Don't confuse a multifunction display with a plain GPS Chartplotter. A multifunction display may be showing the heading based on a FLUXGATE compass, which should be at least as accurate as a magnetic compass once it has been calibrated properly. Without knowing what equipment the OP's boat has, it isn't possible to determine whether his MULTIFUNCTION DISPLAY is outputting a GPS-generated heading or a fluxgate based heading.
Again, using known terrestrial ranges is the only way to learn if the magnetic compass is off or not. Unless the compass was installed without regard to interference, chances are pretty likely that the magnetic compass is going to be more accurate, out of the box, than a GPS-generated heading, at least in regards to where the boat is POINTING.
Originally Posted by pedcab
GPS does not show headings, it shows a course over ground and in any place but a widless pool your heading is always very different from your course...
Some more complete GPS units feature a fluxgate though...And those can show more accurate true and/or magnetic headings...
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