Déclinaison magnétique

Magnetic declination is the angle between true north or astronomic north and magnetic north. This angle arises because Earth's magnetic poles do not align with its geographic poles. 

The magnetic declination varies by location and changes over time due to shifts in Earth's magnetic field.

Declination may be either east or west. Eastern declination occurs when the magnetic north lies east of the true north, while western declination occurs when it lies to the west. 

This variation can affect compass readings, making it crucial for surveyors to account for the declination when determining directions.

The changes in magnetic declination can occur over long periods, known as secular variation, lasting roughly 150 years per cycle. 

Additional variations include annual and daily changes, though these are smaller and often negligible for most applications.

Magnetic declination calculators use location and date as inputs to determine accurate declination values for specific areas, which is essential for updating old survey data or conducting precise land surveys.