heading variation to course being run - odd

[Cutwater28GG]

I drove the boat yesterday for a 4 hour run and I noticed that there was a consistent variance to the heading indication vs the route travelled. always to the left by about 10 degrees. essentially the boat was crabbing sideways according to the Garmin.

This can't have been caused by currents/tides the whole way as I went through the tide change and over many different pieces of water on the journey and the variation was consistently shifted to the left.

Could this be caused by something metal near the digital compass on the boat?
what else could cause this?
ideas?

This photo attempts to show the difference between the boat track and the heading line
I missed magnolia bluff to the west by a lot. if you draw an imaginary line from the dotted track you would see it outside the green mark which matched my eyeball of bow to the green mark

whats also interesting is that the garmin compass direction heading is different to the autopilot heading number (dont have a photo of that but recollection it was also out by 10 degrees or so)

 

[aculverwell]

You will find other threads on this topic-- the general consensus seems to be that the cause is due to magnetic interference with the Garmin CCU. The simple (but temporary) fix-- at least for me-- is to power down the Garmin unit completely (turn off the main House switch) for at least 5 minutes. The factory recommended this approach to me when I first had the issue and it has resolved the issue a number of times, although eventually the compass gets out of whack again.

 

[Red Raven]

See this thread:

http://www.tugnuts.com/viewtopic.php?f=5&t=11050

If the suggestion to power cycle doesn’t work run the sea trial wizard. If it recurs look for something metal or magnetic near the CCU that moves.

Also, you can change the settings to show the GPS Heading vs. boat heading. Since in your photo the boat is aligned with the heading and you said that heading did not match your Jew out the widow it looks like yours is set to show boat heading I prefer to use GPS heading because it will be correct regardless the compass error.