establish position. It is an essential navigational
tool, and every boat should be
equipped with at least one.
How a Compass Works
An irregular and constantly changing magnetic
field surrounds the earth. The poles of
this magnetic field are not in the same location
as the earth’s true geographic poles,
where the longitudinal meridians converge.
The magnetic North Pole is more than 600
miles away from the true North Pole.
Variation
(Earth’s magnetic effect)
Variation is the angle between the true
north geographic pole and the magnetic
North Pole. When the earth’s magnetic
field causes the compass to point west of
true north, the variation is a westerly variation.
Conversely, when the magnetic field
causes the compass to point east of true
north, the variation is an easterly variation.
Note that the variation on our practice
chart is westerly because of its geographic
location. Figure 18-8 illustrates both easterly
and westerly variation.
Variation changes with geographic
position. The variation for a given location
can be found on the chart. It is in the
center of the compass rose closest to the
location. The annual change is also shown.
When used for piloting, variation is always
expressed in whole degrees.
Deviation
(Boat’s magnetic effect)
Metallic objects and stray magnetic fields
created by electrical wires and magnetic
materials aboard a boat can affect the compass
reading. This effect is called deviation.
Careful placement of the ship’s compass
is essential. It should be as far as possible
from electrical wires, instruments, and
metallic materials. Also, make sure you do
not place objects near the compass while
underway. A cell phone can significantly
alter your compass reading. Compasses
can be adjusted to remove most residual
Section 18: Charts 211
31
32
33
34
35
Figure 18-8. Variation. Left vessel has easterly variation and right vessel has westerly variation.