Magnetic declination is a really big deal in some parts of the country. What might be construed as inaccuracy in a compass might actually be due to the extreme angles of the magnetic lines in a given area, to minerals in the rocks and soils, or to whatever you have in your pockets. I stood one day on the highbanks of lower Lake Superior, in an area where many shipwrecks lie on the bottom, and wondered why experienced sea captains would founder on the rocks in a channel that was 17 miles wide. An old ranger told me that in the days before GPS when magnetic compasses were the primary navigation devices, there was so much iron in the rocks that they attracted the compass needle and at night the ships would hit the rocky shores under full power. They built many lighthouses around the lake for this reason.

I have a map showing the magnetic lines in the USA for 2004 (they change constantly as the iron core of the Earth shifts).