Your vessel is proceeding up a channel, and you see a pair of range lights that are in line ahead. The chart indicates that the direction of this pair of lights is 014°T, and the variation is 11°E. If the heading of your vessel at the time of the sighting is 009° per standard magnetic compass, what is the correct deviation?
• Converting true to magnetic using variation (remember "east is least, west is best")
• Relationship between compass, magnetic, true, variation, and deviation
• Using the basic compass error formula: Compass ± Deviation = Magnetic and Magnetic ± Variation = True
• First convert the charted range bearing from true to magnetic using the given variation. Then compare that magnetic bearing to what your compass is showing for the same line (the range) • Ask yourself: when the range lights are exactly in line, what should your ship actually be heading (in true) if you are exactly on that range? • From the difference between the compass heading and the correct magnetic heading, decide whether the deviation is east or west
• Be clear about whether 11°E variation is added or subtracted when going from true to magnetic • Confirm you are using the same line of position for comparison: the range line vs. the ship’s compass heading at that moment • Double-check the sign convention: whether an east deviation makes the compass read higher or lower than magnetic
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