spdep (version 0.3-8)

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)

Arguments

pl

list of polygons of class polylist

row.names

character vector of region ids to be added to the neighbours list as attribute region.id, default seq(1, nrow(x)); if polys has a region.id attribute, it is copied to the neighbours list.

snap

boundary points less than snap distance apart are considered to indicate contiguity

queen

if TRUE, a single shared boundary point meets the contiguity condition, if FALSE, more than one shared point is required

Value

• A neighbours list with class nb

See Also

summary.nb, plot.polylist

Examples

data(columbus)
xx <- poly2nb(polys)
dxx <- diffnb(xx, col.gal.nb)
library(maptools)
plot(polys, border="grey", forcefill=FALSE)
plot(col.gal.nb, coords, add=TRUE)
plot(dxx, coords, add=TRUE, col="red")
title(main="Differences (red) in Columbus GAL weights (black)
and polygon generated queen weights")
xxx <- poly2nb(polys, queen=FALSE)
dxxx <- diffnb(xxx, col.gal.nb)
plot(polys, border = "grey", forcefill=FALSE)
plot(col.gal.nb, coords, add = TRUE)
plot(dxxx, coords, add = TRUE, col = "red")
title(main="Differences (red) in Columbus GAL weights (black)
and polygon generated rook weights")
cards <- card(xx)
maxconts <- which(cards == max(cards))
if(length(maxconts) > 1) maxconts <- maxconts[1]
fg <- rep("grey", length(polys))
fg[maxconts] <- "red"
fg[xx[[maxconts]]] <- "green"
plot(polys, col=fg, forcefill=FALSE)
title(main="Region with largest number of contiguities")