WMM variation being overwritten

[Faraway-dk]

I have been using the WMM plugin for a while, giving the correct value for the area. Now the variation value is being overwritten by variation included in received nmea RMC. Is this correct that it can be overwritten?

History is that I recently added a no 2 MFD to my stng network. I am receiving in Opencpn the nmea183 RMC string from the older RM a50.
Opencpn is not using N2k interface.

[bdbcat]

And are the WMM calculated variation and RMC provided variation significantly different?


Thanks
Dave

[Faraway-dk]

Hi Dave
Thx for your reply
Yes, the variation was significantly different?
The correction should be around East 4.3 and was suddenly it was 0.4.

I discovered by realizing there was a constant heading error/offset.

I assume the wrong variation is a Raymarine issue, may be a setup thing between the Axiom and the A50.
When I setup the Axiom I checked that the auto-variation correction was exactly the same as calculated by WMM plugin.

I am just wondering why the WMM calculation is being modified/changed if Opencpn receives variation info from another source.

[Hakan]

Originally Posted by Faraway-dk

I am just wondering why the WMM calculation is being modified/changed if Opencpn receives variation info from another source.

Plugin WMM is used if no source(GNSS receiver) reports a variation. If any real instrument does report a variation we assume it's a correct value. If the source is old and not updated by the user the device's inbound WMM table will give incorrect results.

If you can manually set the variation from the device to 0.0, or not used, OCPN will though use the WMM plugin instead. That's how this is solved by many using a dated device not applicable for a firmware update.

[Hakan]

RMC string from the older RM a50

If that's the source for the bad variation and you still have position/SOG/COG from other source(s) you can try to filter RMC out if RM a50 is connected by its own OCPN connection.
Use "Ignore sentence" as described here

[Faraway-dk]

Hi Hakan
Thx for the clarification.
Now it makes sense. When I only had the old A50 connected it did not interfere, as it did not provide with a valid variation.
Does it mean, there is no need for the WMM if I can get my MFD,s to output a correct variation. Ocpn will then detect and correct the heading.?

If I over time will switch everything to N2k/stng, will Ocpn pick/use the variation if available?

/Leif

[Hakan]

Leif..
Yes, Ocpn will then detect and correct the heading and thus not use plugin WMM, enabled or not.

If a valid variation is available by N2k it will be used, otherwise WMM as usual.

So I suggest you have WMM enabled all time.

[Faraway-dk]

Hi Hakan
Just disabled WMM in Ocpn.
Entered a manual variation in my Axiom, variation is transfered to the A50 and output as a valid variation in the RMC sentence.
Ocpn is correcting my compass heading. Nice.

Many thanks for help and advice.��