The declination calculator provides declination values for any location on Earth and any time between 1900 and 2025. The calculation is based on the International Geomagentic Reference Field (IGRF) which describes the geomagnetic core field. The calculation for a year in general gives values of sufficient accuracy for the orientation by magnetic compass, it is not necessary to specify an exact day. Strongly magnetized rocks in the Earth's crust at some locations cause local anomalies in declination, which can lead to differences between the actual deviation of a compass needle and the calculated values.
Apart from the declination angle in degrees (°) and minutes (') the declination calculator also provides field intensity, inclination and other commonly used geomagnetic field components. The angle of inclination also is provided in degrees (°) and minutes ('), all other components are given in nanoTesla (nT).
For more details on the IGRF see Working Group V-MOD of IAGA