I have a local marine map dating a few years. It's compass rose says 0 degrees difference between true north and magnetic.
Yesterday I was reviewing some nautical maps of my area on openseamap.org. Therein, I was surprised to see, the variation had changed from 0 to 1.7 degrees west.
There are at least two reasons one might have to equate polar and magnetic north. Either changing map true course to compass course, or changing magnetic bearings to true map bearings.
The thing to remember is that when going from true map to magnetic compass course, east is least and west is best. In other words if you plot a true course on the map, and your area has easterly variation, then subtract the variation from the true which gives the helmsman the compass course to steer.
If taking, lets say, a compass bearing of a lighthouse and transfer it to a map, then add the easterly variation to get the true map bearing.
From map to compass - east is least (subtract), and west is best (add).
From compass to map - the opposite.
Simple
Yesterday I was reviewing some nautical maps of my area on openseamap.org. Therein, I was surprised to see, the variation had changed from 0 to 1.7 degrees west.
There are at least two reasons one might have to equate polar and magnetic north. Either changing map true course to compass course, or changing magnetic bearings to true map bearings.
The thing to remember is that when going from true map to magnetic compass course, east is least and west is best. In other words if you plot a true course on the map, and your area has easterly variation, then subtract the variation from the true which gives the helmsman the compass course to steer.
If taking, lets say, a compass bearing of a lighthouse and transfer it to a map, then add the easterly variation to get the true map bearing.
From map to compass - east is least (subtract), and west is best (add).
From compass to map - the opposite.
Simple