TIDAL DATUM PLANES 95 
stations, the use of a 30 or 31 day month does not introduce the error that accompanies 
such use with tabular values that are better fitted for a 29 day month. In part, too, 
the comparison method tends to correct for disturbing effects of wind and weather. 
Summary 
In the preceding sections it was found that the height to which high water rises 
varies considerably from day to day and from month to month and, in lesser degree, 
from year to year. This variation is of two kinds—the first, in response to changes in 
the phase, distance, and declination of the moon; and the second, in response to the 
variation in sea level. Witb the exception of the variation due to change in longitude 
of the moon’s node, the variations due to changes in the position of the moon balance 
out very largely within a month, so that in a sense the variation due to changes in sea 
level is the primary variation. 
It was found, too, that the rise of high water above half-tide level, from observa- 
tions covering periods of a month or more, may be corrected to a mean value either by 
‘factors derived from theoretical considerations or by comparison with simultaneous 
observations at some suitable primary station. But in this comparison, type of tide, 
and not nearness, determined suitability. In this regard the correction of high water 
to a mean value differs from the like correction of sea level, in which the suitability of 
a station for comparison purposes depends on tbe existence of like meteorological 
conditions. Furthermore, in correcting sea level to a mean value the changes in 
height at two nearby stations were taken as the same, whereas the changes in the 
rise of high water are taken as proportional. 
In general it may be taken that, when corrected to a mean value, a year of tide 
observations will determine the rise of high water above half-tide level correct witbin 
0.05 foot, a month within 0.1 foot, and a day within 0.5 foot. However, in regions of 
large range of tide, and of considerable variation in the rise of high water, a day of 
observations, especially when the factor for correction differs considerably from 1.00, 
may give a value differing by a foot or more from a primary value. 
It should be noted that mean high water is determined with respect to half-tide 
_ level which is itself subject to variations. Hence the accuracy with which the plane 
of mean high water can be determined depends also on the accuracy with which the 
plane of half-tide level is determined. The degree of accuracy in deriving mean high 
water, noted above for observations covering various periods of time, refers only to the 
rise of high water above half-tide level. 
To determine the plane of mean high water from any given series of observations 
the plane of half-tide level must first be determined; then the rise of bigh water above 
half-tide level is corrected to a mean value, and this gives the plane of mean high 
water above the plane of half-tide level. 
