On 12 Mar 2013, at 20:18, Alasdair Campbell wrote:
> Thank you Stuart & James for your responses,
> Stuart, I was not so much interested in what is painted on the ground, but
> what is indicated on the slaved
> gyro. Your comments have made me reconsider my maths.
>
> apt.dat shows a true heading for EGPF R05 of 46.34 (where does one find true
> runway headings?)
> Magvar on 2011 was 4.0W
> (http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task=blogcategory&id=72&Itemid=121.html)
> Annual Rate of Change 0.17°E (source as above)
> So current Magvar =3.66W
> Now I suppose I must ADD? this figure to the true heading? giving 50.00 as
> the magnetic heading for EGPF R05.
> The slaved compass reports 50.03, which is good enough for me, But I wonder
> if, as James suggested, upgrading to
> ww2010 might lose the tiny difference. I may try this on a local repo and see
> what happens.
Yes, you add the variation in the uk at present (I have a mnemonic of "small
map, big world, so add the variation". :] )
I think one your talking about small fractions of a degree you are well into
noise territory and I'd be surprised of updating the magnetic model would make
much difference. Your base data of 4.0W mag variation at EGPF is probably only
accurate +/- 0.25. I had a quick look at my chart and the +4.5W variation line
is not too far to the west.
> Sorry for thinking aloud, but I find this wmm stuff fascinating, though
> complicated.
Yes, it's very interesting. My chart for Ireland specifically warns about the
large variation (+5W) which I'm sure catches people out. And that's before you
have to add variation due to the airframe !
-Stuart
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