World Magnetic Model

[Olly Betts]

On Thu, Apr 11, 2019 at 06:03:39PM +0200, Martin Budaj via Therion wrote:
> On Thu, Apr 11, 2019 at 5:05 PM kevin dixon via Therion <therion at speleo.sk>
> wrote:
> > The last Therion release 5.4.3 is dated 01 Feb 2019.
> >
> > There has in recent years been a significant change of the Magnetic North
> > Pole location, less so for the Geomagnetic South Pole:
> > https://ngdc.noaa.gov/geomag/GeomagneticPoles.shtml
> >
> > Given the above, an out of cycle World Geomagnetic Model WMM2015v2 was
> > released 04 Feb 2019.

Shortly after WMM2015v2 was released I did some comparisons for
Northerly locations on land between WMM2015v2 and IGRF12 for dates at
the end of this year (since IGRF13 is due out in December 2019) and the
largest difference I found was 0.1° which is probably acceptable given
the accuracy of the readings involved (and considering this is only an
issue until this December anyway).

> > When will this update appear in Survex and Therion ?

My current plan for Survex is to address this by updating to IGRF13 in
December 2019.

> Therion does not use WMM at all (as it doesn't contain 20th century
> historical data).
> It uses IGRF model (https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html) which
> hasn't been updated yet.

Survex uses IGRF too, and for the same reason.

It'd be nice to support WMM2015v2 but we would still need to use IGRF
for dates before 2015 and have some way to cleanly transition between
the two (just using IGRF for < 2015 and WMM for >= 2015 would mean that
a series of surveys made from late 2014 to early 2015 could have a
sudden step in the declination corrections, which seems problematic).

I can't speak for Therion, but I'm open to supporting WMM in Survex if
the difference is actually an issue for people.  But if someone wants
WMM support in Survex we at least need a workable plan for what to do
about dates that WMM doesn't handle.  And even better would be a patch
to implement that plan.

Cheers,
    Olly