Simplify a polygon coverage without tearing shared edges

Simplifies polygon boundaries while keeping a shared border between two polygons identical on both sides, so adjacent polygons stay edge-matched with no slivers or gaps – the topology-preserving simplification that a per-feature spatial_map(~ sf::st_simplify(.x)) cannot give, because it simplifies each polygon's copy of a shared border independently. The boundaries of each group are unioned so a shared border is one line, noded into arcs at every junction, each arc simplified once (its junction endpoints pinned), and the arcs re-polygonized; each resulting face inherits the attributes of the source polygon containing it. 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 simplified as an independent coverage. Peak memory is the routing budget during the pass, then one group's geometry while it is simplified – partition on a key whose groups fit in memory. With no by, the whole layer is one coverage.

Usage

spatial_simplify(
  x,
  tolerance,
  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.

tolerance

Distance tolerance for the boundary simplification, in CRS units: vertices that deviate less than this from the simplified line are dropped. Larger values simplify more.

by

Character vector of attribute columns whose groups are each simplified 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.

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 simplified polygons, each carrying its source feature's attributes and the input CRS, backed by temporary .vtr spills removed when the node is garbage-collected.

Details

The simplification is topology-preserving Douglas-Peucker (dTolerance = tolerance) on the noded boundary arcs. 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_map() with ~ sf::st_simplify(.x) for independent per-feature simplification, spatial_smooth() for Chaikin corner-rounding, collect_sf() to materialize as sf.

Examples

p1 <- sf::st_polygon(list(rbind(
  c(0, 0), c(1, 0), c(1, 0.5), 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, 0.5), 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 is simplified once, so the two polygons stay edge-matched.
tbl(f) |> spatial_simplify(tolerance = 0.6) |> collect_sf()
unlink(f)