This line shapefile shows lines of equal annual change (secular variation) in the horizontal component of the total field intensity of the Earth's magnetic field, derived from the International Geomagnetic Reference Field (IGRF) for the epoch 2010.0. Magnetic intensity is the strength of the magnetic field, and is described in terms of a horizontal component and a vertical component. The change in intensity is measured in nanoTeslas per year. This layer is part of the 1997-2014 edition National Atlas of the United States.These data are intended for geographic display and analysis at the national level, and for large regional areas. The data should be displayed and analyzed at scales appropriate for 1:2,000,000-scale data. No responsibility is assumed by the National Atlas of the United States or the U.S. Geological Survey in the use of these data.