My compass is my ultimate source for navigation and I constantly check the accuracy. All of the compasses I have owned included an owners manual that instructed you how to calibrate and also included a non ferrous metal calibrating tool. I would follow the instructions and do it yourself on a calm flat day. Years ago I did mine but I was up the delaware river and I was able to obtain some reference marks of dedicated land objects from a river captain. This my preferred method and is an older way however the GPS can also be used.
I use a hand held GPS and my car to calibrate my compasses. I will get on 55 or 47 and drive till I hit a stretch of road that is dead north or south using the GPS and pull over to the side of the road and get out and away from my car so there is no magnetic interference to set my compass. It seems to work ok.
turn the poachers in & stop the waste
just a little more C&R
let fishin be fun
KILL ONLY WHAT YOU EAT
Anyone that thinks their GPS is "off"...I'd love to know how thay can figure that out. Even most charts and maps have been proven wrong in places based on GPS indications and then comparing with satellite photos....if you think your GPS is wrong...think again.
If you think you can navigate better by compass...refer to para #1.
Navigation has nothing to do with missing barges in the fog...that's dumb luck unless you/they closely follow sound signals, have RADAR or AIS.
And yes...navigation skills are important in case the GPS goes out and probably the OPs concern for calibrating his compass.