9 Use transits to check compass error. Transits are not only used to determine your position line, they can also be used to check your compass error.
On your chart, look for two objects in line.
As you already know, transits can be man-made objects or natural ones.
So, the two objects can be two beacons, two buoys, a building and a light house, or other objects.
With your parallel ruler, determine the True Bearing of the transit and make a note of it.
In this case the True Bearing is 115 degrees.
Now point the bow of your boat towards the two objects.
When they come in transit, make a note of your Compass Bearing, which in this case is 120 degrees.
As the Compass Bearing is more (best) than the True one, the error is West.
To memorize that, remember: Compass best error West, Compass least error East.
To find the error, subtract 120 degrees Compass from 115 degrees True).
The Error of our compass is 5 degrees West.
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