Determining the compass deviation with leading lines:
If is known a good leading lines with the true bearing precisely known, and magnetic variation (declination) from nautical chart, is easy to found the compass deviation.
When we do not know the true bearing of leading line, we must take compass bearings in the 8 courses.
And then make the average of the 8 compass bearings, which will be the magnetic bearing of the leading line.
So, then we can calculate the compass deviation.
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