Most shipmasters making a tortuous passage through a strait or among reefs and islands, after a long passage on one course in a vessel equipped only with magnetic compasses, are worried out of all proportion to the hazards of the manœuvre by reason of not knowing in all certainty what compass errors will pertain on widely different courses. Of course there is the Azimuth Record Book, but the last time similar courses were used could have been in the other hemisphere or under different conditions of trim, &c.

Given time, weather conditions and other suitable opportunity a prudent master will have conducted a swing to ascertain the errors before approaching the stretch of dangerous water. This may not be possible and, again, he knows in his mind that azimuths taken under these circumstances, with little time to waste, do not always conform to the later findings.