Nautical charts rely on
a wide variety of symbols, abbreviations and terms to indicate where the
navigable waters are. Some of the
most basic information on a nautical chart reveals water depth and the location
of dayboards and buoys. Dayboards and buoys are collectively referred to as aids
to navigation, for which the acronym ATON is sometimes used.
Charted water depths
are called "soundings." Soundings
are represented by numbers printed on charts in the areas occupied by water.
The title block on your chart will indicate whether the soundings are in
feet or fathoms (one fathom = 6 feet). Soundings
in fathoms are usually used in the ocean or in waterways where the depths are
great. The soundings are calculated
using the average of the low tides. There
are a few ways to calculate low tide, and no two low tides are exactly the same
level. Due to the use of averages,
it is possible that the water dept may be slightly less than what the chart
indicates.
Another factor related
to water level is the wind, which can push the water in one direction or the
other, changing the depths independently of the tide. When the water level is influenced by the wind, the effect is
called a "wind tide." Wind
tides are not considered in the calculation of charted soundings, since the wind
is unpredictable and unrelated to normal tidal action.
ATONs are illustrated
using symbols and abbreviations. An
example is: Fl G 4s 16ft 4M "7", which marks Garbacon Shoal on the
Neuse River offshore of Oriental. The
previous string of abbreviations is printed next to what looks like a magenta
colored exclamation point with a black dot.
The magenta exclamation indicates that the navigation aid is lighted.
The "Fl G 4s" indicates that the light flashes the color green
every four seconds. The "16ft 4M" indicates the ATON is 16 feet above
the mean high tide water level and is visible from 4 nautical miles in 10 mile
visibility. The "7" is
the number that appears on the ATON. It
is always between quotation marks to distinguish it from soundings and other
things that may be printed on the chart.
If the Garbacon Shoal
ATON was unlighted, the symbol used would be a small green square.
If the Garbacon Shoal ATON was a buoy, the symbol used would be a small
green diamond. There are literally thousands of symbols, abbreviations and
terms used on a nautical chart. Believe
it or not, they are very easy to keep track of.
The Department of
Commerce, the National Ocean and Atmospheric Administration and the National
Ocean Service jointly publish a 99 page reference book called Chart No. 1.
It lists and explains all of the symbols, abbreviations and terms used on
nautical charts. It is available at most marine stores. The standard price of $2.50 is regulated by the federal
government. It should be part of
your collection of marine publications and kept aboard your vessel. It is easy to use and very informative.
Until next time, I wish you clear skies, fair winds, and calm seas!
Captain Larry Walker is the president of World Wide Marine Training, Inc., a US Coast Guard approved facility authorized to give the Captain's license examination up to the 200 Ton Master level.
Visit the World Wide Marine Training website at: www.wegivethetest.com
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