I have Maiana and a USB GPS dongle, both have position, but both have 'null' in magnetic variation??
I used to get Magnetic variation from my GPS sources all last season, but this season there is nothing, it is a new Openplotter install with the latest server version
11 Replies
Do you get RMC or HDG sentences? They carry variation
I will have to check but I think it is getting RMC because I have SOG, COG
All three GPS devices provide a ‘null’ Magnetic variation delta from RMC
So does your RMC data have variation fields filled in?
good question and I just realized I have never looked at in coming NMEA0183 strings, is that something that I can interrogate the logs for? I will be on the boat from tomorrow afternoon until Saturday
if the data is coming from a serial source you can activate debug logging for
signalk:streams:serialport
or you can activate data logging to file and look at the data log contentlooks like it is not in the original message for the GPS dongle:
Jun 11 21:10:43 2025-06-12T02:10:43.832Z signalk-server:events:connection:gps:nmea0183 $GPRMC,021043.000,A,4244.0170,N,08746.7654,W,0.34,48.91,120625,,29
Maiana:
Jun 11 21:18:25 2025-06-12T02:18:25.545Z signalk-server:events:connection:maiana:nmea0183 $GNRMC,021825.000,A,4244.0276,N,08746.7711,W,0.00,0.00,120625,,,D,V1B
not sure why the two devices are not publishing the local Magnetic variation. is there a way we can add it artificially/manually?
can it be done using data fidler and is there an example on what to write?
I tried adding this, and it seemed to work:
{
"context": "vessels.self",
"updates": [
{
"values": [
{
"path": "navigation.magneticVariation",
"value": 0.14137166941
}
]
}
]
}
GitHub
FAQ: Frequently Asked Questions
An implementation of a Signal K central server for boats. - SignalK/signalk-server
Are you sure the GPS should be providing mag variation? I looked into this recently for my Raymarine kit and it said the mag variation can come from the chart or a world magnetic model. Basically it's the MFD that's supplying the mag variation.
I am not sure where it came from last year, but I am missing Heading true as such True Wind which were calculated by Derived data. in looking at some of the issues for Derived Data, there are a few references to this: https://github.com/SignalK/signalk-derived-data/issues/143, https://github.com/SignalK/signalk-derived-data/issues/142 and less so https://github.com/SignalK/signalk-derived-data/issues/131. I wonder if it was buried in one of the messages from OCPN. Last year I think OCPN had a WMM plugin that I am not sure is there on my new rebuilt system. I only forward RMB, XTE and APB so it shouldn't be. This could also be somewhere in Openplotter, I found a thread from back in 2017 whaere it is talked about, but I am not sure if it is done in OP4 - https://forum.openmarine.net/showthread.php?tid=522
FWIW
checked my paths and sources for “magnetic variation”… and it appears quite a number of times, (i do have a number of chartplotters in my system, and for certain each of them are emitting it as a source)
there are two PGNs where it appears, namely 127250 & 127258.
My AIS (which includes “GPS”) does NOT provide it.
However my GS25 (simrad GPS /compass) does provide PGS 127250 to SK with “magnetic variation”.
For certain all the chartplotters are emitting PGN127258 for variation.
not very conclusive i realise … and i did wonder myself about this … but i believe its typically sourced from the chartplotter.