[Therion] declination problem

[Gaëtan PERRIER]

Hi,

I've some problem with the declination. For the coordinate UTM31 398980
4969737 in the log file there is:

output coordinate system: UTM31
meridian convergence (deg): 0.9024
geomag declinations (deg):
  2007.1.1  -0.7738
  2008.1.1  -0.6570

The coordinate UTM31 398980 4969737 are equal to N 44°52'26" E 1°43'15"
rapport to greenwich meridian.

If I go on this site: http://www.ngdc.noaa.gov/seg/geomag/jsp/IGRFWMM.jsp it
gives me for 2007.1.1 a declination of -0°50' so -0.83°. This value is
different in relation to the therion one.
If I try with declimag (http://vtopo.free.fr) it gives me a value as with 
http://www.ngdc.noaa.gov/seg/geomag/jsp/IGRFWMM.jsp

Another quetion: in my centreline there is a line:

date 2007.09.08

Why this date is not used for the declination?

Best regards,

-- 
+--------------------------------------------------------------+
| Gaëtan PERRIER                Nyctalopes Ambidextres & TRIAS |
| gaetan.perrier at speleolot.org       http://www.speleolot.org/ |
+--------------------------------------------------------------+