Dane Watson

Differing Magnetic Headings for three separate planning tools

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Posted (edited)

I am just curious what causes three different navigation route plotting tools to be slightly different even though the routes are identical.

I recently noticed that the degrees calculated by SkyVector, Little NavMap, and AivlaSoft EFB2 are off by a couple degrees - some of the legs are the same while others differ.
Also note that for Little NavMap and AivlaSoft EFB2 I have both with AIRAC Cycle 1903, not sure what SkyVector uses or if this even matters.

Anyone have any thoughts on this phenomena?

See attached screenshots link of my plotted routes for: CZBB WC HUH KORS FHR BVS 3W5 

EDIT - Links 

https://photos.app.goo.gl/6xzokMwt8gBCNS9k8

https://photos.app.goo.gl/Ewm2UZeVoE1ysUMd6

https://photos.app.goo.gl/16f1baQqzEVo1QbD9

 

Edited by Dane Watson
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Posted (edited)

I'm not sure...

Unlike the geographic north/south pole, the magnetic poles aren't a fixed position, but are constantly moving. Because of that, I would assume, that the data is constantly being updated as well. How often, I don't know.

Are you using the same AIRAC - or is this data not part of the monthly navigation database update / AIRAC?

Interesting...

Edited by Anders Bermann

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7 minutes ago, Anders Bermann said:

I'm not sure...

Unlike the geographic north/south pole, the magnetic poles aren't a fixed position, but are constantly moving. Because of that, I would assume, that the data is constantly being updated as well. How often, I don't know.

Are you using the same AIRAC - or is this data not part of the monthly navigation database update / AIRAC?

Interesting...

I am using the same AIRAC (1903) for both Little NavMap and AivlaSoft

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AFAIK, mag declination data is not part of the AIRACs.  In the sim, it's encoded in the MagDec.bgl file, and Herve Sors does publish updated versions of that data every year for use in the sim.

Now as to what the various flight planners are using, I have no idea, but it's possible they have different data sets for the declination data, or that they do their interpolation of declination values between the data points in the declination grid differently (e.g. linear, polynomial, spline).  And for that matter, the method used to compute great circle courses between waypoints can also drive some small variations as well.

Regards

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5 minutes ago, w6kd said:

AFAIK, mag declination data is not part of the AIRACs.  In the sim, it's encoded in the MagDec.bgl file, and Herve Sors does publish updated versions of that data every year for use in the sim.

Now as to what the various flight planners are using, I have no idea, but it's possible they have different data sets for the declination data, or that they do their interpolation of declination values between the data points in the declination grid differently (e.g. linear, polynomial, spline).  And for that matter, the method used to compute great circle courses between waypoints can also drive some small variations as well.

Regards

Thanks Bob for your info.

Like I said I was just curious why there would be a difference

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Lots of fun, this magnetic declination stuff ...

LNM uses the magdeg.bgl from P3D. If the file was updated LNM will use that too.

For X-Plane LNM still uses the included magdec file from Herve Sors since X-Plane does not come with such a thing and simply does its own internal calculation.

VOR, VORDME, VORTAC (not DME and not TACAN) and NDB (P3D, not sure) come with their own magnetic declination from the various simulator databases. These values usually correspond to the last calibration of the navaid in the real world. Sometimes the navaid's magdec is the same as its surrounding (surrounding: value from latest magdec.bgl) and sometimes it is based on a ten-year old VOR calibration which results in a big difference from the actual values around.

You can see this since LNM shows the VOR declination in the tooltip and the value from magdec.bgl in the status bar.

LNM uses the navaid at the end of a flight plan leg to calculate the magnetic course. The following can happen if the flight plan path consists of waypoints and VOR:
* Waypoint course is calculated from magdec.bgl declination
* Next VOR course is calculated from VOR declination (value from navaid database)

If that VOR above has an age old calibration you will still get a straight flight plan line but the magnetic course looks like zig-zag.

To add more fun: All flight plan calculations result in true course and the source data for procedures is based on magnetic course ...

Alex

 

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7 minutes ago, albar965 said:

Lots of fun, this magnetic declination stuff ...

LNM uses the magdeg.bgl from P3D. If the file was updated LNM will use that too.

For X-Plane LNM still uses the included magdec file from Herve Sors since X-Plane does not come with such a thing and simply does its own internal calculation.

VOR, VORDME, VORTAC (not DME and not TACAN) and NDB (P3D, not sure) come with their own magnetic declination from the various simulator databases. These values usually correspond to the last calibration of the navaid in the real world. Sometimes the navaid's magdec is the same as its surrounding (surrounding: value from latest magdec.bgl) and sometimes it is based on a ten-year old VOR calibration which results in a big difference from the actual values around.

You can see this since LNM shows the VOR declination in the tooltip and the value from magdec.bgl in the status bar.

LNM uses the navaid at the end of a flight plan leg to calculate the magnetic course. The following can happen if the flight plan path consists of waypoints and VOR:
* Waypoint course is calculated from magdec.bgl declination
* Next VOR course is calculated from VOR declination (value from navaid database)

If that VOR above has an age old calibration you will still get a straight flight plan line but the magnetic course looks like zig-zag.

To add more fun: All flight plan calculations result in true course and the source data for procedures is based on magnetic course ...

Alex

 

Thanks Alex, very interesting stuff. Didn't realize there was so much to it.

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Found one of these long time ago calibrated VORs. No matter if you use X-Plane, P3D with fsAerodata or Navigraph. It's always more than five degrees difference for this one.

If you look here and search for BTG: https://www.airnav.com/cgi-bin/navaid-info
You will see: "Variation: 21E (1975)"

grafik.png

Alex

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