This line shapefile shows lines of equal annual change (secular variation), in 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 total field intensity is the strength of the field, not divided into its component parts. The change in intensity is measured in nanoTeslas per year. This layer is part of the 1997-2014 edition of the 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 U.S. Geological Survey in the use of these data. National Atlas of the United States. (2011). Magnetic Field - Secular Variation of the Total Field Component for the Epoch 2010.0. National Atlas of the United States. Available at: http://purl.stanford.edu/wc571cd7743. National Atlas of the United States. (2011). Magnetic Field - Secular Variation of the Total Field Component for the Epoch 2010.0. National Atlas of the United States. Available at: http://purl.stanford.edu/wc571cd7743. The total field intensity (F) and the secular variation (FSV, the time rate of change of F) of the IGRF change smoothly over most of the Earth. For more information about the geomagnetic field, please visit <http://www.ngdc.noaa.gov/geomag/faqgeom.shtml>. This layer is presented in the WGS84 coordinate system for web display purposes. Downloadable data are provided in native coordinate system or projection.