Each time a compass is compensated it is necessary to check the
compensation by checking the deviation on various headings. This may be
done by the use of deflector magnets. A more exact method is to swing
the ship in a circle while bearings are taken of a known object on land
and the deviation noted on various headings. The sun is often taken as
a reference point for this purpose.
It is never necessary to swing ship or to correct the Gyro-Compass
for either variation or deviation of any kind. Where a Gyro-Compass
and a magnetic compass are both used on a ship, the ship may be swung
to correct the magnetic compass—the Gyro-Compass furnishing true
headings. The time required is thereby materially shortened.
[Illustration]