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



Joe wrote:
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.


Very good Joe.

Did you read the whole article? Or just the easy words?

Dip angle is changing too, which is why I said a new dev card would be
a good idea. When was the last time you did this, Joe? Steel boats have
big problems with magnetic compasses.


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


What do you do when variation reaches 180 degrees?

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