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post #11 of Old 07-17-2007 Thread Starter
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Originally Posted by sailaway21
... then within at the magnetic ring, you'll see that the direction to magnetic north is 13 degrees to the east of true North.
As you've already seen, you would subtract the 13 degrees East Variation from the 045 and that would give you a magnetic course of 032 degrees Magnetic. (i believe the dog had it backwards, esy to do)
Actually, I had it backwards. SD had it right, but for the wrong reason . (In that the magnetic variation is 13 deg. west in the example.)

When magnetic north is west of true north, it's known as a negative variation (or declination). To correct for magnetic variation, to point to the true bearing from the magnetic bearing, you have to add the inverse of the magnetic variation. Thus, given the compass rose we're using for demo purposes, the magetic variation of which is -13 degrees, to get the true bearing you have to add 13 degrees. E.g.: If you want to point to true north, you have to point to 13 degrees magnetic. (Believe-it-or-not: I figured all this out once, already, when I installed a wind direction indicator for my weather station. My magnetic declination here at home is [well, was] about -7 degrees.)

All of that is confusing. Sapperwhite gave the clearest, most easily understood demonstration of how it works.

Originally Posted by sailaway21
Quiz in the morning, open chart!
I think I'm ready!

I'm going to have to look up the procedure for figuring-out compass deviation.

My wife is asked me earlier "Why are you doing all this? We'll have a GPS." "Because I find it interesting and challenging," I told her. "I want to understand how it's done and be able to do it. It's part of the challenge and fun of boating."


Last edited by SEMIJim; 07-17-2007 at 10:54 PM.
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