Westerly variation requires what adjustment to convert true heading to magnetic heading?

Westerly variation indicates that the magnetic north is to the west of true north. To convert a true heading to a magnetic heading in this case, you must account for this variation by adjusting your true heading to align with the magnetic heading. When dealing with westerly variation, you add the variation value to the true heading. This addition reflects the fact that your geographic location is south of true north and thus the magnetic north appears further to the west.

It's important to understand the concept of variation and its significance in navigation. Variation exists due to the differences between true north, which is the direction along the earth's surface towards the North Pole, and magnetic north, which is the direction that a compass needle points. Since westerly variation involves a movement towards the true north direction, the appropriate method is to add this variation to find your accurate magnetic heading.