[Therion] Declination and meridian convergence

Wolfgang Zillig wolfgang.zillig at web.de
Tue Oct 4 21:57:32 CEST 2011

So in case that magnetic north is 2° east of geographic north, I have to 
correct this in therion by -2 and in survex by +2.
> I am not sure with this. If you measure 0, true bearing would be 2
> (assuming east is 90°), therefore I think you should use 2 in therion
> and -2 in survex. I assume, your situation is the same, as on this
> picture http://en.wikipedia.org/wiki/Magnetic_declination.
OK, I messed this up. You are correct.

>> Question 2:
>> When I enter the measurement data in therion, the meridian convergence
>> is handled automatically (in case a "cs" is specified). However due to historic
>> reasons most of our data is still in survex format. I import the ".3d" file and
>> specify the "cs" during import. I assume that therion is not applying any corrections
>> for the meridian convergence and I need to correct the bearings manually in survex?
> It is probably like that. If you want to have precise coordinates in
> your .3d file according to your CS, all bearing should be corrected
> for meridian convergence. But I am not sure, if you should add or
> substract it from magnetic declination :)

This, depends if you are east or west of the main meridian of the zone 
in the coordinate system. I think I understand the system now, but I'll 
need to draw some figures to explain it. Then I'll put it into the wiki.

Best regards

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