Azimuth local correction

[Ibbis]

Im on 20.6N, 100.4W.
w*w.global-cm.net says
"Azimuth headings are TRUE, not MAGNETIC. Please apply local correction if using a compass."
Example: To get the Galaxy 10R on 123°W I have to point to 54.7 Elevation / 229.8 Azimuth

How I have to correct the Azimuth if I want to point via compass?

 

[Perfesser]

"Azimuth headings are TRUE, not MAGNETIC"

That's trhe reason. a compass does NOT point to true north except along a narrow band in the Central US. The difference is called declination and it is due to the difference between the positions of the magnetic and geographical poles.
Check this chart...
somewhere I have another showing how the NMP has moved since is was first located in the 1800's.

 

[PSB]

Here is some more reading........

http://www.ngdc.noaa.gov/seg/geomag/jsp/struts/calcDeclination

http://www.geo-orbit.org/sizepgs/magmapsp.html

http://en.wikipedia.org/wiki/Magnetic_declination