Compass Primer

                                            How a Compass Works

Information provided by Silva (

The magnetic needle in a compass is attracted by the magnetism of the earth... Magnetic North (MN) is about 800 miles south of True North (TN), the North Pole. The angle of difference is called magnetic declination (variation) and varies from place to place. Topographic maps include diagrams which indicate the angle of difference between True North (=True North) and Magnetic North (see below left). When using a compass and map, you must train yourself to compensate for declination using one of these options:

  • Add or subtract the degrees of magnetic declination provided from the map.

  • Extend the MN line of declination diagram in the map margin. Draw lines parallel to the extension line, approximately 2" apart. Using these lines, the map and compass now reference the MN.

  • Purchase a compass with Geared Declination Correction (such as the Silva Ranger or Explorer Pro models) that allows you to set and leave your degree setting.


Lines of magnetic force vary from vertical at the magnetic poles, to horizontal near the equator, as shown in Figure A. Consequently, one end of a magnetic needle has a tendency to dip down in areas that lie between the poles and the equator. Silva reduced this tendency by placing the center of gravity below the pivot point of the needle. Also, the needle of each compass is counterbalanced for the middle of the magnetic zone where it may be used. A compass needle may show a slight tendency to dip in an area between two zones. Figure B shows how the MN-zone-balance compass needle is affected when used in other magnetic zones.