For folks working with historic deeds and bearing references, there are a number of tools available -
- NOAA's National Geophysical Data Center (NGDC) has an online calculator for magnetic declinations, 1900 to the present: http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp
- NGDC's online tool to compute the seven individual IGRF magnetic field components: http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp
- NGDC's online calculator for historic declinations, back to 1750: http://www.ngdc.noaa.gov/geomagmodels/struts/historicPoint
Additionally, there is a freeware tool available from Resurgent Software, which will compute historic declinations to 1600: http://www.resurgentsoftware.com/geomag.html
Further online tools and resources for geomagnetism and computation:
EDIT: And an additonal one suggested by Chuck Conley from Canada's Geological Survey
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