D of E navigation nightmare

We were out with the Silver Duke of Edinburgh's Award walkers when we noticed this potential compass problem. If you hold the compass next to an iron gate, you can get the wrong direction for North. The compass needle is a magnet. It's magnetic field is strong enough to line up the domains in the iron, turning it into an induced magnet, which attracts the compass needle. Hence the needle will always point towards the gate - giving the wrong direction for North.