The US County Boundary file contains the boundary geographic information for all counties in US. The data is in geojson format which is converted from cartographic boundary files that are simplified representations of geographic areas from the Census Bureau’s MAF/TIGER geographic database. For more details: https://www.census.gov/geographies/mapping-files/time-series/geo/carto-boundary-file.2018.html
Bureau, U. (2019). Cartographic Boundary Files - Shapefile. [online] Census.gov. Available at: https://www.census.gov/geographies/mapping-files/time-series/geo/carto-boundary-file.2018.html [Accessed 2 Oct. 2019].