wrote:
Should be self-explanatory:
http://www.straightdope.com/columns/070112.html

Apologies for the near-sailing related content.

-signed- Injun Ear (formerly known as Eagle Eye)


Deviation is the difference from magnetic north and north shown on a
boats compass due to magnetic influences induced by the boat, not the
shifting of the magnetic poles injun ear.

Shifting of the magnetic field is known as Variation.

You would be better of getting the latest local variation and updating
the local variation listed on the compass rose on your charts.

Joe