Add suport for extracting centeriods

I am using pattern of "simonepri" to start this issue.

Problem

Possibility of extraction of centroids of polygons.

Ideas

For centroids there are few techniques available. Best one according my knowledge is https://github.com/mapbox/polylabel

and second best is postgis pointonsurface https://gis.stackexchange.com/questions/76498/how-is-st-pointonsurface-calculated

and last one centroids of polygon which can be out side a polygon.

Use case

To extract gaps/overlaps from a polygon. First calculate centroids , convert all polygons to lines and convert back to polygon and select polygon which do not have a centroids , inside.

Till now I am using -dissolve with unique ID and -points, but points can be out side of polygon which i do not want.

Can we add this function? I hope that will help others as well.

For the polylabel approach, there is now third-party command line utility "geojson-polygon-labels", presented as a "Command line tool to generate point labels from GeoJSON polygons" https://github.com/andrewharvey/geojson-polygon-labels

You can do the PointOnSurface approach using another third party tool like GDAL, e.g

ogr2ogr ne_50m_admin_0_countries_point_on_surface.shp \
             ne_50m_admin_0_countries.shp \
             -dialect SQLite \
             -sql "SELECT featurecla,scalerank,LABELRANK,SOVEREIGNT,SOV_A3,ADM0_DIF,LEVEL,TYPE,ADMIN,ADM0_A3,GEOU_DIF,GEOUNIT,GU_A3,SU_DIF,SUBUNIT,SU_A3,BRK_DIFF,NAME,NAME_LONG,BRK_A3,BRK_NAME,BRK_GROUP,ABBREV,POSTAL,FORMAL_EN,FORMAL_FR,NAME_CIAWF,NOTE_ADM0,NOTE_BRK,NAME_SORT,NAME_ALT,MAPCOLOR7,MAPCOLOR8,MAPCOLOR9,MAPCOLOR13,POP_EST,POP_RANK,GDP_MD_EST,POP_YEAR,LASTCENSUS,GDP_YEAR,ECONOMY,INCOME_GRP,WIKIPEDIA,FIPS_10_,ISO_A2,ISO_A2_EH,ISO_A3,ISO_A3_EH,ISO_N3,ISO_N3_EH,UN_A3,WB_A2,WB_A3,WOE_ID,N_,WOE_NOTE,ADM0_A3_IS,ADM0_A3_US,ADM0_A3_FR,ADM0_A3_RU,ADM0_A3_ES,ADM0_A3_CN,ADM0_A3_TW,ADM0_A3_IN,ADM0_A3_NP,ADM0_A3_PK,ADM0_A3_DE,ADM0_A3_GB,ADM0_A3_BR,ADM0_A3_IL,ADM0_A3_PS,ADM0_A3_SA,ADM0_A3_EG,ADM0_A3_MA,ADM0_A3_PT,ADM0_A3_AR,ADM0_A3_JP,ADM0_A3_KO,ADM0_A3_VN,ADM0_A3_TR,ADM0_A3_ID,ADM0_A3_PL,ADM0_A3_GR,ADM0_A3_IT,ADM0_A3_NL,ADM0_A3_SE,ADM0_A3_BD,ADM0_A3_UA,ADM0_A3_UN,ADM0_A3_WB,CONTINENT,REGION_UN,SUBREGION,REGION_WB,NAME_LEN,LONG_LEN,ABBREV_LEN,TINY,HOMEPART,MIN_ZOOM,MIN_LABEL,MAX_LABEL,NE_ID,WIKIDATAID,NAME_AR,NAME_BN,NAME_DE,NAME_EN,NAME_ES,NAME_FR,NAME_EL,NAME_HE,NAME_HI,NAME_HU,NAME_ID,NAME_IT,NAME_JA,NAME_KO,NAME_NL,NAME_PL,NAME_PT,NAME_RU,NAME_SV,NAME_TR,NAME_UK,NAME_UR,NAME_VI,NAME_ZH,FC_ISO,FC_US,FC_FR,FC_RU,FC_ES,FC_CN,FC_TW,FC_IN,FC_NP,FC_PK,FC_DE,FC_GB,FC_BR,FC_IL,FC_PS,FC_SA,FC_EG,FC_MA,FC_PT,FC_AR,FC_JP,FC_KO,FC_VN,FC_TR,FC_ID,FC_PL,FC_GR,FC_IT,FC_NL,FC_SE,FC_BD,FC_UA,PointOnSurface(geometry) AS geometry FROM ne_50m_admin_0_countries" \
-lco ENCODING=UTF-8

mbloch / mapshaper