[Therion] Therion uses wrong declination for surveys 'in the future'

[Matthias Keller]

Hi

I just had the following problem: A current survey (with survey dates 
2020.06.27 and 2020.09.19) was compiled using Therion 5.4.4 (from 2019). 
I got the following warning during compilation:

warning -- unable to determine magnetic declination for undated surveys

However, firstly: this warning is wrong, as all surveys are dated correctly.

However I was stunned by some large loop errors in the newest surveys 
(>60cm in only 10 legs). Then, just by some other impulse I updated 
therion to 5.5.1 and suddenly, those errors went down to <10cm and the 
generated models looked differently. Also, the above error message was 
not present anymore.

So i experimented and just added "declination 3 deg" to the two new 
surveys (from June and September) and suddenly, the output generated by 
5.4.4 looked pretty much as the one from 5.5.1

It appears that, when therion cannot determine the magnetic declination 
of a survey (with valid date!), it finally just assumes 0. The problem 
seems to be, that the declination data stored in 5.4.4 didn't let 
therion determine a possible declination for 2020.06.27 and 2020.09.19 
so it just assumed 0. I would have expected therion to at least consider 
the latest known declination which would result to around 3° (in 
Switzerland).

Is this a known issue? It just means that whenever you open a survey in 
some version being 'too old', it will kill your plans...

BTW, Therion 5.4.4 seems to have *some* data for 2020 at least, as it seems:

geomag declinations (deg):
   2019.1.1  2.9201
   2020.1.1  3.0503

but the only surveys from 2020 are the ones from June 2020 and September 
2020, and both were assigned a declination of 0° instead of about 3°. I 
can verify that by adding the declination parameter with "0 deg" to 
those surveys and the output is identical to the one without this parameter.

Thanks

Matt

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