When navigating a plane by chart and compass, the pilot frequently determines the compass course from the true course and the true course from the compass course.
The course is the direction in which the aircraft is flying over the earth’s surface. True course is when it is measured clockwise from the true north. The pilot’s correct true course reading is the magnetic course. To find the magnetic course from the true course, one must add west variation, but subtract east variation. To find the compass course from the magnetic course, add west deviation and subtract east deviation.