magnetic declination

magneettinen deklinaatio puhekieltä The angle between the magnetic meridian and geographic meridian at a given place; a measure of the deviation of magnetic north from true north, expressed in degrees east or west.
2002, Walter A. Schroeder, Opening the Ozarks: A Historical Geography of Missouris Ste. Genevieve District 1760-1830'', footnote, http://books.google.com.au/books?id=7j3NNrHFVR4C&pg=PA159&dq=%22magnetic+declination%22%7C%22magnetic+declinations%22&hl=en&sa=X&ei=W4sFVKm7Bo3s8AXL94JY&redir_esc=yv=onepage&q=%22magnetic%20declination%22%7C%22magnetic%20declinations%22&f=false page 159,

In the Ste Genevieve District, magnetic declination varies significantly within short distances due to iron-rich bedrock, and this might account for magnetic declinations varying from five to ten degrees east in Soulard's surveys.

By the 17th century several instruments had been designed to measure the magnetic declination.

This is corrected by setting the magnetic declination of the compass. This is a correction value given on all navigation maps. Once set with the magnetic declination, a compass will point to the true North Pole also known as the “Geographic North Pole." All places on Earth have a slightly different magnetic declination depending on how you line up with both poles. In an extreme case, if you stood between the two poles and faced the real North Pole, your masgnetic compass would point to the magnetic North Pole behind you—a magnetic declination of 180 degrees!