True and magnetic bearings and headings

[Wbartoszy]

Hello.

I don't understand how the option "Show magnetic bearings and headings" with (or without) the WMM plugin works ...
I have checked both boxes "Show true bearings and headings" and "Show magnetic bearings and headings" (Options | Display | Units).

1. WMM plugin enabled.
I created a route 000° between the isogons 6°E and 7°E (red lines).
Route is 000° and 004° M(agnetic). Difference = + 4°. Why, if mag. variation is, in this place, beetwen 6-7°?

2. WMM plug-in disabled.
Magnetic variation sets to 0.0° (Options | Display | Units | User set magnetic variation) The same route have: 000° / 353° M (-7° difference). Why?

2a. It doesn't matter what value I put into "User set magnetic variation"! (I tried: -25°; 0°; + 25°; 90°; 360° ... etc.) Route 000° always has 353° M. I tried at different places around the world - everywhere: 000° / 353° M.

What am I doing wrong? What I don't understand?

OpenCPN v 5.2, Windows 10.

Regards - Wojtek

[Wbartoszy]

Inside "route properties" box i have different value:

Other place:

[cagney]

I have tested a bit (on Linux) and my conclusion is that when you turn WMM off, you have to restart OpenCPN for the settings to work properly. The other way around, turning WMM on works as expected.
This is not OK and will after discussion here be entered into the tracker.

Looking at your pictures you can actually see an update problem. On your first picture one can see the variation plot even though the option is inactive.

Thomas

[Wbartoszy]

In fact - after OCPN reset - case without WMM - works predictably! Thomas - thank you!

However - with the WMM plug enabled, there is still a mismatch between the magnetic heading in the "road properties field" and the yellow rectangle...


BTW: Do you know what someone might need/usefull this magnetic value for, if we have true value?
A similar question also applies for example to AIS: For what to use "COG M" for = COG of target corrected by declination? ("Course" is of course "COG" - https://gpsd.gitlab.io/gpsd/AIVDM.ht...report_class_a)?
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[cagney]

However - with the WMM plug enabled, there is still a mismatch between the magnetic heading in the "road properties field" and the yellow rectangle...

The yellow rectangle uses the variation at the route leg location, while the properties field uses the variation of the "Own ship location". So when you are actually sailing there is no problem, while it might be confusing for armchair navigation.

More here https://opencpn.org/wiki/dokuwiki/doku.php?id=opencpnpencpn_user_manualptions_se tting:display#units

BTW: Do you know what someone might need/useful this magnetic value for, if we have true value?

I agree with you, so I never uses it. You know that you can turn it off?

The back ground to the introduction of this relatively new feature are quite a few user requests. There are users (mainly from US?) who are used to always use magnetic courses and bearings. One reason is probably to avoid mistakes in conversion. I also think that it started with the use of parallel rulers and paper charts, where you can read the magnetic course directly on the printed compass rose.

Thomas

[LennartG]

Thomas,
another reason to display the magnetic course could be that many autopilots (e.g. Garmin) have a display showing the magnetic course, not COG.
Combined with the fact that deviation also causes a diisplay error of the magnetic course (remember the deviation table?) things just get even more confusing when using the 'magnetic course'.
So: Always use COG to avoid confusion.
/Lennart

[Wbartoszy]

Originally Posted by LennartG

Thomas,
another reason to display the magnetic course could be that many autopilots (e.g. Garmin) have a display showing the magnetic course, not COG.
/Lennart

You're right - autopilots use a magnetic course.
But "Magnetic COG" is NOT equal "magnetic course", and not deviation is a problem here, but leeway and drift.

I know you know, but for terminology clarifyty :
* Compas Course + compass deviation = Magnetic Course (Magnetic Heading);
** Magnetic Course + declination = Heading (HDG; True Course; True Heading);
*** Heading + leeway angle (by wind) = Course Through Water (CTW);
**** Course Through Water + drift angle (by tidal, or river current) = Course over ground (COG).

So, Magnetic Course" is not equal "Magnetic COG". I don't understand for what to use "COG magnetic" and I'm trying to understand.
Habit of USA colleagues - is an explanation (do not teach the old dog new tricks). But I'd like to know (maybe I'll start using it myself - if it's faster, easier, more convenient) where this habit comes from.

BTW: I am trying to translate OCPN (crowdin.com) and without understanding it is impossible. Eg AIS properities: COG and HDG are broadcast by the target AIS transmitter. The "COG" was called "Course" (why?). And after adding "(M) magnetic" we have: "Course (Magnetic)", and this is not "Magnetic Corse" but... "magnetic course over ground". This can be misleading, because it are completely different things (except situation when leeway angle + drift angle = 0).


--

Wojtek

[Wbartoszy]

Originally Posted by cagney

The yellow rectangle uses the variation at the route leg location, while the properties field uses the variation of the "Own ship location". So when you are actually sailing there is no problem, while it might be confusing for armchair navigation.
Thomas

Sorry but this seems not to be the case:
(I don't have a connected GPS or any other sentence source)


Yes, I know: I can turn it off (or not turn on)... :-)


--
Wojtek

[cagney]

I can't repeat your problem....



So there must be something in your setup....
Please attach your opencpn.ini file.

Thomas

[Wbartoszy]

OK. Here you are! (the "pdf" extension is - obviously false)

[cagney]

Wbartoszy

Nothing obvious to me in your file.
You an try a couple of things.

*Turn off all plugins except for WMM and see if it makes a difference. If so, turn them on one by one and see which is the problem plugin.

* Shut down OpenCPn and rename your opencpn.ini file. For example opencpn.ini.bkp. Then restart OpenCPN that will recreate a new default opencpn.ini.
It will be like starting from the beginning again. Test WMM just against the background map, for simplicity. Any changes to your problem?
Once you're done shut down OpenCPN again. Delete opencpn.ini and rename opencpn.ini.bkp to opencpn.ini. Restart and you're back to "normal" again.

Thomas

[Wbartoszy]

First of all: thank you, Thomas, for your time.
Tried both: disabling plugins and creating, a new ini file - nothing helps... :-(
But I noticed that sometimes it's okay...
Here's a video (new ini, no maps or plugins):
https://www.dropbox.com/s/g9bcy9hmnx...59-00.mp4?dl=0

--
Wojtek