Geology

If the compass needle points east or west of the true north, the offset is called east or west declination, respectively. The standard is to use the magnetic north (MN) as a reference for declination, even in the southern hemisphere. To determine the magnetic declination in a study area we can use: (1) Published topographic maps; some maps display an out-of-date declination indicated by the angle between two arrows pointing to the magnetic north (MN) and true north (GN). (2) Published or online isogonic charts, which are available at:

http://geomag.usgs.gov/chartsdo.html, or http://geomag.usgs.gov/models.html.

(3) Online calculator to determine the latest magnetic declination for a given location (latitude and longitude) and year

http://www.geolab.nrcan.gc.ca/geomag/e_cgrf.html

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