Why can\'t we arbitarily choose a direction to be north? Rather than it being a magnetic reading, just say that way back in the \"old days\" (in the history of the planet) someone (a wizened old sorcerer, etc) decided that a compass was a good idea to help people get around and decided that north should be the direction that his workroom window was facing (or a line between the tower and the main statue, etc). He realised that many ways of measuring direction were possible, but they all did the same thing: measured direction, and so chose one of the many, which over time had become the standard directions used by everyone.
A compass helps people navigate, whichever direction that is. That\'s all it needs to be.