The magnetic compass is not infallible. A few years ago, I was walking on Stanton Moor in Derbyshire, close to the ancient Nine Ladies stone circle. A young chap was leading a group of even younger youths, and was attempting to show them how to take a bearing with a Silva compass- he was standing in the middle of the stone circle and using the King Stone, the summer solstice sunrise marker, as a target. He couldn't get a bearing on it at all to match the reading from the map.

I got chatting to him, and he told me about his trouble. I pointed out that he was standing in an area of ancient power, so he could expect some magical disturbance.

I also pointed out that his anorak's front pocket was full of small change, and since the eighties "copper" coins in the UK have been made of plated steel...