Weatherlawyer wrote:

John_H wrote:

Any decent map will have the magnetic correction on it... usually a
small arrow and the number of degrees from TN.

Isn't there a daily variation? And an annual one?

Or by the decade for a significant change!

This one might help....
http://www.scoutingresources.org.uk/compass_magvar.html

A handheld GPS receiver should also do the job.

--
John H