USCG Exam PracticeCompass variation and deviation; T-V-M-D-C
A vessel's compass reads 315°. Deviation is 6°W and variation is 4°W. A navigator computes the true course as 305°. Which statement correctly evaluates this computation?
- ✓ The computation is correct; both westerly errors are subtracted from compass to reach true.Correct
- B. The computation is incorrect; westerly deviation and westerly variation should both be added when converting compass to true, giving 325°T.
- C. The computation is incorrect; deviation is applied first to get magnetic, then variation is applied to get true, and both westerly values are subtracted, giving 305°T — so the answer is actually correct.
- D. The computation is incorrect; only variation is applied when converting compass to true; deviation is used only when converting true to compass.
Why A is correct
Compass 315° – 6°W deviation = Magnetic 309°; Magnetic 309° – 4°W variation = True 305°. Westerly errors are subtracted when converting from compass toward true (C-D-M-V-T: subtract west, add east). The computation of 305°T is therefore correct. 33 CFR §164.78(a)(5) requires the operator to know and apply both corrections.