A Few Tips To Help You Use Your Charts
Knowing how to read a chart and how to use the chart for
navigation (especially Dead-Reckoning) is an important seamanship skill. A few of the key parts of the chart that you
should recognize immediately are: the Compass Rose, the Latitude Scale, what
units the Soundings are in, Scale and Linear Measurements, and Cautions, Notes,
and Warnings
The Compass Rose shows you two rings with degree markings
around the rings. The outer ring (with
the Star at the top of the circle) shows you True North and all of the points
relative to True North. On charts, True
North points to the geographic North Pole and is straight up the chart. The inner ring shows you Magnetic North (the
little arrow) and all of the compass points relative to Magnetic North. On most charts there is a difference between
True and Magnetic. This difference is
called Variance. In the “old days”,
navigators would adjust compass information into True information. That is complicated and can easily lead to
errors. Most of us now use the inner
Magnetic circle because we can directly use those numbers with our compass or
GPS information.
The
Latitude Scale along the left and right sides of the chart show (in Degrees,
Minutes, and tenths of minutes) how far north or south of the equator you
are. The reason that this scale can be
so important is that the navigation saying, “A Mile per Minute”. Each dark or light section on the Latitude
Scale represents one nautical mile. If
you are working on a part of the chart that does not show the Scale and Linear
Measures, you can easily use the Latitude Scale to determine distances
The depth of the water is shown across the chart in many
places with little numbers. It is
important to know what these numbers represent.
For fresh water charts (like Lake Pend Oreille, the Great Lakes, and
many Rivers) the soundings are in Feet.
For salt water charts (like 18421 or the San Juan Islands) the soundings
are in Fathoms. One Fathom equals six
Feet. For Canadian charts (and most of
the rest of the world as well) the soundings are in Meters (Metric). When you move from one chart to another make
sure that you take note of the soundings.
This is particularly important if you are sailing from the San Juan
Islands north up the Strait of Georgia.
The San Juan Islands’ chart shows soundings in Fathoms; the Strait of
Georgia is in Canada, so the soundings are in Meters. There is a big difference between 5Fathoms
and 5 Meters. 5 Fathoms is 30 feet. 5 Meters is about 15 feet (half as deep).
Most charts have typed information in the form of Cautions,
Notes, and Warnings regarding conditions relating to the chart. Chart 18421 of the San Juan Islands has many
of these; including one warning along the West shore of Whidbey Island near the
bottom of the chart that says: “Naval
Air Station small arms range operates 7 days a week. Red flashing light and flags are displayed
during live fire exercises. Use caution
when transiting near the zone.” So, read
all of the “fine print” on the chart.
BTW – the chart for Lake Pend Oreille has a warning near Oden Bay
regarding high voltage lines crossing above the lake.
There is lots of other information on charts. But these five things are excellent starting
points.
Barnacle Bill Holcomb
barnaclebillholcomb@gmail.com
