Region-selection map simplified from KOSTAT 2018 boundary data, shared by southkorea/southkorea-maps. KOSTAT data: free to share or remix per source README. Historical geometry for location selection only; contract queries use API administrative codes.
https://github.com/southkorea/southkorea-maps
https://raw.githubusercontent.com/southkorea/southkorea-maps/master/kostat/2018/json/skorea-provinces-2018-topo-simple.json