Magnetic Declination

Magnetic declination (also called magnetic variation) is the angle between true north (geographic north) and magnetic north (the direction a compass needle points) at a specific location. This angle varies by location and changes over time.

Importance in mining and surveying: - Historical mining claim surveys often used compass bearings - Compass bearings must be corrected for declination to obtain true bearings - Metes and bounds descriptions may reference either magnetic or true bearings - Failure to account for declination can lead to significant boundary errors

Declination values: - In the western United States, declination typically ranges from about 8 to 20 degrees east - Positive (east) declination means magnetic north is east of true north - Negative (west) declination means magnetic north is west of true north - Declination changes over time (secular variation) at approximately 0.1 degrees per year in many locations

Converting bearings: To convert a magnetic bearing to a true bearing: - East declination: add the declination to the magnetic bearing - West declination: subtract the declination from the magnetic bearing

Data sources: - NOAA National Centers for Environmental Information provides current and historical declination values - The World Magnetic Model (WMM) and International Geomagnetic Reference Field (IGRF) provide global models - Historical declination can be calculated for interpreting old survey notes

Modern practice: GPS technology has largely replaced compass surveys for new work, but understanding magnetic declination remains important for interpreting historical claim surveys, survey field notes, and metes and bounds descriptions.