As you cross the New Haven Outer Channel range, you observe the range in line bearing 335.5° per gyrocompass. The helmsman reports that he was heading 100° per gyrocompass, and that the standard magnetic compass read 109° at the time of the observation. What are the gyro error and deviation of the standard magnetic compass on this heading?
• Gyro error = True bearing − Gyro bearing (or vice versa, but stay consistent!) • Relationship between true, gyro, magnetic, and compass headings: T – V – M – D = C • On a range, the true bearing of the range = charted range bearing
• First, use the charted range bearing for New Haven Outer Channel range to find what the true bearing should be, then compare it with the observed gyro bearing 335.5° to determine gyro error and its direction (E or W). • Once you know the gyro error, correct the gyro heading 100° to get the true heading. Then work step‑by‑step from True → Magnetic → Compass to find the deviation on this heading. • Use the reported standard compass heading 109° and remember that deviation is the difference between magnetic heading and compass heading on that course.
• Be consistent with your sign convention: if gyro reads higher than true, is the error east or west? Verify this before choosing. • Check that your computed true heading from the gyro heading and gyro error logically matches the range’s true bearing at the time of the transit. • After you calculate deviation, confirm whether it is east or west by asking: does the compass read higher or lower than the corresponding magnetic heading?
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