# Given an object with geometries of type `GEOMETRY` or `GEOMETRYCOLLECTION`, return an object consisting only of elements of the specified type. Similar to ST_CollectionExtract in PostGIS. If there are no sub-geometries of the specified type, an empty geometry is returned. ```r st_collection_extract( x, type = c("POLYGON", "POINT", "LINESTRING"), warn = FALSE ) ## S3 method for class 'sfg' st_collection_extract( x, type = c("POLYGON", "POINT", "LINESTRING"), warn = FALSE ) ## S3 method for class 'sfc' st_collection_extract( x, type = c("POLYGON", "POINT", "LINESTRING"), warn = FALSE ) ## S3 method for class 'sf' st_collection_extract( x, type = c("POLYGON", "POINT", "LINESTRING"), warn = FALSE ) ``` ## Arguments - `x`: an object of class `sf`, `sfc` or `sfg` that has mixed geometry (`GEOMETRY` or `GEOMETRYCOLLECTION`). - `type`: character; one of "POLYGON", "POINT", "LINESTRING" - `warn`: logical; if `TRUE`, warn if attributes are assigned to sub-geometries when casting (see `st_cast`) ## Returns An object having the same class as `x`, with geometries consisting only of elements of the specified type. For `sfg` objects, an `sfg` object is returned if there is only one geometry of the specified type, otherwise the geometries are combined into an `sfc` object of the relevant type. If any subgeometries in the input are MULTI, then all of the subgeometries in the output will be MULTI. ## Examples ```r pt <- st_point(c(1, 0)) ls <- st_linestring(matrix(c(4, 3, 0, 0), ncol = 2)) poly1 <- st_polygon(list(matrix(c(5.5, 7, 7, 6, 5.5, 0, 0, -0.5, -0.5, 0), ncol = 2))) poly2 <- st_polygon(list(matrix(c(6.6, 8, 8, 7, 6.6, 1, 1, 1.5, 1.5, 1), ncol = 2))) multipoly <- st_multipolygon(list(poly1, poly2)) i <- st_geometrycollection(list(pt, ls, poly1, poly2)) j <- st_geometrycollection(list(pt, ls, poly1, poly2, multipoly)) st_collection_extract(i, "POLYGON") st_collection_extract(i, "POINT") st_collection_extract(i, "LINESTRING") ## A GEOMETRYCOLLECTION aa <- rbind(st_sf(a=1, geom = st_sfc(i)), st_sf(a=2, geom = st_sfc(j))) ## With sf objects st_collection_extract(aa, "POLYGON") st_collection_extract(aa, "LINESTRING") st_collection_extract(aa, "POINT") ## With sfc objects st_collection_extract(st_geometry(aa), "POLYGON") st_collection_extract(st_geometry(aa), "LINESTRING") st_collection_extract(st_geometry(aa), "POINT") ## A GEOMETRY of single types bb <- rbind( st_sf(a = 1, geom = st_sfc(pt)), st_sf(a = 2, geom = st_sfc(ls)), st_sf(a = 3, geom = st_sfc(poly1)), st_sf(a = 4, geom = st_sfc(multipoly)) ) st_collection_extract(bb, "POLYGON") ## A GEOMETRY of mixed single types and GEOMETRYCOLLECTIONS cc <- rbind(aa, bb) st_collection_extract(cc, "POLYGON") ```

st_collection_extract function