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a line on a map connecting points of equal angular difference between magnetic north and grid north
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
A line on a map or chart which joins points of equal angular difference between grid north and magnetic north. See also grid magnetic angle.
Dictionary of Military and Associated Terms. US Department of Defense 2005.