It is important, when sailing to the Arctic Circle, to know where north is.
Traditionally the way to locate north (unless you have natural navigation skills) is the magnetic compass, though as those RYA courses tell you, you must check for variation and deviation.
Variation is a way of taking account of the fact that the magnetic north pole is not located at the Earth's physical pole. Up where we were it was around 20 degrees difference (as can be seen by the pic above from Wikipedia), though in addition the charts often had warnings of local magnetic anomalies.
Deviation is not what your grandma would call what you do on a Saturday night but rather how the ship's compass is being perturbed by ferrous materials and electronics on-board, and was probably similar to the variation.
Given the difference between actual north and compass north we relied a lot on the Raymarine chart plotter's COG (course over ground), which was great until it went into does-not-compute territory.
The problem was that over all the logs in one watch I noticed the COG was around 260 degrees i.e. something like WSW but at the same time the latitude was increasing.
This led to a lot of head scratching, as I'd imagine that COG was calculated using two (lat, long) points and a little spherical geometry, which is pretty straight forward.
So what was going on? Had we entered the mysterious "Iceland Triangle" or something?
To be honest I don't know. Another boat we met reported something similar with their Garmin GPS but the iThings seemed fine. Also sometimes we spotted the chart plotter jump between two GPS locations as if the other electronics was causing interference and that could cause problems.
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