This can be handy.
Some compasses have a device to set magnetic north.
He brought me a sample of a dark igneous rock he found, and it deflected the compass 3 degrees when held next to compass in the office. Seriously, however, there are few places with magnetic anomalies and most of those will affect the compass by only the amount that you might err by anyhow. If you haven't read the Rock Net FAQ and User Tips yet, now would be a good time to do so.