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Re: Compass woes
As someone noted above, your GPS may not have an actual compass (but some do have an internal fluxgate compass). Most GPS units calculate heading from a series of locations over time. So to get a reasonably accurate heading from your GPS you need to be moving at a reasonable speed and in a straight line. Also, most GPS units can give you either true or magnetic heading. The difference between true and magnetic heading is called variation. Variation results from the fact that Earth's magnetic north pole is not located at the geographical north pole.
The fluxgate compass on your autopilot should be correct, assuming it is aligned properly and that it has been calibrated. It is probably set to display your heading in true degrees.
Magnetic compasses can only give you the magnetic heading (plus deviation). All magnetic compasses are subject to the earth's magnetic variation and to the magnetic deviation that is unique to the particular location of the compass on your boat (ferrous metal, such as your engine, and electrical wiring cause deviation). You can make a deviation table by swinging your boat in an arc, lining up landmarks and using a chart to find the actual magnetic headings to those landmarks. The difference between your compass heading and the magnetic heading you get from your chart is the deviation. Deviation varies depending on the direction in which your boat is facing (as your boat turns in an arc, the location of your engine and other ferrous objects relative to magnetic north changes). Therefore, you need to determine deviation for at least the 4 cardinal directions.
I bet if you make a deviation table and you convert all of your headings into a common currency (e.g., degrees true), you will find that all three of your compasses are in pretty close agreement. There will still be some small differences because your autopilot and magnetic compass tells you the heading in which your boat is pointing and the GPS gives you the heading in which your boat is actually moving.
Unless the compass dome is crazed or there is an air bubble inside the dome (indicating that oil has been lost), I wouldn't bother removing it from the boat for servicing.
Last edited by Damon Gannon; 10-29-2019 at 06:42 PM.