[Therion] Calculating Magnetic Declination

[David A. Riggs]

I just spent far too long trying to get automatic calculation of
magnetic declination to work, and I'm posting my findings so that
others don't make the same mistake, and so that therion (an already
great piece of software) can be improved accordingly. I've found
that...

Specifying the fixed coordinate before the coordinate system causes
therion to silently fail to apply declination. In other words, this is
wrong:

   date 2007.06.31
   fix A0 626333 4320333 2620
   cs UTM17

Specifying the coordinate system before the fixed coordinate causes
therion to properly calculate magnetic declination. This is correct:

   date 2007.06.31
   cs UTM17
   fix A0 626333 4320333 2620

Therion 0.5.1 does this completely silently, with no notification that
declination has been applied at all. Therion 0.5.0 prints the
following (rather confusing) message in therion.log:

   output coordinate system: EPSG:32617
   meridian convergence (deg): -0.9190
   geomag declinations (deg):
     2007.1.1  -8.9944
     2008.1.1  -9.0224

My recommendations are that therion should:

- Ignore the order of 'cs' and 'fix', apply declination as long as
both are included
- Explicitly state that declination has or has not been applied,
regardless of configuration, so users are aware of this
- Emit a warning if partially or incorrectly configured for magnetic
declination calculation

Thanks very much!

Dave Riggs
Morgantown, WV

-- 
David A. Riggs <david.a.riggs at gmail.com>