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Dissolve geometries by group

Source: R/spatial.R
spatial_dissolve.Rd

Unions the geometries within each by group into a single feature (the GIS "Dissolve" tool), optionally summarising attributes. Unlike the streamed per-batch verbs, dissolve needs every geometry of a group together to union them, so it rides the partition tier: x is spilled once and routed into one disjoint shard per group in a single bounded pass, then each shard is read in and unioned with sf. Peak memory is the routing budget during the pass, then one group's geometries while it is unioned – partition the input on a key whose groups fit in memory. With no by, the whole layer dissolves into one feature.

Usage

spatial_dissolve(
  x,
  by = NULL,
  ...,
  geom = "geometry",
  crs = NA,
  .fun = NULL,
  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 dissolve within: one output feature per distinct combination of their values. NULL (default) dissolves the entire layer into a single feature.

...

Further arguments passed to sf::st_union() (e.g. is_coverage = TRUE).

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.

.fun

Optional named list of attribute summaries. Each element is a function taking the group's data.frame and returning a length-1 value; the list name becomes the output column (e.g. .fun = list(total = function(d) sum(d$pop))). Default NULL keeps only the by columns and the dissolved geometry.

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 group – the by columns, any .fun summaries, and the dissolved geometry – backed by temporary .vtr spills removed when the node is garbage-collected, carrying the input CRS for collect_sf().

Details

Geometry travels through the engine as hex-encoded WKB in a string column and the CRS is carried on the returned node; use collect_sf() to materialize. On projected or unprojected planar data each group is unioned natively on the GEOS C API straight off the hex-WKB column; geographic coordinates with spherical geometry on (sf::sf_use_s2()), or any extra sf::st_union() arguments (e.g. is_coverage = TRUE), union through sf instead. The sf package is an optional dependency (Suggests).

See also

spatial_overlay() to split overlaps apart rather than merge them, offload() for the partition tier this rides on, collect_sf().

Examples

nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)
nc$band <- nc$SID74 > 5            # an attribute to dissolve within
f <- tempfile(fileext = ".vtr")
write_vtr(data.frame(
  band = nc$band, BIR74 = nc$BIR74,
  geometry = sf::st_as_binary(sf::st_geometry(nc), hex = TRUE)
), f)

# Merge the counties into two features by `band`, summing births.
merged <- tbl(f) |>
  spatial_dissolve(by = "band", crs = sf::st_crs(nc),
                   .fun = list(births = function(d) sum(d$BIR74)))
collect_sf(merged)
unlink(f)