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2010. National Geophysical Data Center. This line shapefile shows lines of equal vertical intensity of the Earth's magnetic field, derived from the International Geomagnetic Reference Fie... National Atlas of the United States.
1972, 1973, 1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, and 1999. Geological Survey (U.S.). This line shapefile portrays the major roads in the United States, Puerto Rico, and the U.S. Virgin Islands. The file was produced by joining the i... Geological Survey (U.S.).