Boat heading from mag compass?

[rszemeti]

I have a tilt compensated magnetic compass that ouputs NMEA "HCHDM" sentences (and some rate of turn info as well) ...



Can OpenCPN use these to display the correct boat heading? I have trid in the past using GPS for boat heading and it is poor at slow speed and totally useless when stopped ...



I also noticed there appears to be a "True Compass" visual widget, but it does not seem to work with mag compass data, even though OpenCPN clearly knows the variation ... I elected not to send "true" compass data out of the compass to the NMEA bus as that would require the compass knowing about variation .. which seems like a bad plan.

[nohal]

Originally Posted by rszemeti

I have a tilt compensated magnetic compass that ouputs NMEA "HCHDM" sentences (and some rate of turn info as well) ...



Can OpenCPN use these to display the correct boat heading? I have trid in the past using GPS for boat heading and it is poor at slow speed and totally useless when stopped ...



I also noticed there appears to be a "True Compass" visual widget, but it does not seem to work with mag compass data, even though OpenCPN clearly knows the variation ... I elected not to send "true" compass data out of the compass to the NMEA bus as that would require the compass knowing about variation .. which seems like a bad plan.

Yes, HDM should be supported as a source of heading.

[rszemeti]

Originally Posted by nohal

Yes, HDM should be supported as a source of heading.

After a bit more investigation, it seems it does alter the heading, but only if I click on the display or zoom it to get an update.


Maybe I'll connect the GPS as well and see if that helps get the screen to auto-update.

[Andreas29]

Hi,

@rszemeti: You can use the NMEA-datastream window (under connections) to see if and how often the MAG-sentence will send over the network to OC.

Depending from the output there you can plan your next steps.

OC can use MAG-sentences without problems.

Regards

Andreas

[rszemeti]

It is sent every 0.5 second, but it looks like OC only updates the screen if there is a GPS position change.



When I add GPS info to the stream, it starts to update, but compass data on it's own does not. I get two lines on the screen, I guess one is heading, the other VMG, so it looks like the info is being accepted, I think it is OK.

[Hakan]

Originally Posted by rszemeti

.......... but compass data on it's own does not. I get two lines on the screen, I guess one is heading, the other VMG, so it looks like the info is being accepted, I think it is OK.

The heading is not a position so the boats position is of course not updated if only heading is received. The heading is though rotating the boat symbol to received heading even without a position.


The two lines are heading and GPS course, COG. There's appearance can be controlled by Options->Ships->Own ship->...Predictor length.

Håkan

[Andreas29]

Hi,

not tested in OC 5.0 yet (I´ll test this at the next two week ends) but with oc 4.8.8 it doesn´t matter if there is a GPS sentence in the nmea datastream or not.

HDM always used and with WMM enabled it will shown true heading fast updated on the chart.

Regards

Andreas

[rszemeti]

On OC5.0 it looks like the HDM / boat orientation is not updated in the absence of position updates from what I can see.

[Hakan]

Here's O v5.0 and only heading is received. Boat orientation is nicely updated.
Håkan