VI. HALF-TIDE LEVEL 
Definition 
The plane of half-tide level, or mean tide level, as it is sometimes called, is defined 
as lying exactly halfway between the planes of mean high water and mean low water. 
It is thus a plane lying close to mean sea level, and frequently the two are taken as 
synonomous. As accurate datum planes, however, the two must be carefully 
distinguished. 
Strictly, the plane should be designated as that of ‘‘mean half-tide level,” rather 
than “‘half-tide level’? in consonance with the distinction between sea level and mean 
sea level. No confusion, however, results from the dropping of “mean,” since the 
context clearly indicates the sense in which the term is used, whether to designate the 
half-tide level for a short period of time, as a day, week, or month, or as a datum plane. 
Prior to the invention of the automatic tide gauge the recording of the tide through- 
out the 24 hours of the day was a matter of considerable expense. It was therefore 
customary to observe the tide only near the times of high and low water. This per- 
mitted a tabulation of the high and low waters but not of the hourly heights. Half- 
tide level could be determined from such tabulations, but not mean sea level; and as 
a rule the earlier determinations were those of the plane of half-tide level. 
While the tabulation of the hourly ordinates is necessary in the harmonic analysis 
of the tide, the only datum plane derived from such tabulation is the plane of mean 
sea level. From the high and low water tabulation, however, not only is the plane 
of half-tide level determined, but also the various high-water and low-water planes. 
Moreover, since mean high water and mean low water are symmetrical with respect 
to half-tide level, a determination of the one is also a determination of the other. It 
is therefore customary to derive these high-water and low-water datum planes with 
regard to half-tide level. 
Variations in Half-tide Level 
The tide oscillates about sea level, high water and low water being, respectively, 
the maximum and minimum of the oscillation. And, on the average, the rise of high 
water above sea level is approximately the same as the fall of low water below sea level. 
Since half-tide level lies halfway between high water and low water, it follows that 
it must vary in much the same way as sea level. 
This conclusion is borne out by an examination of daily, monthly, and yearly 
values of half-tide level. The variations in sea level discussed in the previous section 
may be taken to represent also the corresponding variations in half-tide level. It is 
unnecessary, therefore, to go into a detailed discussion of the variations in half-tide 
level. It will be sufficient to note in summary that, like sea level, half-tide level at 
any point on the coast varies from day to day, from month to month, and from year 
to year. From one day to the next, half-tide level may vary by a foot or more, and 
within the same year two values of daily half-tide level may differ by 5 feet or more. 
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