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The compass error found by comparing a charted range's true bearing with the gyro/magnetic bearing observed when the marks are in line is most reliable because a range:

Explanation

When the two marks of a charted range are exactly in line, the true bearing between them is known precisely from the chart, giving a clean reference to compare against the compass and determine error — a standard way to check deviation underway.

Authority: Bowditch (Pub. 9), compass checks

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