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/