Your ship is entering port from sea, and you sight a pair of range lights. When in line, they bear 315° per standard magnetic compass. The chart shows that the range bearing is 312°T, and that variation is 6°W. What is the deviation of your compass at the time of the sighting?
• True–Magnetic–Compass (T-M-C) relationship and how variation and deviation fit into it • Difference between true bearing on the chart and bearing by the ship’s compass • Sign convention for east and west variation/deviation (whether they are added or subtracted)
• First, convert the charted range bearing from true to magnetic using the given variation. What is the correct math step with 6°W variation? • Next, compare that magnetic bearing to the 315° compass bearing you observed. Is the compass reading higher or lower than magnetic? • From that difference, decide whether the compass error is east or west, then identify which part of that error is deviation (since variation is already known).
• Be clear about the rule: “West is best, East is least” (how does this help you remember whether to add or subtract variation/deviation?) • Double-check whether you are going from true to compass or from compass to true – the direction changes the sign of the correction. • Verify that you are only solving for deviation, not total compass error; you already know variation from the problem statement.
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