An internal function to make basemap code more readable
define_shapefiles(limits)A numeric vector of length 4: The first element defines the minimum longitude, the second element the maximum longitude, the third element the minimum latitude and the fourth element the maximum latitude of the bounding box.
A list containing the correct shapefile and a logical statement whether the limits were supplied as decimal degrees.
This is an internal function, which is automatically run by the basemap function.