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US County boundaries from the US Census Bureau's MAF/TIGER geographic database. Reprojected using Albers Equal Area.
data(us_aea)
A SpatialPolygonsDataFrame object
SpatialPolygonsDataFrame
United States Census Bureau. Data accessed from the US Census Bureau website: http://www2.census.gov/geo/tiger/GENZ2010/gz_2010_us_050_00_20m.zip; accessed 06 April 2018