How do i find true south using a magnetic compass?
First off, I’ll clarify one point given by the previous respondents, since one has already attempted to do this and himself got it wrong! There are two systematic errors with a magnetic compass. The first one is magnetic deviation which is attributable to the presence locally of large metallic objects. This is difficult to predict and is significant on things like ships and aircraft. However, the effect should not be noticeable if you are simply holding a pocket compass in the middle of the countryside. The second is what most are referring to here: magnetic declination or variation (both terms are correct: which has prevalence depends on which field you are in). This is an error due to the fact that the north and south magnetic poles are not at the same place and the geographical poles, and indeed their location varies over time. To compensate for this you need to apply a known offset to the bearing indicated by the needle. The US National Geophysical Data Center release updated mathe
