On passage on a small yacht as crew I had occasion to disagree with the owner about the bearing of a navigation mark. My reading using binoculars (with an incorporated bearing compass) was 160o and hers, with the type of hand bearing compass used by many sailors, which hangs around the neck and is held up to the eye, was 200o. When I used her compass the reading was again 160o.
There had to be some local deviation about her person and, as a joke, I suggested her glasses. After a few moments she said that she had glasses which had “clip” on sunglasses, which were magnetically attached! It transpired that her glasses frames were magnetised and this was the local deviation which she had not noticed before!
In the absence of a crosscheck this could have led to a serious incident.
Current training suggests if all else fails use a hand held compass!
The RYA has been informed and intends to raise awareness of the issue.