that heading. When converting true heading to
magnetic, subtract easterly errors and add westerly
Before we go any further, you must know how
distances are measured along the circumference of a
circle. Measurement along a meridian, a perfect circle,
is expressed in degrees of arc. These degrees of arc may
be transformed into linear measurement. The compass
card is the best example of circular measurement in
degrees of arc.
Whatever the size of the card, its circumference
always contains 360°. Each degree contains 60 minutes
('), and each minute contains 60 seconds ('').
MAGNETIC COMPASS ERROR
LEARNING OBJECTIVE: Explain magnetic
compass error, including variations and
Most of the time the magnetic compass does not
point directly north. Usually, there is a difference of
several degrees. This difference, known as compass
error, is made up of variation and deviation.
The true North Pole and the magnetic north pole are
not located at the same spot. This variation causes a
magnetic compass needle to point more or less away
from true north. The amount the needle is offset is called
variation because the amount varies at different points
on Earth's surface. Even in the same locality variation
usually does not remain constant, but increases or
decreases at a certain known rate annually.
The variation for any given locality, together with
the amount of annual increase or decrease, is shown on
the compass rose of the chart for that particular locality.
The compass rose shown in figure 2-4 indicates that in
1990 there was a 14°45' westerly variation in that area,
increasing 1' annually.
To find the amount of variation in this locality in
1995, count the number of years since 1990 (in this case
5); multiply that by the amount of annual increase;
(which here gives you 5 X 1', or 5); add that to the
variation in 1990 and you have a 1995 variation of
Figure 2-4 Compass rose.
Remember: If the annual variation is an increase,
you add; if it is a decrease, you subtract.
Variation remains the same for any heading of the
ship at a given locality. No matter which way the ship is
heading, the magnetic compass, if affected only by
variation, points steadily in the general direction of the
magnetic north pole.
The amount a magnetic compass needle is deflected
by magnetic material in the ship is called deviation.
Although deviation remains a constant for any
given compass heading, it is not the same on all
headings. Deviation gradually increases, decreases,
increases, and decreases again as the ship goes through
an entire 360° of swing.
The magnetic steering compass is located in the
pilothouse, where it is affected considerably by
deviation. Usually the standard compass is topside,
where the magnetic forces producing deviation are not
as strong. Courses and bearings by these compasses
must be carefully differentiated by the abbreviations
psc (per standard compass), pstgc (per steering
compass), and pgc (per gyrocompass). The standard
compass provides a means for checking the steering
compass and the gyrocompass.
Some ships may have another magnetic compass,
also known as the emergency steering compass, located
at the after steering station, when that station is topside.