34 seamanship secretsto go, and you have a true direction.
All meridians of longitude on the chart
point toward the true, or geographic,
North Pole. Th us, to measure true direc-
tions, we measure the angle clockwise
from a meridian to a course or bearing
line drawn on the chart.
Magnetic directions. Most
small craft use a magnetic compass.
Th is instrument consists of a compass
card with a small magnet attached to
its bottom. The card floats in liquid
inside an airtight case so that it can
turn without friction, and the magnet
rotates the card until it lines up with
the magnetic North Pole, which is
some distance from the true North
Pole. Now imagine pointing one arm
toward the magnetic North Pole and
the other in the direction you want to
go. Unless you happen to be standing
in a spot from where the magnetic and
true north poles line up, you’ll find
that this magnetic course angle diff ers
from the true angle (i.e., the angle of
your course in degrees from the true
North Pole). Th e diff erence between
the two angles can vary from 1 degree
to more than 50 degrees, depending
on where you are located! Th is diff er-
ence, called variation, changes when
we move from one point to another on
the Earth’s surface.LOCATION, LOCATION,
LOCATION—VARIATION
EXPLAINED
If we sail from Miami to Bermuda,
the variation from the beginning to
the end of our passage will change
by around 12 degrees. If we make aA Primer on Plotting Terms
When you plot lines on a chart, they
might be referred to as courseline
(course) or trackline (track). Many
navigators use these interchangeably.
In this book, we will use the following
terms.
Trackline (or track), abbreviated
TR. The plotted direction. You plot the
trackline (or track) onto the chart. This
is the track you want the boat to follow
from point A to B. But sometimes wind
or current push the boat to one side
or the other. In that case, you have to
steer a different course (see next). To
label a trackline, precede the direction
of the line with “TR”; for example,
“TR—233M” means that the trackline
direction from A to B is 233 degrees
magnetic.
Compass Course (or steering course).
The course steered by the person on the
wheel or tiller. If it is found that the boat
is no longer on the TR, the navigator
changes to a new compass course (or
steering course), to return the boat to
the trackline. If you steer a compass
course different from the trackline, plot
it onto the chart. Precede the course
with a “C”; for example, “C—225M”
means that the steering course is 225
degrees magnetic.
Course or Courseline. A general term
that covers both steering course and
trackline (or track). You plot a course
onto the chart. The line itself is the
trackline. Or, you could measure the
trackline with parallel rules or protractor
to fi nd the magnetic course. Then again,
someone might ask what course you
are steering. You would tell them what
compass course you are steering. Many
navigators precede all directions with a
C on the chart. We will prefi x all tracks
with “TR” and compass courses with
“C” to eliminate confusion.