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Old April 26, 2013, 06:13 PM   #19
David_S
Senior Member
 
Join Date: February 11, 2008
Location: New Zealand
Posts: 149
Some useful comments, thanks.

Thanks for the comments and stories, everyone. Had a few chuckles and could relate to some of the incidents.

I agree that the best navigation tool is a topo map in a waterproof bag. I use MemoryMap to print off an A4 sheet of the area I am going to and put it in a ziplock bag. I also print off a second copy which I mark up with our route and intentions and leave with my wife.

I have tested the compass with all the things I carry close to it in my pack. Nothing obviously affected it. Not surprising seeing that Rangefinders etc are mostly plastic or stainless steel which are not magnetic materials. I too suspected the GPS but the compass barely twitched with the GPS whether on or off.

Before taking the photo I did wander around with the compass a bit to see if that made a difference. But nothing. We then pulled out the other one which worked fine.

I haven't "stroked" mine yet as I want to take it along to our next volunteer SAR training session as an example. We use compasses occasionally for such things as maintaining the line of a search or taking cross bearings on the location of a missing person identified by a cry or by light.

Quote:
Now you've made me nervous
I agree with you 603Country. The main purpose of the post is to make people aware that compasses can reverse polarity and to check them before use. There is one indication that reversal may have occurred which will be apparent in the field. If you place the compass on a flat surface and the needle is tilted badly so that it is touching the card or close to it then polarity reversal may have occurred.

For those who don't know, the reason is that the Earth's magnetic lines of force are not parallel to the surface of the Earth. A compass needle tries to align itself with the lines of force so that in the northern hemisphere the north end of a compass needle will point downward and in the southern hemisphere upwards. (Google "angle of dip")

I believe compass manufacturers compensate for this with the bearing alignment so that the needle will swing freely (this is the reason why Northern hemisphere compasses don't work well in the southern hemisphere). So if your compass polarity is reversed the needle will drag indicating a reversal may have occurred. We commented on this at the time without realising the significance - in my original photo the faulty compass is the one with the white lanyard.

Does this make sense?

David
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