COG incorrect Compared to Garmin Chartplotter

[jeremyjed]

I've just installed my signal k server on a Victron Cerbo GX. Everything is working great, but I've noticed that my COG is consistently off by exactly 10 degrees compared to what is displayed on my Garmin Chartplotter. I've looked though the documentation and the configuration settings, but I haven't found where I can account for this discrepancy.

How to I set a permanent 10 degree variance for my COG?

[bradharley]

Do you have more than one compass or do you have an offset set in the chart plotter?Brad Harley. — Sent from my iPhoneOn Nov 6, 2024, at 17:44, JeremyJed ***@***.***> wrote: I've just installed my signal k server on a Victron Cerbo GX. Everything is working great, but I've noticed that my COG is consistently off by exactly 10 degrees compared to what is displayed on my Garmin Chartplotter. I've looked though the documentation and the configuration settings, but I haven't found where I can account for this discrepancy. How to I set a permanent 10 degree variance for my COG? —Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[jeremyjed]

I'm not sure what you mean by having more than one compass. Looking at the Data browser I have:

navigation.courseOverGroundTrue | 5.7566 | rad | 11/06 19:53:57 | n2k-on-ve.can-socket.1  (129026)

navigation.courseOverGroundTrue | 0 | rad | 11/06 19:53:54 | signalk-starlink

navigation.courseOverGroundTrue | 5.756600062734873 | rad | 11/06 19:53:55 | venus.com.victronenergy.gps.socketcan_can0_vi0_uc109024

The 5.7566 radians shows on my Aquamaps display as 330 degrees. But on my Garmin Chartplotter, and my physical compass, it reads 340 degrees. I don't have any offset on the chartplotter because that's matching the physical compass.

Also to verify when underway looking at Aquamaps my vessel moves at an angle because of this 10 degree variance.

[sbender9]

Yeah, you must have some offset set on the Garmin. COG should not normally match a physical magnetic compass. COG is a "true" heading, not magnetic. And it's not where your boat is pointing, it's where it is going.

You may be able to use the calibration plugin to adjust it, otherwise could definitely be done in node-red.

[sberl]

5.7566 radians is equal to 329.83 degrees.
Where do you live? What is the magnetic variation there? I suspect you are seeing the difference between true and magnetic direction here?
Also, as Scott says about COG is not the same as heading. Heading is the direction the boat is pointed. Course is the direction you are moving. Course and heading may be different because of currents, or leeway.

Either can be expressed as a true or magnetic direction.

navigation.courseOverGroundTrue is the true direction in which you are moving. A magnetic compass shows the magnetic direction the boat is pointed. that would be navigation.headingCompass.

There's also navigation.headingMagnetic which is headingCompass adjusted for compass deviation (errors introduced by magnetic stuff on your boat)

And there is navigation.headingTrue, which is navigation.headingCompass adjusted for magnetic variation, which is the difference between magnetic and true north based on your location.