1. Digital data grids for the Magnetic anomaly map of North America [electronic resource] Yale Public content 2003. 2004-08-23T18:55:07Z. 2011-03-03T23:28:56Z. North American Magnetic Anomaly Group. U.S. Dept. of the Interior, U.S. Geological Survey ; For sale by USGS Information Services. English. Public. North America.