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Old 18-03-2014, 15:04   #1
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Magnetic variation test inquiry

My dear forum friends.

Is here anyone having a GPS close to a zero magnetic variation line and are willing to make me a small test.
I've a theory that no GPS in automatic variation mode will message a variation value less than 0.2 degree, plus or minus, and would like to have it tested.
The test can be done while crossing a zero line watching the NMEA signal, like the GPRMC, or why not record it using the VDR plugin.

The reason for my request is that my dated GPS, Garmin 120, no longer reports the variation. In automatic mode it reports 0.2 degree but I can manually force it to 000.
Now the fabulous Dashboard of OCPN can use the WMM plugin to make the true heading but only if the GPS variation is N/A, e.g. NAN. I've change my dashboard copy to do the same thing when the GPS variation is less then 0.1. It,s not a big problem to keep that updated on every new OCPN version coming from our team but if I can get rid of that it would be convenient. And also may I not be the only one having dated equipment. It could be that many of us using OCPN is not the folks buying the latest gadgets all the time?
If my theory happens to be correct It would be possible to implement my changes to the official OCPN Dashboard? And to a sufficient likelihood be convinced that we not force OCPN to use the theoretical WMM value instead of a possibly more accurate GPS value if the GPS would actually message a variation of 000.0 degrees.

I would greatly appreciate your participation.
Thanks Håkan
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Old 19-03-2014, 15:10   #2
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Re: Magnetic variation test inquiry

Håkan,

seems not that easy:

first there are not that many places where one might test with a zero variation -
Oran (Algeria), Ebro delta-Valencia Spain, Belgium, Norwegian westcoast, Missisippi delta, North coast Yucatan, Pacific coast of Nicaragua and Peru. A point at the Westcoast of Australia, Malacca straight and three or four points in Malaysia and Phillipines. Somalia (happy testing) and nearby to Karachi.

And second: which GPS do display magnetic variation? It's foreseen in the RMC message but for example neither SIRF nor uBlox do implement those in their basic chipsets.
Would be interesting to see a list of GPS sensors which produce this data.

Hubert
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Old 19-03-2014, 16:28   #3
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Re: Magnetic variation test inquiry

Håkan

That's it then! Your summer cruise has a purpose, and the destination is Stavanger!
While you're there, please give us a report on the restoration of "Wyvern" .

Thomas
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Old 20-03-2014, 01:33   #4
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Re: Magnetic variation test inquiry

Hubert, Thomas ..Thanks for your participation.
Hubert I know this is not so easy. That's why I tried to involve the OCPN team where the most unexpected knowledge can be found. I'm also aware of the declination lines and as mentioned I'm not that far from a zero position my self. But that's the basic of this purpose. My GPS' inbound table is out of date. So Thomas I may go to Stavanger but not to test the GPS but more for other reasons. The old ship "Wyvern" is what I read safely returned home from Sweden. It could be worth a glance or more. But the nice equally named creature could maybe be noticed here at dark Saturday nights as well.
The second:
Well I thought if my dated Garmin has the function to do the same calculations as our WMM plugin there are more gadgets having the same feature. But it's maybe not so common? There are for sure no GPS sensors with an inbound WMM table. If it's available the navigator's software has to do it. And then they may have to trust the same table as we do in our WMM plugin?

One of the reasons for my discussion here is that Dave R once mentioned that his GPS (or navigator) reported another variation value then our WMM and therefore it could be risky to switch to the WMM table if the GPS really reports a variation of 0.0 degr when at such a position.
But I still claim no GPS, or navigator, will ever report a value less then 0.2 degr. So my search here was to find a way to verify that. But instead of doing it by an experiment it could also be that there is someone here with knowledge of the program in a navigator and can verify or refuse my assertion.

But my problem still remains. Since my GPS does include a value for variation in its RMC message, correct or not, Dashboard will never use our WMM table. And I can’t switch it of manually. Only force it to “000”.

