You wish to check the deviation of your standard magnetic compass. You find a natural range that you steer for and note that the gyrocompass heading is 034°, and the heading by standard magnetic compass is 026°. The gyro error is 1°W. Variation is 9°E. What is the deviation for that heading?
• The relationship between gyro heading, magnetic heading, and compass heading • The standard formula: True ± Variation = Magnetic ± Deviation = Compass (TVMDC) and how gyro error fits in • How to correctly apply west and east errors ("west is best, east is least") when correcting or applying errors
• First, correct the gyro heading using the given gyro error 1°W to find the true heading. Are you adding or subtracting a west error? • Next, use the given variation 9°E to move between true and magnetic. Are you going from true to magnetic, or magnetic to true? • Once you have the magnetic heading, compare it with the standard magnetic compass heading 026°. How does the difference tell you the deviation, and is it east or west?
• Be sure you are clear about the order of conversions: Gyro → True → Magnetic → Compass, and where deviation fits in • Double‑check whether you are applying an error (to get an observed reading) or correcting an error (to get the true value); the sign (+/−) may reverse • Verify that the final deviation sign (E or W) agrees with the direction you had to move from magnetic to compass (or vice versa)
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