This localised field in which the compass has to function on a boat is produced by ferrous material in its construction, equipment, engines, electrical installations, etc. These cause the needle to deviate from the magnetic meridian along which it is supposed to lie.
Eli bala ligciniweyo apho ikhampasi kufuneka isebenze khona kwisikhephe yenziwe yimathiriyeli yentsimbi ekudityanisweni izixhobo i-injini nase kufakelweni kombane njalo njalo, ezi zibangela ukuba inaliti inxaxhe ukusuka kumazibuthe weMerdiyani apho ifanele ukubo ilala khona.