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Old 03-02-2014, 12:16   #46
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Re: OpenCPN Beta Version 3.3.1328 Released

A short comment on "variation":

Usually,

- a Westerly Variation is indicate with a negative number (ex. 12W == -12 )

- a Easterly Variation is indicate with a positive number (ex. 12E == 12 )

A magnetic value will equal to the true value minus the variation, thus

Magnetic Bearing = True Bearing - variation
(variation can be positive o negative)

Look like that the WMM plugins display the variation correctly (at least in my understanding), but Opencpn do a wrong calculation of Magnetic values adding, instead of subtracting, the value of variation.

Just my 2 cents....
Kind regards,
Roberto
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Old 03-02-2014, 12:24   #47
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Re: OpenCPN Beta Version 3.3.1328 Released

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Pavel, my communication skills are not great. This has to be as plain as I can make it.
It's an old rose-1988, but you get the idea.
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Old 03-02-2014, 12:30   #48
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Re: OpenCPN Beta Version 3.3.1328 Released

I 100% agree with rpedriali in his remarks about plus and minus. The values are correct but OpenCPN calculate it wrong according to my knowledge.

Lars
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Old 03-02-2014, 12:36   #49
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Re: OpenCPN Beta Version 3.3.1328 Released

What about including he possibility to ad a deviation table for the boat and get the correct course to steer on the compass.

Lars
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Old 03-02-2014, 12:45   #50
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Re: OpenCPN Beta Version 3.3.1328 Released

Ufff... Kind of a relief that my books were not wrong The fix is easy - https://github.com/OpenCPN/OpenCPN/b...art1.cpp#L5221
Re deviation table: If it's not in Flyspray yet, someone has a homework...

Pavel
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Old 03-02-2014, 12:51   #51
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Re: OpenCPN Beta Version 3.3.1328 Released

Quote:
Originally Posted by nohal View Post
Happy...
Are you really saying that the WMM model is totally wrong??? It is very hard to believe for me that http://www.ngdc.noaa.gov/geomag/WMM/...010_D_MERC.pdf is totally wrong...

Pavel
looks like it....at least in application to TRUE values

But maybe they are considering it as an x/y grid I think so an Easterly "declination" would be in the positive quadrant...Thomas will know how to explain it.
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Old 03-02-2014, 12:56   #52
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Re: OpenCPN Beta Version 3.3.1328 Released

Happy...
Variation is simply variation, it does not change it's value depending on being applied to true or magnetic, what changes is whether you use substraction or addition.
Starting from true, going to magnetic, substraction has to be used - there simply is a bug in OpenCPN core, not in the plugin or in the model itself...

Pavel
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Old 03-02-2014, 13:11   #53
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Re: OpenCPN Beta Version 3.3.1328 Released

yeah, I was looking for a complicated answer but that pdf is simply about applying to the MAGNETIC compass to get the TRUE...backwards to what is wanted in O...

so, that's not done in the plugin? and therefore not in the plugin flyspray?

added: or I should say backward to what O is doing..
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Old 03-02-2014, 13:24   #54
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Re: OpenCPN Beta Version 3.3.1328 Released

OpenCPN can use 3 sources of the variation: GPS, WMM plugin or your input. All these three methods provide just the bare number to the core, nothing more.
The internal data is always TRUE from the GPS and for what you plot on the chart and in all the internal calculations and has to be converted to MAGNETIC for display if the user selects to do so. This calculation was wrong until a few minutes ago.
Where you are, the variation is 17E (=+17) and it has to be substracted from the true heading to get a magnetic one.
Anyway, now fixed, so in the next beta the magnetic value displayed will be just your boat's deviation off of what you read on your compass when you plot it in OpenCPN...

Pavel
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Old 03-02-2014, 13:27   #55
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Re: OpenCPN Beta Version 3.3.1328 Released

Happy..
My location for the example was south-west Greenland.
And following this link: Magnetic declination calculator - Results
The declination at your point should be some 17 degr?? And that corresponds to the lines in O.

Edit:
Sorry - it's the same thing twice over. I didn't saw Pavel's last before posting
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Old 03-02-2014, 14:12   #56
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Re: OpenCPN Beta Version 3.3.1328 Released

Am I talking through my hat here or isn't it correct as it is now?
In my picture here I want to go straight to north. That is my compass should apart from deviation show 2 or 3 degr East??? (there's rounding discrepancy between the two windows but never mind) Or are we just talking different languages?
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Old 03-02-2014, 14:24   #57
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Re: OpenCPN Beta Version 3.3.1328 Released

Quote:
Originally Posted by Hakan View Post
Am I talking through my hat here or isn't it correct as it is now?
In my picture here I want to go straight to north. That is my compass should apart from deviation show 2 or 3 degr East??? (there's rounding discrepancy between the two windows but never mind) Or are we just talking different languages?
Your picture show a variation of +2.8 ~= + 3 (3° E), so to go straight to north you have to follow 357° on your compass.....

Regards,
Roberto
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Old 03-02-2014, 14:28   #58
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Re: OpenCPN Beta Version 3.3.1328 Released

Quote:
Originally Posted by Hakan View Post
Am I talking through my hat here or isn't it correct as it is now?
In my picture here I want to go straight to north. That is my compass should apart from deviation show 2 or 3 degr East??? (there's rounding discrepancy between the two windows but never mind) Or are we just talking different languages?
Thanks Happy and Nohal.
I was flabbergasted testing this again and seeing it being back to front!
@Håkan Just get an old paper/raster chart out and look at one of the compass roses displaying both true and magnetic.

Thomas
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Old 03-02-2014, 14:38   #59
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Re: OpenCPN Beta Version 3.3.1328 Released

Quote:
Originally Posted by cagney View Post
Thanks Happy and Nohal.
I was flabbergasted testing this again and seeing it being back to front!
@Håkan Just get an old paper/raster chart out and look at one of the compass roses displaying both true and magnetic.

Thomas
Me too. I've been happily looking at that plugin thinking, Hey! that's cool! and just being pleased the number is there and automatically subtracting it.
So really, it was "Correct" until now.
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Old 03-02-2014, 16:11   #60
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Re: OpenCPN Beta Version 3.3.1328 Released

Yes -- that old trap again. + and - long time ago....
Now I've changed the signs in my branch and it's so nice.
Very good you started this Happy..
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