Tide Differences Tables
These tables provide information for calcu- primary reference tide table. A minus sign (–)
lating the tides at additional locations along indicates that the time should be subtracted
the east coast, Bermuda and the Bahamas. from the primary reference table time.
This information is keyed to the tide tables for The two Height columns give the height
the primary reference stations found on the differences for high and low tide that should
preceding pages. be applied to the tide heights listed in the
Caution: The time and height values in primary reference table. Either multiply *,
the table are average differences derived add +, or subtract – , as indicated. The first
from comparisons of simultaneous tide ob- difference is applied to high-tide; the second,
servations at the secondary location and its to low-tide. If parentheses enclose the height
reference station. Because these values are differences, the tide height from the primary
constant, they may not always provide for the reference table must be multiplied by the first
daily variations of the actual tide, especially number inside the parentheses, and the result
if the secondary station is some distance from further modified by the second number as
the primary reference station. Therefore, al- noted -- add or subtract.
though the application of the time and height Apply to derived heights to chart sounding
differences will generally provide fairly accu- to obtain water depths at tide. This tidal data
rate approximations, they cannot result in pre- is based on the tidal datum of the reference
dictions as accurate as those at the reference station, which in turn is related to the local
station. The reference station values are based chart datum in use. The chart datum in U.S.
upon much larger periods of analysis. waters is Mean Lower Low Water.
T
ides
How to use the table
Range provides average spring range
(monthly large tides) in feet for the tide differ-
Tide differences are organized geographi-
ence station.
cally, beginning with stations in Nova Scotia,
Clearance under bridges and overhead
then following the Atlantic coast south, the
power lines is measured from Mean High
Gulf coast westward, and ending with Ber-
Water (MHW), which is not listed in these
muda and the Bahamas.
Tide Tables. However, MHW for many loca-
In the Place column, locate the desired tide
r
e
n
c
es
tions is listed in the Tidal Information block
difference station. Then note the name of the
on NOAA charts. Add this value to the listed
i
ff
e
primary reference tide station listed in bold
bridge clearance and you have the clearance

D
type under the Differences heading above the
at Mean Lower Low Water, the chart datum.
station you have chosen. The first reference
Subtract the present height of the tide (inter-
above your chosen station is the table from
T
ide
polated or calculated using the height of tide
which you calculate your differences.
at any time table) to establish clearance.
Position denotes approximate latitude and
Note: In the table, dashes (– –) are substi-
longitude in minutes and tenth of a minute.
tuted for data that is unknown, indeterminate,
Under the Differences heading, the two
unreliable, or not applicable.
Time columns give the time differences in
Caution – Cape Cod Canal
hours and minutes to be added or subtracted
Predictions of the times of low water at the
to the times given in the primary reference
Cape Cod Canal must be used with caution
table for the desired day. The first column
because of the peculiarities in the behavior
gives the time difference for high tide, and
of the tide. The tide may be practically at a
the second column gives the difference for
stand for as much as two hours before or after
low tide. A plus sign (+) indicates that the
the predicted times of low water, the levels at
time should be added to the time listed in the
other than high and low water times cannot
be obtained in the usual way as in Table 3
(Height of Tide at Any Time).
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