Some Basic Chart Knowledge
If you are sailing in an area that you are not familiar
with, it is good advice to have a NOAA chart for the area. And, to have some basic knowledge regarding
what you see on the chart.
Just for starters: tan is land; light green is a tide flat
(land at low tide, water at high tide); light blue is for shallow water; and
white is for deeper water. How deep the
water is will be shown by numbers printed on the water. Soundings (water depths) will be in Feet for
fresh water areas; Fathoms (6 feet) for salt water areas; and Meters (metric)
on nearly all charts of foreign waters.
The Soundings are identified on the chart at the lower right corner and
the middle of the left top.
There will be a “distance scale” that shows distances that
you can use a divider to figure out how far it is from one place to another.
The Latitude Scale along the left and right margin of the
chart show the degree latitude north or south of the equator. The earth is divided into 90 equal parallels
which equal one degree north or south of the equator. When you look at the latitude scale, you will
see the degrees and minutes (as well as tenths of a minute). One minute of latitude is exactly the same distance
as one nautical mile (a nautical mile equals a minute of arc). So, you can use the latitude scale instead of
the distance scale if the latitude scale is closer to the part of the chart you
are working on.
You’ll find Anacortes, WA lies right on 48 degrees 30
minutes north latitude. And that
Bellingham straddles 48 degrees 45 minutes of north latitude.
Don’t confuse the latitude scale with the longitude scale
found at the top and bottom margins of the chart. Longitude is the east or west distance from
the Prime Meridian (0 degrees – which crosses through the observatory at
Greenwich, England. These meridians
connect the geographic north and south poles.
Longitude is measured in degrees and minutes… but because all of the
meridians taper to the poles, the distance is much greater at the equator, and
smaller as you get closer to the poles.
If you take NOAA Chart 18421 and measure the length of a longitude minute
at the bottom of the chart, you will find that it is only seven tenths of a
minute of latitude (or about 600 yards shorter).
In the United States the compass rose shows compass
points. There are three rings on the
compass rose. The outer ring shows a
compass that points to True North (the geographic north pole). The next inner ring shows a compass that
points to the magnetic north pole. The
magnetic north pole wobbles around the Arctic Ocean – and is presently near the
northern coast of Canada. Because of
this, if you are in the middle of the United States, these two rings are about
the same… but as you travel to the east or west coast, the variance between
True North and magnetic north increases.
In the San Juan Islands, the variance is about 18 degrees.
The third ring (most inner ring) shows what sailors refer to
as points on the compass. There are
eight points for each 90 degrees.
Nearly all charts show the aids to navigation (or buoys,
lighthouses, and day shapes). If the aid
to navigation has a light, the aid is highlighted by a purple exclamation mark
on its side or upside down. There is
lots of information for each aid. Is the
light white (or another color)? Is there
numbers or letters painted on the aid?
Is there a sound device (bell, horn, etc.)? And so on…
If you are not familiar with charts and the information
found on the chart, take a look at John Rousmaniere’s book “The Annapolis Book
of Seamanship” and NOAA Chart #1 (a book all about chart symbols).
barnaclebillholcomb@gmail.com
barnaclebillholcomb@gmail.com