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compasses need "swinging" calibrating every year as they do not all point the same. large amount of iron in the rocks or a seemingly harmless metal like aluminium can change the lines of isogonic flux around an object not to mention the magnets in the motor drives of the mount so use another method like the ones mentioned eariler also do not forget that you need to be around 13-14 degrees on the east side of south to account for local deviation
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