At 2016 you sight N. Dumpling Light in line with Latimer Reef Light (Fl 6 sec, 55 ft) bearing 079°pgc. At the time of the bearing the helmsman reported he was steering 164° pgc and 172° per standard magnetic compass. What is the deviation for that heading?
• Compass error = variation + deviation and the relationship between pgc (per gyro compass) and psc (per standard compass) headings • How to use a charted true bearing (from a range of two lights) to find gyro error, then relate that to standard compass error • Sign convention: West is Best, East is Least for converting between true and compass directions
• First, from the chart, determine the TRUE bearing of N. Dumpling Light when it is in line with Latimer Reef Light. Compare that true bearing with the observed 079° pgc to find GYRO error: is the gyro reading too high or too low? • Once you know gyro error, compare the gyro heading (164° pgc) with the standard compass heading (172° psc). From their difference, decide whether the standard compass is reading higher or lower than the gyro, and what that says about COMPASS deviation. • Use the local variation from the chart for that area and date. Combine variation and the compass error you’ve just found to isolate deviation on that heading. Check the sign (E or W) carefully using a consistent rule.
• Confirm the true bearing of the range line between N. Dumpling Light and Latimer Reef Light from the chart; don’t guess it. • Look up the correct magnetic variation (including annual change) near those lights for the exam chart’s year. • Track signs step-by-step: when you convert between true, gyro, magnetic, and compass, write whether each error is E or W, and double-check whether the compass or gyro is reading too high or too low.
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