[Therion] North arrow and magnetic declination

[Olly Betts]

On Thu, May 28, 2020 at 10:20:50AM +0200, Martin Sluka via Therion wrote:
> > 27. 5. 2020 v 23:39, Bill Gee <bgee at campercaver.net>:
> > I am well aware of how Therion processes declination when going
> > through the survey data.  I take the trouble to make sure every set
> > of survey data has a date associated with it so that the
> > declinations can be properly calculated.
> >  
> > As for putting declination on the map ... I have never done that and
> > do not plan to start.   I think it is not useful to have it on a
> > map.  In 20 years (or whenever) it will be wrong.  It is better to
> > just leave it off.
>
> Declination up to 1 degree could be not a problem. But there are
> regions where declination is more than 10 degree.

It's more than 25° in southern NZ:

https://www.gns.cri.nz/var/ezwebin_site/storage/images/home/our-science/land-and-marine-geoscience/earth-s-magnetic-field/declination-around-new-zealand/nz-magentic-declination/75157-1-eng-GB/NZ-magentic-declination.png

> And I know a false theory about development of two huge parallel caves
> instead of only one system because the author of that theory just
> omitted inclination of surveys of upper and lower part of that cave
> from upper and lover entrances (they were not connected). 

I suspect it took CUCC longer to connect Eislufthöhle to Kaninchenhöhle
because we weren't correcting for declination back then (the declination
in the area used to be essentially zero and the models to calculate it
weren't easily available so we left it to sort out later, but gradually
it increased to the point where it actually made a difference).

> Softwares for cave data offer the automatic calculation of declination
> (and grid convergence), so there is not reason not to use it. Therion
> calculates declination according each day from the model.

I don't think anyone has suggested in this thread that one shouldn't
correct for declination.

> Maybe there should be the date when a map was generated under North
> sign in such case.

If you're going to put a Magnetic North arrow on, then I agree it really
ought to be clear what date it's for.

Bill's original message was saying that therion has options for North
arrows which show both geographic and magnetic north, but *DON'T* say
what date the shown magnetic north is for:

https://mailman.speleo.sk/pipermail/therion/2020-May/008254.html

That seems like a bug to me.

Cheers,
    Olly