Magnetic declination, also known as magnetic variation, is the angle between magnetic North, the direction of the magnetic North Pole indicated by a magnetic compass, and true North, the direction of the North Pole. Because the magnetic North Pole is not at the North Pole, it is necessary to compensate for the difference when using a compass for navigation. Modern navigational compasses will usually include a declination adjustment that can be set to make the compass indicate true North instead of magnetic North.
Magnetic declination varies from place to place and over time. A prediction of the current magnetic declination for a given location can be obtained on-line from a web page  operated by the National Geophysical Data Center, a division of the National Oceanic and Atmospheric Administration of the United States.