Where should SK get magnetic variation from?

[Littlechay]

I'm trying to see if I can generate a true heading for OCPN somehow. 

I think one way should be to generate the full HDG NMEA 0183 sentence. This included fields for magnetic deviation and variation. There is Signal K key for magnetic variation but it is empty; or rather zero value. 

Another way would be to calculate true heading but we still need a source of variation (and ideally deviation). 

Where would/should Signal K get magnetic variation from? 

Am I missing something obvious?

RPi 3
OP 0.11.9a 

MPU9150
BME280
an RTC and some 1W temp sensors and a few switches and relays hanging of GPIOs

[Sailoog]

On OP 0.8.0 there were these options and we have still to adapt them to 0.10.0. Look at menu tools>calculate, there is an approach but is not working yet.

We calculate:
- Variation from date and position
- True heading from magnetic heading and variation
- Rate of turn from heading variation
- True wind from either STW or SOG

And we set accuracy controls

We generate signal k data but you will be able to convert to NMEA with NMEA generator.

All this is the next task, working on it

[Sailoog]

Something like this:

   

[Littlechay]

Something like this:

That looks like it will do the job .. thanks I look forward to trying it