Magnetic Compass Error
Before we use a magnetic compass aboard a ship, we must first correct
for the magnetic influences that make the compass deviate from true or
The first influence is variation, which we have already covered. The
second is deviation.
Deviation may be defined as the amount that the compass is deflected
from the magnetic meridian because of the effects of the ships iron.
This is where permanent and induced magnetism come in to play.
Also known as hard-iron magnetism, permanent magnetism is created in
the ships structure during the building process. The ships structure
gains its own unique magnetic field based on the angle that the keel was
Also known as soft-iron magnetism, induced magnetism varies according
to the intensity of the component of Earths field in which it was
The amount of deviation varies as the ship changes course. The ships
magnetic effects may be corrected by the proper placement of various
The process of correcting for deviation error is called swinging ship.
The navigator and QM gang will swing the ship through 360 degrees,
stopping each 15 degrees and comparing the compass heading against a
properly functioning gyrocompass. The results are recorded on the
magnetic compass deviation table.
Example: While swinging ship and steady on course 015° by gyro, the
magnetic compass reads 016°. It should read 015°; the 1° difference is
the amount of deviation. In this case, it is labeled westerly deviation
The next topic deals with the magnetic compass deviation table. From
there we will look at degaussing, and then you will learn how to
perform compass calculations to correct for variation and deviation.