The United States Census Bureau defines a shapefile as "A shapefile is a geospatial data format for use in geographic information system (GIS) software. Shapefiles spatially describe vector data such as points, lines, and polygons, representing, for instance, landmarks, roads, and lakes". Shapefiles are used extensively to store spatial information and can be used to plot data on maps. We can easily download many different shapefiles for the USA counties or states from the USA Census Bureau website (www.census.gov). The shapefiles are downloaded as a folder, which comprises files with various extensions such as .shp, .dbf, .prj, .shx, and .xml. For this recipe, we will only use the .shp extension.

The shapefile package in R can be used to read a shapefile, add the processed data to our shapefile, and then save it in the shapefile format. When you save a shapefile in R or any other software, it will create other files that support the shapefile. However, for this recipe...