Karl Westman of Ocean City, New Jersey, asks:
“Is it all right to use magnetic headings on my chartplotter to adjust a new compass?”
Tim Bartlett replies:
In theory, definitely not. But in practice I’d have to give you a very guarded “maybe.” The problem, of course, is that your heading is the direction your boat is pointing in. Your chartplotter/GPS unit cannot know this. It only knows the direction the boat is moving, known as your “track” or “course over ground.”
And there can be a big difference. For example, if you are doing 6 knots with a 3-knot current on your beam, there’s a difference of almost 30 degrees between your heading and your track. Buf if you should be lucky enough to have a perfectly calm day and there is no tide or current to affect the boat’s course over the ground, the track shown by your chartplotter should be the same as your compass heading if the compass is adjusted properly.
A good way to check your compass is to aim your boat at a fixed landmark or buoy, and then take the magnetic bearing of the landmark by placing your chartplotter’s cursor on it. Then compare that bearing with the compass heading. As long as the landmark is more than a mile or so away, the bearing shown by the chartplotter should be the same as the compass heading, give or take a degree.