Compasses and Magnetism
Understanding deviation, variation and applications of compass error are critical for many chart plotting problems. The most common error I consistently see in students is misapplication of deviation to three-body fix problems. Remember that deviation is based on your ship's heading, and not where you are looking. This video discusses compass correction using the acronym "True Vampires Make Dull Companions at wakes" or "Can Dead Men Vote Twice at elections."