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 186° T, and the variation is 11° W. If the heading of your vessel at the time of the sighting is 193° per standard magnetic compass, what is the correct deviation?
• Compass error = Difference between a true direction and what your compass shows • Relationship between variation, deviation, and compass error** • Converting between true, magnetic, and compass headings using the correct algebraic signs (East/West)
• Start from the charted range bearing (given in TRUE) and combine it with the local variation to find what the MAGNETIC bearing of the range should be • Compare that expected magnetic bearing with what your ship’s COMPASS is actually steering to find the total compass error • Use the rule "Compass error = Variation + Deviation" (with proper signs) to solve for deviation, then decide if it is East or West
• Be sure you are clear on whether variation 11° W is added or subtracted when going from TRUE to MAGNETIC • Check whether a compass that reads higher or lower than the magnetic bearing indicates East or West deviation • Before choosing an answer, verify that your final deviation sign (E or W) is consistent with both the numeric difference and the rule Error East, Magnetic least; Error West, Magnetic best
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