Déclinaison magnétique

True north aligns with Earth's rotational axis, while magnetic north follows Earth's magnetic field direction. These do not coincide because the magnetic poles do not align with the geographic poles. This angular difference is magnetic declination, which varies by location and changes over time due to shifts in Earth's magnetic field.

Magnetic declination causes compass readings to deviate from true north, potentially leading to significant directional errors. Surveyors must account for declination when determining directions to ensure accurate measurements. Errors in compensation can result in dangerous inaccuracies, especially in navigation, aviation, and land surveying applications where precision is critical.

Eastern declination occurs when magnetic north lies east of true north, while western declination occurs when magnetic north lies west of true north. The direction depends on the observer's location on Earth. Understanding whether declination is east or west is essential for correctly adjusting compass readings and survey instruments.

Earth's magnetic field is dynamic, causing declination to vary. Long-term changes called secular variation occur over cycles lasting approximately 150 years due to slow movements of molten iron in Earth's outer core. Smaller annual and daily variations result from external factors like solar activity, though these are usually negligible for most practical applications.

Magnetic declination calculators provide precise values by inputting location and date. These tools are essential for updating historical survey data and conducting new measurements that account for changes in Earth's magnetic field. By using calculators, surveyors align instruments correctly with true north and minimize errors from natural magnetic field variability.

Surveyors incorporate magnetic declination into angle measurements to convert magnetic bearings to true bearings. Accurate declination values ensure that traverse measurements and directional calculations reflect true north rather than magnetic north. This correction is critical when performing design example traverse angle computations to maintain survey accuracy and consistency.

Magnetic declination varies significantly by geographic location because Earth's magnetic poles are offset differently relative to the geographic poles at different positions on the surface. A declination value accurate for one location may be incorrect for another. This location-dependent variation is why surveyors must use calculators or reference current declination data specific to their project area.