On 9/14/07, Gaëtan PERRIER <gaetan.perrier@speleolot.org> wrote:

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

Therion calculates in fact the same value as the above mentioned calculators, but the date given in the log file is not correct. Formerly just the year used to be printed (meaning mid-year, or July 1) before the declination value. By an oversight the mid-year declination value is still being printed, even if the date has been changed to January 1. It will be fixed soon. Anyway, the loop closure algorithm uses correct decliantion values.

Another quetion: in my centreline there is a line:

date 2007.09.08

Why this date is not used for the declination?

The exact date is used for calculations, just the log files lists yearly declination values. Martin