Just out of curiosity the other day I decided to 'compare' my main and my backup compass. Neither is a Silva, but both are made by reputable manufacturers. Worryingly there was adifference of around 7 degrees between the two.
My question then is how do I know which one is correct (if either). Is there a way or do I need to go and get another one?
My guess (and that's all it is) is that with a difference of 7 degrees, it's quite likely that both compass needles are as far out of kilter as it's possible for them to be. You'll probably find that the 'correct' direction for magnetic north is between the two of them. The more compasses you can compare, the more likely you'll find one that's close to being accurate.
By the way... don't do anything daft like putting two compasses together to compare them, as that's guaranteed to throw the needles completely out!
First, make sure there's no other obvious magnetic influence where you're checking. Most obvious one would be the second compass, but any big lumps of iron/steel can cock things up. Aside from that, check that one hasn't been declination corrected, which if available as a feature will be a small screw somewhere on the protractor to allow you to offset it by a fixed amount.
To check a compass, use a map. Go to a known location on it and take bearings on other known points, comparing your results to what the map tells you they ought to be.
To check a compass, use a map. Go to a known location on it and take bearings on other known points, comparing your results to what the map tells you they ought to be.
Pete.
Take a minimum of three bearings, a good place to do this, if possible is from an OS trig point - which will not have moved since the map was printed. Beware of using objects that are not necessarily static (eg watch out if taking a bearing on a house corner that the house hasn't had an extension built since the map was printed).
