compasses get messed up if you are on or near any rock that has significant iron content. I have had them mess up in areas of metamorphic rock too. Rocks that have been re-heated often become magnetic. If I think a compass is wrong I take a good look at the rock around me. Avoid setting a compass on a rock to take a reading. There are also magnetic flares from the sun that can cause problems. There actually is a government agency (cannot remember which) out of Colorado that puts out magnetic forecasts. I used to run magnatometer lines and would have to check the forecast each day. There were some days that I just could not get reliable data.