Håkan
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Old 20-03-2014, 01:52   #5
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Re: Magnetic variation test inquiry

By the way ---
This is how I sort my incorrect RMC value out:
Code:
In dashboard_pi.cpp. Rows around 855

if( mPriVar >= 3 ) {
                        if( !wxIsNaN( m_NMEA0183.Rmc.MagneticVariation ) ) //Hakan
                         {
                            if(0.1 < m_NMEA0183.Rmc.MagneticVariation )
                            {
                                    mPriVar = 3;
                                    if( m_NMEA0183.Rmc.MagneticVariationDirection == East )
                                        mVar = m_NMEA0183.Rmc.MagneticVariation;
                                    else if( m_NMEA0183.Rmc.MagneticVariationDirection == West )
                                        mVar = -m_NMEA0183.Rmc.MagneticVariation;
                                        mVar_Watchdog = gps_watchdog_timeout_ticks;

                                 SendSentenceToAllInstruments( OCPN_DBP_STC_HMV, mVar, _T("\u00B0") );
                            }
And then the HDT will be updated from the WMM's table
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Old 27-03-2014, 11:36   #6
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Re: Magnetic variation test inquiry

Hakan
Should this become a item in traker?
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Old 28-03-2014, 15:00   #7
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Re: Magnetic variation test inquiry

Quote:
Originally Posted by rgleason View Post
Hakan
Should this become a item in traker?
Well - Rick. I don't know-- There is, as far I know. no one else complaining about this "failure". What I refer to is the behaviour of an old GPS navigator where its WMM table is out of date and you in the same time have a fluxgate compass sending heading data into OCPN.
Then I claim we can take the risk assuming a variation of zero is a result of a dated navigator. But Dave R didn't really agreed at our last discussion.

But if you think its a good idea I could put my Git Diffs together and make a Flyspray item?

Håkan
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Old 28-03-2014, 15:13   #8
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Re: Magnetic variation test inquiry

It's a bit exotic isn't it? If somebody is able to test it I would be curious about the results, but I think it should not be in the software.
If there is a need for that very specific situation you have delivered the patch here.
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Old 28-03-2014, 16:11   #9
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Re: Magnetic variation test inquiry

bcn..
What's exotic? The combination of an old navigator and a fluxgate where the WMM_pi is needed for the variation or crossing the zero line?
Sorry, I couldn't follow you due to my lacking English.
Håkan
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Old 28-03-2014, 16:36   #10
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Re: Magnetic variation test inquiry

FWIW, the trend in NMEA2000 is the gps no longer supplies magnetic variation, there are better devices, i.e. real computing devices to store/calculate variation. In NMEA0183 days, the gps did so for convenience, which is no longer required with NMEA2000 are it's a multi-talker network and no need for compound data structures to mask lack of network functionality.
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Old 29-03-2014, 01:34   #11
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Re: Magnetic variation test inquiry

Dot Dun
I think I understand your point about nmea 2000 advantage and simpler data structure, but we don't all have new stuff. Eg my radar and a pilot is 20 years. nmea0183 is our reality, so for that reason I think Hagans patch has merit. Hakan don't know if others will have same problem, but a tracker entry with Patch and your explanation will help. It could have medium or low priority but someone will find their answer! So yes, please, use tracker. Your piece is am example of the strength of opencpn. Thanks

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Old 29-03-2014, 01:36   #12
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Re: Magnetic variation test inquiry

P's this would not be a bug but a feature request or solution.

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Old 29-03-2014, 01:46   #13
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Re: Magnetic variation test inquiry

Håkan,

my impression is that the configuration you have (old GPS required for the fluxgate that does not show M variation any longer correctly) might be one that will not be found frequently ("exotic" ).

Which GPS delivers M_variation in the RMC message - we know about the Garmin you are employing? Which else? When do they time out?
The other parameter would be the predicted behavior for these devices when in a zone with var<0.2 which has to be confirmed yet if I understand you correctly.

Kind regards

Hubert
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Old 29-03-2014, 02:01   #14
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Re: Magnetic variation test inquiry

Quote:
Originally Posted by rgleason View Post
..........Your piece is am example of the strength of opencpn. ......
Yes!...exactly. This lovely open source philosophy and the outstanding collection of coders in combination with this forum is great. You can try to share and convince others that you have a good idea and may have it implemented. But you can also make small additions to fit your own needs and with a little extra work, or pleasure?, still take advantage of new program versions.
I'll put my patches to Flyspray.
Thanks Håkan
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Old 29-03-2014, 02:11   #15
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Re: Magnetic variation test inquiry

Quote:
Originally Posted by bcn View Post
Håkan,

my impression is that the configuration you have (old GPS required for the fluxgate that does not show M variation any longer correctly) might be one that will not be found frequently ("exotic" ).

Which GPS delivers M_variation in the RMC message - we know about the Garmin you are employing? Which else? When do they time out?
The other parameter would be the predicted behavior for these devices when in a zone with var<0.2 which has to be confirmed yet if I understand you correctly.

Kind regards

Hubert
Thanks for the explanation Hubert. Yes I can agree that my combination of old and new stuff can be a bit odd. It may be a result of money savings and "Do it yourself".
The latter part is a better way of describing my questions to the forum. Thanks. /Håkan
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