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VOR radials north reference


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cloudless
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Hi everyone,

new SkyDemon user here.

I'm a bit confused because the way I learned VORs is that they are referenced to magnetic north. At least to a certain amount, as they are not updated every time the variation changes.

But it seems SkyDemon defaults to referencing them to something else? For example, see the KRH VOR in south-west germany:


On the right side is an ICAO chart pulled from DFS AIS with the 360° radial in red and the SkyDemon one copied from the left picture in black.

The one from skydemon
a.) doesn't match with the one provided by the DFS
b.) isn't directly 360° true north either, it seems more like 355° or something
c.) the difference between them could be the variation (something link 2-3° here)

So is the data of the VOR wrong?

I had expected SkyDemon to provide, display and reference radials of VORs according to "magnetic". In parenthesis because, as noted above, their might be a slight discrepancy by the VORs not always updating their reference with every variation change.

Thanks for the clarification,

cloudless

grahamb
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cloudless - 8/22/2022 9:58:46 AM
Hi everyone,

new SkyDemon user here.

I'm a bit confused because the way I learned VORs is that they are referenced to magnetic north. At least to a certain amount, as they are not updated every time the variation changes.

But it seems SkyDemon defaults to referencing them to something else? For example, see the KRH VOR in south-west germany:


On the right side is an ICAO chart pulled from DFS AIS with the 360° radial in red and the SkyDemon one copied from the left picture in black.

The one from skydemon
a.) doesn't match with the one provided by the DFS
b.) isn't directly 360° true north either, it seems more like 355° or something
c.) the difference between them could be the variation (something link 2-3° here)

So is the data of the VOR wrong?

I had expected SkyDemon to provide, display and reference radials of VORs according to "magnetic". In parenthesis because, as noted above, their might be a slight discrepancy by the VORs not always updating their reference with every variation change.

Thanks for the clarification,

cloudless

Does it matter if the VOR roses are not exactly oriented? You're hardly going to plot a line from them with a chinagraph pencil are you? Tongue To me, the compass rose's only purpose on a moving map app is to draw your eye to the location of the VOR.

Edited 8/22/2022 4:39:55 PM by grahamb
cloudless
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Is this a real question? Of course it matters: the orientation of the VOR rose is the visual representation of the north reference.

So if I pull up my ICAO chart and read a course of radial 45° magnetic outbound it seems to be a legit questions why the radial 45° of the same VOR in SkyDemon has an offset to the one provided in the official chart - or am I missing something?

In fact, the orientation of the VOR rose is the only way to get the real radial from a VOR (besides pulling up the data of the variation used by the VOR in some database).

EDIT:

So I re-checked/measured with the help of the SkyDemon map ruler and it seems that the VORs are oriented according to true north and point b.) in my listing was wrong.

The difference of the radial to the one provided by the official DFS map source could be indeed attributed to the variation in the area. Since VORs are oriented to magnetic north (to a certain degree) rather than true north, I would still prefer to have them oriented that way. I don't know about elsewhere but at least in switzerland and germany it is common for them to be displayed that way.

I mean just think about what happens if you fly in an area of 90° E variation (and say the VOR is updated to this variation) but your SkyDemon VOR north reference still points to true north:

- If you fly, say, radial 180 outbound of the VOR (the real one) you would fly a true course of 270°
- Now you want that radial on your SkyDemon map. You "Track Radial" and input 180° (as this is the value you turned your OBS in the cockpit to). But as far as I can see it, SkyDemon takes that as a true north value. So in your SkyDemon map you would get an actual southbound course whereas the course you actually fly is a westward one.

Edited 8/22/2022 8:42:16 PM by cloudless
grahamb
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cloudless - 8/22/2022 8:08:23 PM
Is this a real question? Of course it matters: the orientation of the VOR rose is the visual representation of the north reference.

So if I pull up my ICAO chart and read a course of radial 45° magnetic outbound it seems to be a legit questions why the radial 45° of the same VOR in SkyDemon has an offset to the one provided in the official chart - or am I missing something?

