poly2nb: Construct neighbours list from polygon list

Description

The function builds a neighbours list based on regions with contiguous boundaries, that is sharing one or more boundary point. The current function is in part interpreted and may run slowly for many regions or detailed boundaries, but from 0.2-16 should not fail because of lack of memory when single polygons are built of very many border coordinates.

Usage

poly2nb(pl, row.names = NULL, snap=sqrt(.Machine$double.eps),
 queen=TRUE, useC=TRUE, foundInBox=NULL)

Value

A neighbours list with class nb. See card for details of “nb” objects.

Arguments

pl: list of polygons of class extending SpatialPolygons, or an sf or sfc object containing non-empty (multi-)polygon objects
row.names: character vector of region ids to be added to the neighbours list as attribute region.id, default seq(1, nrow(x)); if pl has row.names, they are used instead of the default sequence.
snap: boundary points less than snap distance apart are considered to indicate contiguity; used both to find candidate and actual neighbours for planar geometries, but only actual neighbours for spherical geometries, as spherical spatial indexing itself injects some fuzzyness.
queen: if TRUE, a single shared boundary point meets the contiguity condition, if FALSE, more than one shared point is required; note that more than one shared boundary point does not necessarily mean a shared boundary line
useC: default TRUE, doing the work loop in C, may be set to false to revert to R code calling two C functions in an n*k work loop, where k is the average number of candidate neighbours
foundInBox: default NULL using R code or st_intersects() to generate candidate neighbours (using snap= if the geometries are not spherical); if not NULL (for legacy purposes) a list of length (n-1) with integer vectors of candidate neighbours (j > i) (as created by the poly_findInBoxGEOS function in rgeos for clean polygons)

Author

Roger Bivand Roger.Bivand@nhh.no with contributions from Micah Altman

Examples

Run this code

columbus <- st_read(system.file("shapes/columbus.shp", package="spData")[1], quiet=TRUE)
col.gal.nb <- read.gal(system.file("weights/columbus.gal", package="spData")[1])
coords <- st_coordinates(st_centroid(st_geometry(columbus)))
xx <- poly2nb(as(columbus, "Spatial"))
dxx <- diffnb(xx, col.gal.nb)
plot(st_geometry(columbus), border="grey")
plot(col.gal.nb, coords, add=TRUE)
plot(dxx, coords, add=TRUE, col="red")
title(main=paste("Differences (red) in Columbus GAL weights (black)",
 "and polygon generated queen weights", sep="\n"), cex.main=0.6)
# poly2nb with sf sfc_MULTIPOLYGON objects
sf_xx <- poly2nb(columbus)
diffnb(sf_xx, xx)
sfc_xx <- poly2nb(st_geometry(columbus))
diffnb(sfc_xx, xx)
xxx <- poly2nb(as(columbus, "Spatial"), queen=FALSE)
dxxx <- diffnb(xxx, col.gal.nb)
plot(st_geometry(columbus), border = "grey")
plot(col.gal.nb, coords, add = TRUE)
plot(dxxx, coords, add = TRUE, col = "red")
title(main=paste("Differences (red) in Columbus GAL weights (black)",
 "and polygon generated rook weights", sep="\n"), cex.main=0.6)
cards <- card(xx)
maxconts <- which(cards == max(cards))
if(length(maxconts) > 1) maxconts <- maxconts[1]
fg <- rep("grey", length(cards))
fg[maxconts] <- "red"
fg[xx[[maxconts]]] <- "green"
plot(st_geometry(columbus), col=fg)
title(main="Region with largest number of contiguities", cex.main=0.6)
nc.sids <- st_read(system.file("shapes/sids.shp", package="spData")[1], quiet=TRUE)
system.time(xxnb <- poly2nb(nc.sids))
system.time(xxnb <- poly2nb(as(nc.sids, "Spatial")))
plot(st_geometry(nc.sids))
plot(xxnb, coordinates(as(nc.sids, "Spatial")), add=TRUE, col="blue")
sq <- st_polygon(list(rbind(c(0,0), c(1,0), c(1,1), c(0,1), c(0,0))))
sq2 <- sq + c(0,1)
sq3 <- sq + c(1,0)
sq4 <- sq + c(1,1)
gm <- st_sfc(list(sq, sq2, sq3, sq4))
df <- st_as_sf(gm, id=1:4)
plot(st_geometry(df))
text(st_coordinates(st_centroid(gm)), as.character(df$id))
unclass(poly2nb(df, queen = FALSE))