Build the shared-edge topology of a polygon coverage

Decomposes a polygon coverage into the arcs of its planar topology: each border is returned once, tagged with the polygons on either side. Where the raw boundaries of adjacent polygons each carry their own copy of a shared edge, this nodes the unioned boundaries so a shared border is a single arc carrying the identifiers of both neighbours – the "build topology" of a GIS, and the adjacency a dissolve or a coverage edit needs. An internal arc names two faces; an outer arc names one and leaves the other side NA. Like spatial_dissolve() it rides the partition tier: x is spilled once and routed into one disjoint shard per by group in a single bounded pass, and each group is treated as an independent coverage. Peak memory is the routing budget during the pass, then one group's geometry while its arcs are built.

Usage

spatial_topology(
  x,
  id = NULL,
  by = NULL,
  geom = "geometry",
  crs = NA,
  face_cols = c("face1", "face2"),
  flush_rows = NULL
)

Arguments

x

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

id

Optional name of a column of x whose value identifies each polygon in the face columns. NULL (default) uses the 1-based feature order within the group.

by

Character vector of attribute columns whose groups are each built as an independent coverage. NULL (default) treats the whole layer as one coverage.

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.

face_cols

Length-2 character vector naming the two output columns that hold the identifiers of the polygons on either side of each arc. Default c("face1", "face2").

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 arc – the arc geometry plus the two face-identifier columns (and any by columns) – carrying the input CRS and backed by temporary .vtr spills removed when the node is garbage-collected.

Details

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_polygonize() to rebuild faces from arcs, spatial_dissolve() to merge geometries by group, collect_sf() to materialize as sf.

Examples

p1 <- sf::st_polygon(list(rbind(c(0, 0), c(1, 0), c(1, 1), c(0, 1), c(0, 0))))
p2 <- sf::st_polygon(list(rbind(c(1, 0), c(2, 0), c(2, 1), c(1, 1), c(1, 0))))
f <- tempfile(fileext = ".vtr")
write_vtr(data.frame(
  id = c("a", "b"),
  geometry = sf::st_as_binary(sf::st_sfc(p1, p2), hex = TRUE)
), f)

# The shared edge appears once, tagged with both neighbours.
tbl(f) |> spatial_topology(id = "id") |> collect()
unlink(f)