Magnetic Declination

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:

• http://geomag.usgs.gov/models/
  
  
• http://geomag.org/models/index.html
  
  

EDIT: And an additonal one suggested by Chuck Conley from Canada's Geological Survey

• http://geomag.nrcan.gc.ca/apps/mdcal_e.php