Magnetic Declination

Magnetic declination is the difference in direction between magnetic north, where a compass needle points, and true north. BLM's Glossary of Surveying and Mapping Terms defines it as "the bearing on a given date (reckoned east or west from the north branch of the celestial meridian plane) of magnetic north as determined by the positive pole of a freely suspended magnetic needle which is subject to no transient artificial disturbance."

The glossary defines magnetic north as "the direction indicated by the north end of a magnetized needle under influence of the earth's magnetic field and free of local magnetic disturbance." The words "on a given date" matter, because declination changes. The glossary calls that change magnetic variation, "regular or erratic change in magnetic declination," and cautions that "old field notes use Var. as the declination of the compass."

The PLSS runs on the true meridian

Public land surveys are not based on magnetic north. The Land Ordinance of 1785 directed surveyors to "run and note all lines by the true meridian" (Manual section 2-1), and federal law still requires "north and south lines run according to the true meridian" (43 U.S.C. 751). BLM's Manual of Surveying Instructions (2009) states that the direction of each line "is determined with reference to the true meridian as defined by the axis of the earth's rotation" (section 2-3). BLM's Specifications for Descriptions of Land (2017) say the principal meridian is run with reference to "true north," not magnetic north.

The magnetic needle in survey history

The Manual lists the instruments historically used to set the direction of PLSS lines: "the magnetic needle, solar compass, transit with solar attachment, and direct altitude or hour angle observations on the Sun, Polaris, or other stars." Several methods remain valid, but "only the use of the needle compass is unequivocally prohibited" (section 2-4).

How declination is recorded

• The field notes of a survey include "the magnetic declination, including the observed local attraction within the area of the survey," and the average value is shown on the plat (section 3-223)
• The north arrow on a plat shows the true meridian, and "the mean magnetic declination over the area surveyed, when determined, is shown at the base of the north arrow" (section 9-81)
• Mineral surveys "must be made with an instrument by which the meridian may be determined independently of the magnetic needle," with directions referred to the true meridian, and the mean declination, "when observed," recorded in the field notes (section 10-121)

Why it matters for mining claims

Official plats and mineral surveys give bearings from true north. A bearing read from a compass is from magnetic north unless it has been corrected. When a location notice, mining claim map, or older record gives bearings, mining claim research has to establish which north they use before plotting them against PLSS corners. BLM's Specifications list a "basis of bearings" among the elements of a metes-and-bounds description, and their example states one "referenced to the true meridian."