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:


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!


1 Answer 1


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)

  • 1
    $\begingroup$ 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, 2018 at 14:18
  • $\begingroup$ @svoop: correct $\endgroup$ Mar 15, 2018 at 14:39
  • $\begingroup$ Great, thanks a bunch! $\endgroup$
    – svoop
    Mar 15, 2018 at 18:08
  • $\begingroup$ Link is now paywalled. $\endgroup$
    – Ralph J
    Sep 16, 2020 at 21:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.