Reliability of MAGVAR in Facility records

[MSFS 2024] Questions & Community Discussions SimConnect
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Q: can/should I use the MAGVAR content of SimConnect reported Facility Records (Apt,VOR, etc)
or replace them with WMM2025 calculated values while collecting them.

I use the Magdec.bgl file to tag 2020 content but this is no longer deployed in the streaming FS2024 version.

Reason: I’ve compared the returned values with calculated ones from WMM2025 and found a lot (thousands…) to be not matching.

Some report 0 at locations where there is certainly not 0 deviation
Some report with maybe?? an inverted sign
Others randomly different from WMM2025

e.g. Lat,Lon, Ms MAGVAR, WMM2025 calculated

MAGVAR_DEVIATION;44.383600205183;-78.1581848859787;0;-11.0715479378851
MAGVAR_DEVIATION;43.2886255905032;-78.7097279727459;0;-10.5187832646883
MAGVAR_DEVIATION;42.7933054044843;-78.2921665906906;0;-10.6708271887006
MAGVAR_DEVIATION;43.0512373521924;-75.2276627719402;0;-12.389075153028
MAGVAR_DEVIATION;44.5673834905028;-75.6383331120014;12.7242126464844;-12.5063123921588
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For airports, it will usually be overridden by the scenery airport, so it is IMO not useful/reliable. For VORs, it is the station declination, and VORs are not often calibrated to the actual magnetic north; in some cases they have not been re-calibrated since the 60s or earlier. You should use that value for any radials relating to that VOR to get the correct true heading.

In terms of the magvar model for dynamic magvar values, MSFS uses WMM2025 currently, and it’s updated periodically. JS instruments can use Facilities.getMagVar(lat, lon) to get the MSFS magvar model value, but I don’t know there’s an equivalent for WASM.

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Thanks… and no there is no call for MagVar at a specific location in WASM or SimConnect that I know.

As I use the MagVar outside the Sim and only to plot lines and shapes on a chart I may just load the database with WMM values or calculate them on the fly.

Did some data analysis and it shows that Facility records are a mixed bag with regards to matching the model data.
I see that VOR would need to match their chart values in order to plot the radials as per chart, however for the others I think I have to pull the magvar from another source than the facility record, some are off by 15 deg… ILSs have suspicuous numbers like 12.000, 14.000 etc …

→ not so reliable - for my purpose at least

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Yes, I would agree for both of those points, and that is what the default avionics do (using Facilities.getMagVar as the other source). The sim magvar database (which Facilities.getMagVar returns in JS) is built from WMM2025, so you will be safe using that.

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