Types of Tides and Reference Planes
According to the characteristics of the tidal pattern occurring at a
particular place, tides are classified as semidiurnal, diurnal, or mixed.
In semidiurnal tides, there are two high and two low tides each tidal
day, and they occur at fairly regular intervals. Usually, there are only
relatively small variations in the height of any two successive high or
low waters. Tides on the Atlantic coast of the United States are
representative of this pattern.
In diurnal tides, there is only one high and one low tide each tidal day.
The water levels on succeeding days usually do not vary a great deal.
In the United States, diurnal tides occur along the northern shore of the
Gulf of Mexico.
In mixed tides, the tidal pattern is characterized by wide variations in
heights of successive high and low waters. There are usually two high
and two low waters each day, but occasionally the tide may become
diurnal. In the United States, mixed tides occur along the Pacific Coast,
Alaska, and Hawaii. If information for water depths, heights, elevations
of topographical features, aids to navigation, bridge clearances, and so
forth are to be meaningful when printed on nautical charts, standard
reference planes for their measurements must be used. For this reason,
standard reference planes for these measurements have been established.
Generally speaking, heights and elevations are given on a chart in
reference to a standard high-water plane, while heights of tide and
charted depths of water are given with respect to a standard low-water
plane (see fig. 7-2). The charted depth is simply the vertical distance
from the low-water reference plane to the ocean bottom; its the depth
figure you see printed on nautical charts. The charted height is the
vertical distance above the water measured from the high-water
The mean range of tide is the vertical distance between the high water
and low-water reference planes used, and represents the average range
of tide at a given location.
You should remember that the water level
will sometimes be below the reference plane. Put another way,
sometimes the actual depth of water can be less than the charted
depth. You will recognize this situation because there will be a minus
sign (-) placed before the height of tide shown in the Tide Tables. In
this case, you subtract the value of the height of tide from the charted
depth to find the actual depth of water.