[Therion] Declination and meridian convergence

Stacho Mudrak lists at group-s.sk
Tue Oct 4 19:44:02 CEST 2011

> Question 1: Is it correct that in therion and in survex the declination is specified exactly the opposite way?

Yes, it is like that. You are right, there should be explicit warning
in the manual.

> Therion handbook page 18:
> true bearing = measured bearing + declination
> Survex 1.2.1 manual page 19:
> (true bearing)=((bearing)-(declination zero err))
> 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.

> 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 :)


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