Magnetic declination is the angle between true north, which aligns with the Earth's rotational axis, and magnetic north, which follows the direction of the Earth's magnetic field. This discrepancy exists because the magnetic poles do not coincide with the geographic poles. The value of magnetic declination depends on the observer's location on Earth and is subject to changes over time due to the dynamic nature of the Earth's magnetic field.

The declination is called eastern when magnetic north lies east of true north. Conversely, the declination is called western when magnetic north is positioned west of true north. Understanding and compensating for this variation is essential for accurate compass readings. Errors in accounting for magnetic declination can lead to significant and sometimes dangerous directional inaccuracies, especially in fields like navigation, aviation, and land surveying.

The Earth's magnetic field changes over time, leading to variations in magnetic declination. Long-term changes, known as secular variations, occur over cycles lasting approximately 150 years and result from slow movements of molten iron within the Earth's outer core. Smaller variations, including those that happen annually or daily, are influenced by external factors such as solar activity, though these are usually minor and negligible for most practical applications.

Magnetic declination calculators provide precise values by considering location and date, making them indispensable tools for modern surveying and navigation. These calculators ensure that historical survey data can be updated accurately and that new measurements account for changes in the magnetic field. By using these tools, surveyors and navigators can align instruments correctly with true north, minimizing errors caused by the natural variability of the Earth's magnetic field.