True and Magnetic North
Date: 04/08/2002 at 02:10:33 From: Fabrice Subject: True and Magnetic North Hi Doc, I am aware that true north is different from magnetic north (the difference being the magnetic declination). True north is calculated from the latitude/longitude and magnetic north is measured using a compass. On the other hand, I have no idea how to convert one to another. Could you help me? Cheers, Fabrice
Date: 04/08/2002 at 09:08:13 From: Doctor Rick Subject: Re: True and Magnetic North Hi, Fabrice. Magnetic declination (the deviation of a compass from true geographic north) is very complicated. The major cause of magnetic declination is the fact that the magnetic north (or south) pole is not at the geographic pole. We can do a rough calculation of this effect if we have the location of the magnetic pole. The compass will point roughly toward this point, and there is a formula to find the (true) bearing from your location to this point. See this item in our archives: Bearing between Two Points http://mathforum.org/library/drmath/view/55417.html The magnetic poles drift over years. The current location of the magnetic north pole is somewhere around 78.3 deg N, 104.0 deg W. Use these coordinates for point B and your own location for point A when using the formula on the page above. The actual situation is more complicated; for instance, the south magnetic pole is not directly opposite the north magnetic pole. Here is a Web site I found that computes the magnetic declination given a location, elevation, and year. Be sure to see the FAQ there; it explains that even this calculator cannot take everything into account, only about 90% of the magnetic effects on a compass. Compute Values of Earth's Magnetic Field (NOAA) http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl The magnetic declination is the true bearing corresponding to a compass (magnetic) bearing of 0 degrees. To convert a compass bearing to true bearing, add the magnetic declination; to convert a true bearing to compass bearing, subtract it. - Doctor Rick, The Math Forum http://mathforum.org/dr.math/
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