Great question — answering as the UphillTech team:
Declination | NWCG provides a good explanation and description. They start with a map (in this case of the USA, with possibly out of date values) that shows lines connecting locations with equal declination: For instance, if you were in California (left coast) the declination is about 16°E. That means that the compass is pointing too far to the right (East). Or, if you were in Delaware (right coast) the declination is about 10°W. That means that the compass is pointing too far to the left (West). I currently live in Albuquerque New Mexico which currently has a magnetic declination of 8.00°E. Your compass is pointing 8° east of where it should be pointing. So to go due north, you should be...
Full answer by Ray Butterworth: https://outdoors.stackexchange.com/a/29097 (CC BY-SA 4.0).
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