How local magnetic fields affect your steering compass. How to measure and and how to allow for it. Move seemlessly from ºCompass to ºMagnetic to ºTrue and back again. This is the same for Day Skipper and Yachtmaster. The only difference is that Day skippers are not required to allowed for deviation in their exercises, course work or their assessment. They simply need to know what it is. Yachtmasters, however will be working with deviation throughout their course and their assessments.