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Old 30-05-2023, 00:36   #1
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Location: Stockholm, Sweden
Boat: Beneteau Oceanis 411 Clipper
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Heading and COG arrows dont match

Hi,
Starten tovuse OpenCPN in real life and very happy sp far.
One annoying detail is that the heading and COG arrows are consistently in different directions. The header arrow matches reality pretty well, but COG is always more left. No tide, wind or other external force that would cause thus.

Picking up GPS and other data from my Raymarine network over WLAN.

Why is this? How can I fix it?
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Old 30-05-2023, 00:45   #2
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Re: Heading and COG arrows dont match

OpenCpn just displays what it receives from your instruments. Heading is usually derived from a heading sensor and COG is derived from your GPS data. They will differ if your heading sensor is not calibrated or if the boat is moving sideways due to current or leeway.
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Old 30-05-2023, 00:47   #3
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Re: Heading and COG arrows dont match

Thanks.
Yes I understundom.
In this case the hearing arrow is close to really and COG is consistently to the left. No currentbor Wind that can explain it....
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Old 30-05-2023, 01:42   #4
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Re: Heading and COG arrows dont match

If you look at your picture above it looks like the left one is correct as the track suggests that the boat has recently turned slightly south. This would suggest heading sensor miscalibration which is quite common.
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Old 30-05-2023, 02:49   #5
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Re: Heading and COG arrows dont match

jacob..
To see what you receive you could arrange a Dashboard with instruments like my picture and Mag and true heading will be clear as the local variation.
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Old 30-05-2023, 04:42   #6
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Re: Heading and COG arrows dont match

Quote:
Originally Posted by AedanC View Post
If you look at your picture above it looks like the left one is correct as the track suggests that the boat has recently turned slightly south. This would suggest heading sensor miscalibration which is quite common.
Good feedback. I change the autopilot computer incl. Heading sensor at the end of last season. Could probably need a calibration.
Thanks.
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Old 30-05-2023, 04:43   #7
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Re: Heading and COG arrows dont match

Quote:
Originally Posted by Hakan View Post
jacob..
To see what you receive you could arrange a Dashboard with instruments like my picture and Mag and true heading will be clear as the local variation.
Thanks, will do this. Together with sensor calibration.
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Old 30-05-2023, 04:51   #8
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Re: Heading and COG arrows dont match

I guess that has been explained on previous messages.

first of all O is displaying the data given from your devices, not magic, just communication of data among instruments.
The Heading is the data given by a "heading Device", e.g. Fluxometer, compass with interface device, gyro compass, etc.
Being fluxometer and compass should be calibrated (compass adjustment either seaman way "adjusted") or at least compared, I mean if you are not certified to adjust your compas you can compare the true course with the magnetic one knowing the deviation at each course.
adjustment should be carried out by a official compass adjuster and sometimes is not cheap at all.
most of the worldwide flags don't accept the adjustment done by yourself if not certified. Of course there is another ones that accept it.

gyro compass is the most trustable way of getting the heading because is based in the true north, it must be calibrated with the latitude and also the speed, but most of these input are feed by GNSS, having all these data automatically corrected, installation uses to be big and expensive and must required annual maintenance by service engineer.

about the COG is "graphically" calculated by GNSS considering actual and past positions so is really good source of how vessel is moving over ther earth.
could be consider as source for comparing with the magnetic course and calculate deviations.
but sometimes it get to confusion because small deviations in the positioning could lead in "big alterations" of course giving as not really a true picture of the vessel's track.
much better to make some kind of filter in positioning for this calculation not at every second and doing it on a "longer" period which give us much clear picture of the track.
from your picture you have to understand more or less that you're moving across the COG, maybe you can check it much better saving the track and you'll see that vessel has been in the past COG track instead of the heading track.

Also you have to consider that IMO recommends to use as RADARs input data the Heading and the STW and doesn't recommend to use COG & SOG.
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Old 01-06-2023, 10:26   #9
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Re: Heading and COG arrows dont match

Quote:
Originally Posted by jakobmagnusson View Post
Hi,
Starten tovuse OpenCPN in real life and very happy sp far.
One annoying detail is that the heading and COG arrows are consistently in different directions. The header arrow matches reality pretty well, but COG is always more left. No tide, wind or other external force that would cause thus.

Picking up GPS and other data from my Raymarine network over WLAN.

Why is this? How can I fix it?
I observe several errors of appreciations:

1- In the photo, the COG appears to the right of the HDG, not to the left.
2- This effect is totally normal in your position. At Finnboda you currently have a magnetic variation (declination) of 9.05 degrees East. That means that to the magnetic angle, you must add 9.05 to obtain the HDT (True Heading), and that will be more similar to the COG if there are no currents or leeway.

https://www.ngdc.noaa.gov/geomag/cal.../magcalc.shtml
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