Join Date: Oct 2002
Location: Phuket, Thailand
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Re: Magnetic Variance
Dean, we have this problem (Difference between mag north and true north) because we have a habit of drawing maps with north up however the magnetic north and south poles are the ends of the magnetic field around the earth. The magnetic field is created by magnetic elements in the earth's fluid outer core and this molten rock does not align perfectly with the axis around which the earth spins. Since we do have are maps with north straight up we have an ability to make the compass work with a map. Magnetic variation or declination is the answer. So depending on what point of the earth you are (or going) we have this east/west rule. In aviation we learned "east is least, west is best". So...if you have a true course of 100 degrees and you have a west variation of 4 degrees west... 100 degrees + 4 degrees = 104 degrees is your compass course. Remember that these is a difference between a course and a heading. Now with East variation you would subtract the 4 degrees for a course of 096 degrees. Now you need to add or subtract winds and water currents of your magnetic heading.
Hope this helps,
Part time PADI instructor, part time sailor. Full time having a blast.
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Last edited by jerryrlitton; 01-30-2014 at 07:56 PM.