In surveying, declination refers to the angular difference between magnetic north and true north at a specific location on Earth’s surface. It’s a crucial concept because compasses, which are fundamental tools in surveying, align themselves with magnetic north rather than true north.
Imagine Earth’s magnetic field as a giant bar magnet with its poles slightly offset from the geographic poles. This offset causes magnetic north to vary depending on your location. Declination is the angle between the direction a compass needle points (magnetic north) and the direction towards the geographic North Pole (true north).
There are two types of declination:
East Declination: This occurs when magnetic north is located to the east of true north. To correct for this, you would subtract the declination angle from your compass reading to obtain the true bearing.
West Declination: This happens when magnetic north is situated to the west of true north. In this case, you would add the declination angle to your compass reading to get the true bearing.
Declination is not constant; it varies depending on your location and changes over time due to the Earth’s magnetic field fluctuations. Surveyors use up-to-date declination charts or online calculators to determine the declination value for their specific location and time.
Understanding and accounting for declination is essential in surveying to ensure accurate measurements and mapping. By applying the appropriate correction, surveyors can convert magnetic bearings obtained from their compasses into true bearings, which are referenced to true north, providing a consistent and reliable framework for their work.