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DREZLER

compass declination

4 posts in this topic

I just bought a marine compass for my pro guide. It turns out that they don't always point north because magnetic poles are always moving. However, after going on the NOAA HSOforum, it turns out that andover, mn has a declination of less than one degree! It's my lucky day! I entered some other areas in the country and they can be off 15 degrees or more. That would sure suck!

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Not to burst your bubble, but unless you fish only in Andover, declination changes by time and location, here in Portage I have almost 3 degrees west declination. Deviation is the most important thing you need to be aware of, that is the influence of metal objects and magnetic fields on board your boat that will cause an incorrect reading. We are required to "swing" our compasses and have deviation tables at the helm as the number will be more or less depending on direction. Dependence on a compass heading has been almost eliminated by modern electronics, but is still the best and only sure way to navigate off shore and out of site of land.

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THANK YOU FOR THAT REPLY. I DIDN'T HAVE A GREAT UNDERSTANDING OF HOW IT ALL WORKS BUT I AM STARTING TO GET IT. SO IT ALMOST SEEMS LIKE ANYWHERE IN MINNESOTA I COULD PLAN ON THE COMPASS BEING OFF ONLY AS MUCH AS ABOUT 3 DEGREES? THAT SHOULDN'T BE MUCH OF A PROBLEM IF I'M JUST USING IT AS A BASIC REFERENCE.

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That is not entirely true, while declination is something to be aware of, it is of the least importance. Deviation is most important, influences from metal objects in the boat, electronics mounted too closely to the compass, geological anomilies are far worse. To get a better idea of deviation, hold a metal object, like a buck knife for instance, away from the compass, slowly move the knife towards the compass while watching the compass card, it will begin to rotate off of its heading, that is deviation. If you have a marine radio, gps, or depth finder near the compass turn them on one at a time and watch to see if the compass card begins to move, your compass can be a long way off without ever knowing it, although you can compensate it to eliminate slight deviation from known sources with a brass screwdriver on the adjusting screws. Sometimes moving the sources of trouble are the only solution.

Good luck

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