Has anyone heard the term, "magnetic disturbance"? Here's a bit of info from the Green Bay sectional aeronautical chart (published by the FAA), which covers the BWCA.

"Magnetic disturbance of as much as 12 degrees exists at ground level between Tower and Ely" . . . as much as 18 degrees . . . from Duluth to Grand Marias".

I'm certain it has to do with the iron ore in the region. Compass error will be constant in a given area, therefore local deviation can be recorded and noted on your maps. Plot a straight line course and read the heading from your GPS. Subtract the difference of the GPS heading to what the compass reads to get the deviation.

-YB