Variation
Variation (magnetic variation or declination) is the angular difference between true north and magnetic north at a given location, caused by the fact that the earth’s magnetic poles do not coincide with its geographic poles. Variation is shown on aeronautical charts as isogonic lines (lines of equal variation).
Applying Variation
Variation is applied to convert true courses and headings to magnetic: East variation is subtracted; west variation is added. Mnemonic: “East is least, west is best.” Example: True course 090°, variation 15°W → Magnetic course 090° + 15° = 105°. In Canada, variation is generally westerly and ranges from about 5°W in the east to 30°W or more in the western Arctic.
Isogonic Lines
Isogonic lines on VFR charts connect points of equal magnetic variation. The agonic line is the specific isogonic line where variation is zero — it passes roughly through eastern Ontario into the US midwest. East of the agonic line, variation is westerly; west of it, variation is easterly (rare in Canada).
On the TC Written Exam
Variation is fundamental to navigation. Know: east subtract, west add; how to convert true to magnetic heading; the TVMDC chain.