I've searched the forum but can't find an answer to this.  The course between two VORs in Little Navmap is different from the course shown in the MSFS Nav Log between those same two VORs.

For example:
From LVL (112.90) (calibrated declination 8 degrees West)
To SBV (110.40) (calibrated declination 5 degrees West), 54nm away

LNM shows the true course as 261 and the magnetic course as 266.
MSFS shows the course as 271.

It was my understanding that LNM uses the magnetic declination information in MSFS's magdec.bgl file.  So why do MSFS and LNM show the course differently?  What am I missing?

LNM uses the magdec.bgl file from MSFS but this data is fixed for 2006. It does not include drift rates and does not allow to extrapolate values for future data opposed to the WMM.

The rules LNM uses to display magnetic course are here: https://www.littlenavmap.org/manuals/littlenavmap/release/2.6/en/FLIGHTPLAN.html#magnetic-declination

I did not check the numbers but for your example: Traveling towards west: LVL -> SBV
LNM will use the declination of LVL inbound LVL to calculate the magnetic course. Then the declination of SBV when outbound LVL and inbound SBV.
After SBV it depends. If there is another VOR after SBV in the plan it will use the declination of this next VOR. If the next leg is terminated by a waypoint it will use the declination of SBV.

You can also disable the calculation in options and let LNM fall back to the world magnetic model or magdec.bgl instead of using the VOR declinations.

Magnetic course is a difficult thing. There are not only VOR with a declination different than the environment. There are also different courses at the start and end of a great circle line and also different declinations at the start and end of this line.

Alex

 

 

That's very helpful. Thanks very much for explaining and for the link to the manual.