Speaking of compasses, geographers went through three revelations regarding "tricks" of the magnetic field of the Earth. FIrst was the realization that magnetic North did not correspond to true North, because the magnetic pole is offest by about ten degress from true, requiring local corrective angles. Second, was that the magnetic pole is not a fixed location; it wanders gradually, requiring the corrective angle to be update every decade or so. Third, that in numerous locations there are magnetic anomolies that have nothing to do with the Earth's magnetic field, in their presence it is better not to pay any attention to your compass.