shapefile_details       Details of the shapefiles included in this
                        package.
shp_all_us_congressional_districts
                        Shapefiles detailing the Congressional
                        Districts of the USA, designed for subsetting
                        to the contiguous USA.
shp_all_us_counties     Shapefiles detailing the Counties of the USA,
                        designed for subsetting to the contiguous USA.
shp_all_us_states       Shapefiles detailing the states of the USA,
                        designed for subsetting to the contiguous USA.
