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

[Bruce Mutton]

Hi Matt

I think these issues have been raised previously.

The error message for dates later than those included in the Therion model was inaccurate, as you point out.  I ‘think’ that in recent versions this may have been replaced with a more meaningful message.

 

And then the question of whether it is better for Therion to extrapolate future declinations, or set them to zero?

Shades of failing gracefully or abruptly, I guess.  I could take either side, but I think my preference would be for Therion to set declinations beyond those modelled to zero.  This way it is more likely that users will detect the problem (as you have), and either upgrade to a more recent version of Therion, or manually set the declination for each affected survey, using the declination statement.

 

Bruce

 

From: Therion <therion-bounces at speleo.sk> On Behalf Of Matthias Keller
Sent: Monday, 5 October 2020 21:40
To: therion at speleo.sk
Subject: [Therion] Therion uses wrong declination for surveys 'in the future'

 

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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