In fact, the orientation of the VOR rose is the only way to get the real radial from a VOR (besides pulling up the data of the variation used by the VOR in some database).

EDIT:

So I re-checked/measured with the help of the SkyDemon map ruler and it seems that the VORs are oriented according to true north and point b.) in my listing was wrong.

The difference of the radial to the one provided by the official DFS map source could be indeed attributed to the variation in the area. Since VORs are oriented to magnetic north (to a certain degree) rather than true north, I would still prefer to have them oriented that way. I don't know about elsewhere but at least in switzerland and germany it is common for them to be displayed that way.

I mean just think about what happens if you fly in an area of 90° E variation (and say the VOR is updated to this variation) but your SkyDemon VOR north reference still points to true north:

- If you fly, say, radial 180 outbound of the VOR (the real one) you would fly a true course of 270°
- Now you want that radial on your SkyDemon map. You "Track Radial" and input 180° (as this is the value you turned your OBS in the cockpit to). But as far as I can see it, SkyDemon takes that as a true north value. So in your SkyDemon map you would get an actual southbound course whereas the course you actually fly is a westward one.

I just find it surprising that you would actually use the VOR rose to determine an exact track, when both the PLOG and the ruler will give you both True and Magnetic bearings accurately.

If you load the charts for a country with a large magnetic variation like the US, you’ll see that VOR roses do tilt towards Mag North, it’s just that the orientation is not 100% exact. I fail to see how that is in any way significant in a tool like SD. 

Edited 8/23/2022 5:43:29 AM by grahamb
cloudless
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Okay I'll tell you how this whole thing came about or better my use case:

Sometimes I use radials as a boundary which I don't want to cross since on the "other side" is an airspace. So I did my flight planning and read the radial out of the physical chart. Then I simulated the flight in SkyDemon and did the "track radial" thing, inputting the value.

But now the radial in SkyDemon crossed the airspace instead of passing at its boundary. So I used the SkyDemon map ruler to get the values of the originally wanted radial pulling the ruler to the airspace boundary. SkyDemon told me that I wasn't to far off with my plotted radial on the physical chart. Since it gave me also the true "track" of the radial, I tried to input true north in the "track radial" and the radial came closer to the one I had on my physical map.

For me as an engineer "closer" is still far off, so that made me curious.

It might be that the remaining difference between the radials comes due to the difference between each full degree - the airspace is a good distance away from the VOR so a degree more or less can give quite some lateral distance.

So after this discussion - in essence - only one issue seems to remain (at least it's an issue for me, but please correct me when I'm wrong):
- the "track radial" function uses true north radial values and not magnetic ones.

pgroell
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Some tests show that the VOR's are referenced to the magnetic north.
See the screenshot from the US (variation 12W) or the one from France (variation 2E).
But we don't know the variation Skydemon uses (I suspect a calculated model) neither do we know the one your chart uses (I know charts that get all the informations updated yearly except for the variation which only gets updated every three or four years, for example the French ICAO VFR chart published April 2022 uses a variation chart dated January 2020).
This could be an explanation to the small discrepancies you are seeing.

Pascal



Tim Dawson
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VORs are aligned to magnetic north, but they don't move on their own; instead the engineer who sets them up and maintains them will periodically adjust the internal setting.

VOR KRH is currently set to 0.80W in the digital data we get for radio navigation aids in Germany. This does differ to what AIP Germany section ENR 4.1 says (1.0E), so I will raise a query to see whether the AIP or the digital data is right, and get one of them corrected. This will probably take a couple of AIRAC cycles.

cloudless
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Thanks for all the replies and research and good to know that the direction is indeed oriented to magnetic north. Lets see what the query brings for results.

I hope this discussion isn't viewed as picking peanuts - at least that wasn't my intention. It was just something I noticed and wanted to discuss with others.

Tim Dawson
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DFS have confirmed that their VOR declination data is very out of date, disappointingly. They expect to correct this in late December.

Thank you for bringing it to our (and their) attention.

Tim

GO

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