Merge sliver polygons into a neighbour

Cleans a polygon coverage by absorbing every feature whose area is below max_area into an adjacent feature (the QGIS "Eliminate Selected Polygons"): the sliver removal a per-feature transform cannot do, because the target a sliver merges into is one of its neighbours, not the sliver itself. Each small feature is joined to the neighbour it shares the longest border with (or the largest-area neighbour, with into = "largest_area"); chains of slivers collapse so a connected run of small features flows to its single largest member, whose attribute row survives. A small feature with no neighbour is kept unchanged, so nothing vanishes. 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 cleaned as an independent coverage. Peak memory is the routing budget during the pass, then one group's geometry while its slivers are merged – partition on a key whose groups fit in memory. With no by, the whole layer is one coverage.

Usage

spatial_eliminate(
  x,
  max_area,
  by = NULL,
  into = c("longest_border", "largest_area"),
  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.

max_area

Area threshold in CRS units squared: a feature smaller than this is a sliver and is merged into a neighbour. Larger values absorb more.

by

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

into

How to pick the neighbour a sliver merges into: "longest_border" (default) the neighbour sharing the longest boundary, or "largest_area" the neighbour with the greatest area.

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 the cleaned coverage – one row per surviving feature, each carrying its (largest member's) attributes and the input CRS, backed by temporary .vtr spills removed when the node is garbage-collected.

Details

Adjacency and shared-border length are sf/GEOS (sf::st_intersects, sf::st_boundary, sf::st_intersection) and expect projected or unprojected planar data; max_area is in CRS units squared. 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 merge geometries by attribute, spatial_simplify() for coverage-preserving simplification, spatial_topology() for the shared-edge adjacency, collect_sf() to materialize as sf.

Examples

big    <- sf::st_polygon(list(rbind(
  c(0, 0), c(10, 0), c(10, 10), c(0, 10), c(0, 0))))
sliver <- sf::st_polygon(list(rbind(
  c(10, 0), c(10.3, 0), c(10.3, 10), c(10, 10), c(10, 0))))
f <- tempfile(fileext = ".vtr")
write_vtr(data.frame(
  id = c("keep", "sliver"),
  geometry = sf::st_as_binary(sf::st_sfc(big, sliver), hex = TRUE)
), f)

# The thin sliver is absorbed into the square it borders.
tbl(f) |> spatial_eliminate(max_area = 5) |> collect_sf()
unlink(f)