 25 year period represented, the 
lowest and highest yearly values differed by 0.66 foot at Boston, 0.38 foot at Los Angeles 
and 0.66 foot at Pensacola. 
Comparison of the diagrams of Figure 48 with the corresponding diagrams of yearly 
high water in Figure 42 brings out some resemblance but also some differences. And 
since low water like high water is affected by changes in sea level, we can eliminate the 
sea-level effects from yearly low water by subtracting the heights of these yearly low 
waters from the corresponding yearly values of sea level. Figure 49 shows the results 
in diagrammatic form. 
UI25) WSO) £S5 1940 1945 
4.8 
Boston 
4.9 
0.6 Pensacola 
On, 
Fie. 49.—Variation of yearly low water in relation to yearly sea level. 
