Freedom From Checking Deviation

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.