Navigation General · Compass Error
The most reliable method to determine the deviation on a new or altered compass heading is to:
- AEstimate it by interpolating from the nearest two entries in the deviation table
- BCompare the compass bearing of a known object to its computed true bearing✓ Correct
- CUse the chart compass rose inner ring
- DAsk the vessel's pilot for the local deviation value
Explanation
Deviation can be found by comparing the compass bearing of an object (range, lighthouse, celestial body) whose true bearing is known. The difference (true minus compass) equals total compass error; remove variation and the remainder is deviation.
Authority: Bowditch (Pub. 9)
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