TIDAL DATUM PLANES 99 
latter effects can be very largely eliminated by subtracting the height of each low water 
from the instantaneous height of sea level. The resulting heights of the low waters 
at the three stations for the same month of October are shown in Figure 45. The 
instantaneous height of sea level for any low water is derived by averaging the 24 hourly 
heights of the tide which center about the time of that particular low water. 
The times of the moon’s phases, extreme semimonthly declination and of apogee 
and perigee are plotted at the top of Figure 45. The periodic fluctuations in the height 
of low water in relation to the moon’s position now become clearer. For the semidaily 
and mixed types of tide Figure 45 shows that low water falls lower at the times of new 
1 5 10 IS) 20 BS) 30 
Om n@ S JA Beh tine 
Atlantic City a Sap 
| 4 
3 
Los Angeles 
Pensacola | 
Fic. 45.—Low waters, Atlantic City, Los Angeles, and Pensacola, October 1947 referred to instantaneous sea level. 
