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AIXM (the XML-based exchange model for AIP by Eurocontrol and european AIS) defines the following properties for a VOR:

  • codeTypeNorth
    A code indicating the direction of the "zero bearing" provided by the station, values: "TRUE" (true north) or "MAG" (magnetic north).
  • valDeclination
    The angular difference between the direction of the "zero bearing" indicated by the station and the direction of the True North.
  • valMagVar
    The local angular difference between the True North and the Magnetic North.
  • dateMagVar
    The year the magnetic variation was measured.

Now say we have a VOR with the following values:

<codeTypeNorth>MAG</codeTypeNorth>
<valMagVar>0.27</valMagVar>
<dateMagVar>2018</dateMagVar>

Am I correct with the following statements?

  • valMagVar is a positive number, therefore magnetic north is east of true north.
  • codeTypeNorth is "MAG", therefore the VOR is aligned towards the magnetic north. Since the magnetic variation is +0.27°, the "zero bearing" is equally +0.27° east of true north and therefore the (undeclared) valDeclination must be +0.27° as well.

Please don't comment on the practical relevance of this question. I'm developing and documenting OFMX (a derivative of AIXM) we will use as our future well-defined exchange format for https://openflightmaps.org and I want to be sure to be 100% sure I explain the terms "magnetic variation" and "magnetic declination" accurately in the OFMX documentation.

Thanks a lot for your help!

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Since the magnetic variation is +0.27°, the "zero bearing" is equally +0.27° east of true north and therefore the (undeclared) valDeclination must be +0.27° as well.

You are assuming that the VOR alignment will always be kept bang-up-to-date with the actual measured magnetic variation. Sadly, this is not the case! There is actually a considerable amount of work involved in re-aligning a VOR (it is not just a matter of tweaking a knob!) and re-alignment is only performed rather rarely.

Also note that the terms 'variation' and 'declination' are NOT synonymous in this context - 'variation' refers to the actual measured difference between true north and magnetic north; 'declination' refers to the difference between VOR zero-bearing and true north. These are NOT generally exactly the same.

For example: UK AIP ENR 4.1 (PDF)

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  • $\begingroup$ In other words: The declination is the "correction" from the re-alignment of the VOR while variation is the actual difference between true and magnetic north. And theoretically, right after a re-alignment, variation and declination should be equal, but will diverge until the next re-alignment is made. Correct? $\endgroup$ – svoop Mar 15 '18 at 14:18
  • $\begingroup$ @svoop: correct $\endgroup$ – Fiddlesticks Mar 15 '18 at 14:39
  • $\begingroup$ Great, thanks a bunch! $\endgroup$ – svoop Mar 15 '18 at 18:08

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