Determining the compass deviation using reciprocal bearings:
The compass deviation can be determined by reciprocal bearings, if ashore does not exist leading lines and nor faraway objects.
The principle of the method is the following:
When using this method is required two observers, one ashore and one on board of the vessel.
They simultaneously observe a bearing from ship to shore and another from shore to ship.
From ship to shore will be a compass bearing. And from shore to ship will be a magnetic bearing, because this is influenced only by terrestrial magnetism.
The magnetic bearing from shore to ship shall convert to reverse magnetic bearing.
The difference between the two bearings will be the compass deviation.
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