Merge contiguous line segments into maximal lines

Sews the line segments of each group into the longest possible linestrings (sf::st_line_merge, the line counterpart of a dissolve): segments that meet end to end become one chain, and each chain is emitted as its own row. Where a plain union of lines returns a single multilinestring of all the parts, this joins the parts through their shared endpoints; at a crossing where more than two segments meet the merge is ambiguous and the segments stay separate. 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, then each group's segments are merged together. Peak memory is the routing budget during the pass, then one group's geometry while it is merged. With no by, the whole layer is merged at once.

Usage

spatial_line_merge(
  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 merge within: one set of maximal lines per distinct combination of their values. NULL (default) merges 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 maximal merged line, carrying the by columns and the input CRS, backed by temporary .vtr spills removed when the node is garbage-collected.

Details

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

See also

spatial_dissolve() to union geometries by group, spatial_explode() for the opposite direction (multipart to single part), collect_sf() to materialize as sf.

Examples

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

# The three end-to-end segments become one line.
tbl(f) |> spatial_line_merge() |> collect_sf()
unlink(f)