Source: https://raw.githubusercontent.com/nvkelso/natural-earth-vector/master/geojson/ne_50m_admin_0_countries.geojson Clues: https://medium.com/@mbostock/command-line-cartography-part-2-c3a82c5c0f3 Example:
ndjson-split 'd.features' < ne_50m_admin_0_countries.geojson \
| ndjson-filter 'd.properties.SUBREGION == "South-Eastern Asia" || d.properties.SUBREGION == "Southern Asia" || d.properties.SUBREGION == "Northern Africa" || d.properties.REGION_WB == "Sub-Saharan Africa" || d.properties.REGION_WB == "Oceania" || d.properties.REGION_WB == "Latin America & Caribbean"' \
| ndjson-filter 'd.properties = {ISO_A3: d.properties.ISO_A3, ISO_N3: d.properties.ISO_N3, REGION_UN: d.properties.REGION_UN, SUBREGION: d.properties.SUBREGION, NAME: d.properties.GEOUNIT, REGION_WB: d.properties.REGION_WB, CONTINENT: d.properties.CONTINENT }' \
> ne50_studyarea.ndjson
geo2topo -n countries=ne50_studyarea.ndjson > ne_50m_6f_countries.studyarea.topo.json
topomerge land=countries < ne_50m_6f_countries.studyarea.topo.json > ne_50m_countries_land.studyarea.topo.json
topo2geo land=land.studyarea.geo.json < ne_50m_countries_land.studyarea.topo.json