XI. OTHER TIDAL DATUMS 
Principal Datum Planes 
The six datum planes discussed in the preceding pages, namely, mean sea level, 
hali-tide level, mean high water, mean low water, lower low water, and higher high 
water, constitute the principal tidal datum planes. They are more easily determined ° 
than other tidal datum planes, and from a given series of tide observations they can 
be derived with a greater degree of precision. 
Other tidal datum planes have at times been used. Thus, the planes of monthly 
lowest low water and spring low water and the Indian tide plane have been used in 
hydrographic surveying and in tide predictions. To determine accurately such datum 
planes directly from observations requires a much longer series of observations than is 
necessary for any of the principal planes, for spring tides or tropic tides occur but twice 
a month and monthly lowest low water but once a month. As a rule, however, ap- 
proximate determinations of such planes are quite satisfactory, especially if their 
relation to mean sea level or half-tide level is stated. 
When the use of some datum plane other than one of the principal datums is 
found of advantage, it is desirable that it be defined with reference to one of these 
principal datums. Thus, if a plane below mean low water or mean lower low water 
is to be used, it is best to define it by its distance below either of these datums or mean 
sea level rather than seek some secondary tidal datum which approximates it. 
Several datum planes have, however, been used heretofore, and it is proposed here 
to discuss them briefly. 
Monthly Lowest Low Water 
When a datum plane is desired which will be so low that most low waters will be 
above it, the plane of monthly lowest low water has sometimes been used. As its name 
signifies, it is the plane determined by the average height of the lowest low waters of 
each month over a considerable period of time. 
This plane has sometimes been called the plane of extreme low water or of storm 
low water, but objections may be urged against both of those designations. Calling 
the lowest low water of each month an extreme low water is obviously arbitrary, while 
calling it a storm low water is even more arbitrary, for the lowest low water of a month 
is frequently not due to storms. The term monthly lowest low water is self-explan- 
atory and definitely refers to the low water which, during the month in question, 
falls to the lowest level. 
Within a year, the heights of monthly lowest low water may vary considerably. 
Yearly averages of these monthly lowest low waters, however, will vary by less than 
a foot, and a three-year average will not differ by more than a quarter of a foot from 
a mean based on a number of years. 
Along the Atlantic coast of the United States, the plane of monthly lowest low 
water is below mean low water by the following amounts: from Maine to Rhode 
Island, about 2 feet; from New York to Georgia, about 1} feet; Florida, about 1 foot. 
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