There is a very nice program for calculating local geomagnetic deviation available from the NOAA National Geophysical Data Center, known as the Geomagnetic Field Synthesis Program. The source is available for download and the website given below also allows the program to be executed without source download.
http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl I use this program to determine the deviation for a specific site for solar eclipse observation. Among other camera systems, I set up a time-lapse video camera on a clock-driven polar equatorial mount. Since this is done during the daylight hours, I use a good magnetic compass and the deviation value from the above website to set the right ascension axis of the polar mount to the azimuth of true north. I use a precision level and protractor to set the elevation angle. While not perfect, the alignment has been adequate for a four-hour observation. The website program also provides for calculating the change in deviation over a period of time; dates from 1900 to 2000 are supported now. Below is an example of the program output for my home location over the past ten years. (You may need to widen your mail viewer window to see the full width) Model: DGRF90 Latitude: 33.84 deg Longitude: -117.87 deg Elevation: 287.00 ft Range of Interest: 1/1/1990 - 12/31/1999, step 1.00 -------------------------------------------------------------------------- Date D I H X Y Z F (yr) (deg) (deg) (nt) (nt) (nt) (nt) (nt) 1990.00 13d 51.8m 59d 1.0m 25248 24513 6049 42047 49045 1991.00 13d 50.5m 59d 0.1m 25231 24498 6036 41994 48991 1992.00 13d 49.2m 58d 59.3m 25213 24484 6023 41942 48937 1993.00 13d 47.9m 58d 58.4m 25196 24469 6010 41890 48883 1994.00 13d 46.6m 58d 57.5m 25179 24455 5996 41837 48830 1995.00 13d 45.3m 58d 56.7m 25162 24440 5983 41785 48776 1996.00 13d 44.7m 58d 55.9m 25141 24421 5974 41729 48717 1997.00 13d 44.1m 58d 55.1m 25120 24401 5965 41673 48658 1998.00 13d 43.5m 58d 54.4m 25099 24382 5955 41617 48599 1999.00 13d 42.9m 58d 53.6m 25078 24363 5946 41561 48541 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2000.00 13d 42.4m 58d 52.8m 25057 24343 5937 41505 48482 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - dD dI dH dX dY dZ dF (min/yr) (min/yr) (nT/yr) (nT/yr) (nT/yr) (nT/yr) (nT/yr) 2000.00 -0.6 -0.8 -21.0 -19.4 -9.2 -56.1 -58.9 -------------------------------------------------------------------------- The website has a very good explanation of the model and of each parameter in the table above. There is also a link to a website that can provide local coordinates. I don't present this as the perfect solution to the magnetic north thread seen on this reflector recently. Every approach has its limitations and the best selection certainly depends on the application. This has worked for me for my solar eclipse observing in some very strange and remote parts of our Earth. -- Best wishes, Larry McDavid W6FUB Anaheim, CA (20 miles southeast of Los Angeles, near Disneyland)
