magnetic declination
Technical term in geophysics and navigation; the angle between magnetic north and true north at a given location.
The map shows the value of magnetic declination.
偏角 is a broader term for 'declination' or 'angle of deviation' in astronomy and physics, while 磁気偏差 specifically refers to magnetic declination.
Compound of 磁気 (magnetism) and 偏差 (deviation, declination).