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Regarding magnetic variation and magnetic declination:

At one time I thought that it would be sufficient to now the location
of the magnetic North Pole and calculate the magnetic variation by its
angular difference to the geographical North Pole using spherical
trigonometry. I realised then that the result couldn't be correct for
my location. It's at this stage when I learned that the magnetic field
of the earth isn't regularly aligned like longitude lines and rather
shows remarkable anomalies at certain locations on the globe. It is
for this reason that charts or tables may possibly only be valid for
certain parts of the globe.

The following links are quite ilustrative:
Regarding the movement of the magnetic North Pole see
http://gsc.nrcan.gc.ca/geomag/nmp/long_mvt_nmp2_e.php
and regarding the different magnetic declinations on the earth see
Fig. 2 here http://gsc.nrcan.gc.ca/geomag/field/index_e.php

Marcel

   

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