VORs and isogonic lines

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Can somebody kindly explain why on charts the 360 due north pointers on VORs
(which supposedly points to magnetic north) do not align with the nearest
isogonic line (which is also supposedly magnetic north)? I know that the
heading of isogonic lines varies considerably depending upon the area, but
in the San Francisco area for example the difference between the VOR and
isogenic headings are considerable. My problem is that I have read that when
plotting a direct A to B course you draw a line, align the protractor with
the grid lines, then adjust the protractor angle reading with the isogonic
variation. But the result differs from that if you use a parallel ruler
against the nearest VOR. Am I missing out on something obvious here?
Thanks. John

 

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Archived from groups: alt.games.microsoft.flight-sim (More info?)

"John Lee" <jandj.lee@ntlworld.com> wrote in message
news:2HmXd.444$Gz.118@newsfe1-gui.ntli.net...
> Can somebody kindly explain why on charts the 360 due north pointers on
VORs
> (which supposedly points to magnetic north) do not align with the nearest
> isogonic line (which is also supposedly magnetic north)?

The magnetic variation on a VOR is manually set by the maintainer.
Sometimes it's set close to the actual local variation; other times, it's
off by several degrees. Sometimes they are purposely not reset as
frequently as the local magnetic variation changes so that familiar airways
do not suddenly (or continuously) change.

I can't speak for the civilian side, but for military flyers, the IFR
Enroute Supplement lists the variation for each VOR in the
Airport/Facilities Directory. That way you eliminate the discrepancies
between what you measure on the chart and to which variation the VOR is
actually set.
--
Chris
http://www.mcmartinville.com

 

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Archived from groups: alt.games.microsoft.flight-sim (More info?)

"John Lee" <jandj.lee@ntlworld.com> schrieb im Newsbeitrag
news:2HmXd.444$Gz.118@newsfe1-gui.ntli.net...
> Can somebody kindly explain why on charts the 360 due north pointers on
> VORs (which supposedly points to magnetic north) do not align with the
> nearest isogonic line (which is also supposedly magnetic north)? I know
> that the heading of isogonic lines varies considerably depending upon the
> area, but in the San Francisco area for example the difference between the
> VOR and isogenic headings are considerable. My problem is that I have read
> that when plotting a direct A to B course you draw a line, align the
> protractor with the grid lines, then adjust the protractor angle reading
> with the isogonic variation. But the result differs from that if you use a
> parallel ruler against the nearest VOR. Am I missing out on something
> obvious here? Thanks. John
>
Maybe there's some misunderstanding in the wording. Isogonic lines do n ot
point to magnetic north. An isogonic line means that all points on this line
have the same magnetic variation. So if you search for the next isogonic
line to your VOR look at the value of the isogonic line (must be a value
indicating a E or W variation). The VOR should then be "misaligned" relative
to true north by the indicated value.
--
Oskar
(retired captain)
Remember, in the great scheme of things, we're all small potatoes...