[Therion] Declination handling imprecise?

[Benedikt Hallinger]

Hello,
i found out that the declination handling seems to be somewhat inaccurate.
The produced error is marginal and negligible with small caves, however i am 
currently working on a system with >100km and this produces errors of >10m 
there.

What i observe is the following:
- Therion seems to use the first day of any given year referenced by "date" 
command.
- If a survey was taken at the end of a year it seems that the 1.1 of the 
following year will be used (i assume this happens at mid-year).
- This approach looses at most half a years magnetic drift.
This alone is acceptable as the data by itself is already not that accurate.

However if you have several centerlines in the survey, each with different 
date-commands, therion seems to calculate the average of all dates and then 
uses the calculated declination for that date (applying the rule above, so 
taking the "closest 1.1.").
This is then applied to all shots of the whole survey.


A possible solution to this seems, to get the correct declination for each 
date and explicitely write suitable "declination" commands that override the 
"date"-calculated values. This then gives the proper results.
I attached a test survey where one can see this easily. The western survey 
line shows the date-behviour, the right one the explicit declination-commands 
active. You can see this at the northernmost survey legs, the error on 2km is 
13m corresponding to roughly ~0.65% error introduced additionally to the 
measurement errors itself.
I got the values by the GFZ-Potsdam website 
(http://www-app1.gfz-potsdam.de/cgi-bin/igrf.pl?Year=2013&Month=01&Day=3&alt=600&latd=47&latm=32&lond=13&lonm=37), 
but note, that the declination has to be converted to decimal degrees.



Am i right with my assumptions and is my only solution to this to introduce 
explicite declination-commands everywhere?
Would this easily be fixable in therions code?


With best Regards,
Benedikt Hallinger
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