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Explode multipart geometries into single-part features

Source: R/spatial.R
spatial_explode.Rd

Streams a lazy vectra query and splits every multipart geometry into its component single-part geometries: a MULTIPOLYGON becomes one row per polygon, a MULTILINESTRING one row per linestring, a MULTIPOINT one row per point, and a GEOMETRYCOLLECTION one row per member (recursively). The attributes of the source feature are copied onto each part. Already single-part geometries pass through unchanged, as does an empty geometry (kept as one row). This is the streaming counterpart of the QGIS "multipart to singleparts" tool and of sf::st_cast() to a single-part type.

Usage

spatial_explode(
  x,
  geom = "geometry",
  crs = NA,
  out_geom = NULL,
  part = NULL,
  flush_rows = NULL
)

Arguments

x

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

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.

out_geom

Name of the output geometry column. Defaults to geom (or "geometry" when coords is used).

part

Optional name of an integer column numbering the parts within each source feature, 1-based. Default NULL adds no such column.

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 single-part features, backed by temporary .vtr spills (removed when the node is garbage-collected) and carrying the input CRS.

Details

One batch is decoded, exploded, and spilled at a time, so peak memory tracks one batch and its parts, not the whole layer.

See also

spatial_map() for per-feature transforms, spatial_dissolve() to merge features the other way, collect_sf() to materialize as sf.

Examples

mp <- sf::st_multipolygon(list(
  list(rbind(c(0, 0), c(1, 0), c(1, 1), c(0, 1), c(0, 0))),
  list(rbind(c(2, 2), c(3, 2), c(3, 3), c(2, 3), c(2, 2)))))
f <- tempfile(fileext = ".vtr")
write_vtr(data.frame(
  id = 1L,
  geometry = sf::st_as_binary(sf::st_sfc(mp), hex = TRUE)
), f)

# One row per polygon, attributes copied, parts numbered.
tbl(f) |> spatial_explode(part = "part_id") |> collect_sf()
unlink(f)