id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
54	Automatic Declination adjustment	Wookey	Olly Betts	"If surveys are dated and geolocated than a magnetic  instrument correction can be automatically calculated.

As Survex has recently gained geolocation capabilities this is now a practical option. Therion has done this for a long time using the IGRF model coefficients and its own C++ code. 
Here is a C-library that may well do the trick: http://geomag.org/models/dmsp.html

"	enhancement	closed	minor	1.2.21	cavern		fixed	declination magnetic