I can only understand that the 20 degrees is the maximum variation between latitude and variation in the earths magnetic field, horizontal at the equator, perpendicular at the poles but not necessarly following the curvature of the earth?
WARNING & DISCLAIMER:
SELECT AND USE OUTDOORS AND SURVIVAL EQUIPMENT, SUPPLIES AND TECHNIQUES AT YOUR OWN RISK. Information posted
on this forum is not reviewed for accuracy and may not be reliable, use at your own risk. Please
review the full WARNING & DISCLAIMER about information on this
site.