Based on the fact that this database is free and public domain, apparently it's possible for such data to have a restrictive license. So, I'm wonder what the so

Based on the fact that this database is free and public domain, apparently it's possible for such data to have a restrictive license. So, I'm wonder what the sources of this data are? Is it compiled from public resources from the governments listed, determined and measured by hand by visiting the countries (with a GPS device in hand :P), or something else? I couldn't find this information on github.