With a few exceptions, the 'compass card' on a GPS is driven by running calculations of your course over the ground. It has no knowlege of which way the boat is pointing. Come to think of it, a wet compass has no idea of which direction you are moving, it just shows, for better or for worse, which way you are facing! However, if you have a chartplotter
input, your GPS can probably be set to show what a remotely mounted electronic compass is saying. This little black wart can be mounted below decks, far away from any magnetic field generating objects such as engines, motors, speakers or just nearby power cables
. Here I allow for the obligatory caveate than it is and electronic device, dependant on enthusiastic and participating electrons.
Option two is a remotely mounted VHF radio with a RAM mic, haning on a hook somewhere more than 3 feet away from the compass. A word of caution, many wheel
steering systems use wire rope
something like a motorcycle chain to connect the wheel
to the rudder
quadrant. Some bycycle chain is made from a grade of stainless that can be magnetized by the ships field over a long period of time, so when you are checking the compass of an older boat you should turn the wheel lock to lock and look for a change in compass heading.
In airplanes we had to consult a compass correction card because the error was not the same on every heading. We apparently don't have as bad a problem as airplane cockpits, where you couldn't hear a radio speaker five feet away.