[Therion] Calculating Magnetic Declination

[Stacho Mudrak]

Quoting "David A. Riggs" <david.a.riggs at gmail.com>:
> 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

It is exactly like this. CS should be specified before coordinates in given
system are used. You are right, it is nowhere stated in the thbook, we will try
to write it there explicitly, to avoid confusions. Thanks for pointing out.

> 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

This is very strange - we did not touched this code. I am not sure about 0.5.1,
but latest snapshot that I have, generates this output.

> My recommendations are that therion should:
> 
> - Ignore the order of 'cs' and 'fix', apply declination as long as
> both are included

Well, this is a little bit problem. CS is handled same way for all commands,
including centerline. Sometimes it is needed to switch coordinate system within
single command (example from our experience: if we have a surface and usually 3D
grind is in one CS and bitmap in another). But even in centerline it has happen
to us. But one think we can do - generate a warning, if there is CS
specification in centerline, but stations are fixed before it.

> - Explicitly state that declination has or has not been applied,
> regardless of configuration, so users are aware of this

This should be the case - if it is not - it is a bug and needs to be fixed.
Could you please try latest snapshot, if it is still the case?

> - Emit a warning if partially or incorrectly configured for magnetic
> declination calculation

I will put it into TODO list. Thanks for pointing out.

Regards, S.