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k nearest neighbours of a streamed layer, with distances

Source: R/spatial.R
spatial_knn.Rd

Streams a large left side x through the engine and, for each feature, finds the k nearest features of a small resident layer y, returning one row per (left, neighbour) pair with the neighbour's rank, identifier, and distance. Where spatial_join() with sf::st_nearest_feature attaches only the single nearest match, this returns the top k and the distances themselves – the nearest-k query and the building block of a distance matrix. The billion-row left stream never materializes; y (the candidate neighbours) stays resident.

Usage

spatial_knn(
  x,
  y,
  k = 1L,
  geom = "geometry",
  coords = NULL,
  crs = NA,
  y_id = NULL,
  id_col = "neighbor",
  dist_col = "distance",
  rank_col = "rank",
  out_geom = NULL,
  flush_rows = NULL
)

Arguments

x

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

y

An sf or sfc object: the resident candidate-neighbour layer.

k

Number of nearest neighbours to return per left feature (capped at the number of y features). Default 1.

geom

Name of the input geometry column holding hex-WKB or WKT strings. Default "geometry". Ignored when coords is given.

coords

Optional length-2 character vector naming the x and y coordinate columns to assemble point geometry from (e.g. c("x", "y")), for inputs such as tiff_extract_points() output. The coordinate columns are retained.

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.

y_id

Optional name of a column in y whose value identifies each neighbour in the output. Default NULL uses y's 1-based row index.

id_col, dist_col, rank_col

Names of the output columns holding the neighbour identifier, the distance, and the 1-based rank (1 = nearest). Defaults "neighbor", "distance", "rank".

out_geom

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

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 (left, neighbour) pair – x's columns (geometry included) plus the rank, neighbour identifier, and distance – backed by temporary .vtr spills (removed when the node is garbage- collected) and carrying the input CRS.

Details

Distances are sf's sf::st_distance(): planar (CRS units) on projected or unprojected planar data, great-circle (metres) on geographic coordinates with spherical geometry on (sf::sf_use_s2()). Each batch forms its left-by-y distance matrix, so y should be the small side; when y carries no CRS it inherits the stream's. The left geometry rides through unchanged (replicated once per neighbour). The sf package is an optional dependency (Suggests).

See also

spatial_join() for a nearest-feature attribute join, collect_sf() to materialize as sf.

Examples

nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)
towns <- sf::st_centroid(sf::st_geometry(nc))[1:5]
towns <- sf::st_sf(town = nc$NAME[1:5], geometry = towns)

set.seed(1)
pts <- sf::st_coordinates(sf::st_sample(nc, 100))
f <- tempfile(fileext = ".vtr")
write_vtr(data.frame(id = seq_len(nrow(pts)), x = pts[, 1], y = pts[, 2]), f)

# The two nearest towns to each point, with distances.
tbl(f) |>
  spatial_knn(towns, k = 2, coords = c("x", "y"), crs = sf::st_crs(nc),
              y_id = "town") |>
  collect() |> head()
unlink(f)