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Build polygonal faces from a line network

Source: R/spatial_topology.R
spatial_polygonize.Rd

Forms the polygons enclosed by a set of lines (the QGIS "Polygonize", GEOS Polygonize): the inverse of taking polygon boundaries. The lines of each group are unioned and noded so every crossing becomes a shared vertex, then the faces of that planar arrangement are returned, one per row. A pile of lines that does not close any area yields no faces. Like spatial_dissolve() and spatial_construct() it rides the partition tier: x is spilled once and routed into one disjoint shard per by group in a single bounded pass, then each group's lines are polygonized together. Peak memory is the routing budget during the pass, then one group's geometry while its faces are built – partition on a key whose groups fit in memory. With no by, the whole layer yields one set of faces.

Usage

spatial_polygonize(
  x,
  by = NULL,
  geom = "geometry",
  crs = NA,
  flush_rows = NULL
)

Arguments

x

A vectra_node (from tbl(), tbl_tiff(), any verb chain, ...). It is consumed by the stream.

by

Character vector of attribute columns to polygonize within: one set of faces per distinct combination of their values. NULL (default) polygonizes the whole layer at once.

geom

Name of the input geometry column holding hex-WKB or WKT strings. Default "geometry". Ignored when coords is given.

crs

Coordinate reference system of the input geometry, in any form sf::st_crs() accepts (EPSG integer, WKT, proj string). Defaults to the CRS the upstream node carries, or unknown.

flush_rows

Transformed rows buffered before a spill flush. Larger values mean fewer, bigger temporary files. Defaults to getOption("vectra.spatial_flush", 5e5).

Value

A vectra_node of one row per face, carrying the by columns and the input CRS, backed by temporary .vtr spills removed when the node is garbage-collected.

Details

Each face is new geometry built from the whole group, so it carries the by columns only, not the attributes of any single source line. Geometry travels through the engine as hex-encoded WKB in a string column and the CRS is carried on the returned node; the noding is sf/GEOS and expects projected or unprojected planar data. The sf package is an optional dependency (Suggests).

See also

spatial_split() to cut existing polygons by a blade, spatial_construct() for hulls and tessellations, spatial_dissolve() to merge geometries by group, collect_sf() to materialize as sf.

Examples

grid <- sf::st_sfc(
  sf::st_linestring(rbind(c(0, 0), c(2, 0))),
  sf::st_linestring(rbind(c(0, 1), c(2, 1))),
  sf::st_linestring(rbind(c(0, 2), c(2, 2))),
  sf::st_linestring(rbind(c(0, 0), c(0, 2))),
  sf::st_linestring(rbind(c(1, 0), c(1, 2))),
  sf::st_linestring(rbind(c(2, 0), c(2, 2))))
f <- tempfile(fileext = ".vtr")
write_vtr(data.frame(
  geometry = sf::st_as_binary(grid, hex = TRUE)
), f)

# The four unit cells enclosed by the grid of lines.
tbl(f) |> spatial_polygonize() |> collect_sf()
unlink(f)