Although the magnetic field of the Earth lies roughly north and south, the Earth's magnetic poles do not coincide with its geographic poles, which are used in the construction of aeronautical charts. Consequently, at most places on the Earth's surface, the direction sensitive steel needles that seek the Earth's magnetic field will not point to true north, but to magnetic north.
Furthermore, local magnetic fields from mineral deposits and other conditions may distort the Earth's magnetic field, and cause additional error in the position of the compass' north-seeking magnetized needles with reference to true north.
The angular difference between magnetic north, the reference for the magnetic compass, and true north is variation. Lines that connect points of equal variation are called isogonic lines. The line connecting points where the magnetic variation is zero is an agonic line.
To convert from true courses or headings to magnetic, subtract easterly variation and add westerly variation.
Reverse the process to convert from magnetic to true.
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