tri2nb: Neighbours list from tri object
In spdep: Spatial Dependence: Weighting Schemes, Statistics and Models

Description Usage Arguments Details Value Author(s) See Also Examples

The function uses the deldir package to convert a matrix of two-dimensional coordinates into a neighbours list of class nb with a list of integer vectors containing neighbour region number ids.

1	tri2nb(coords, row.names = NULL)

`coords`	matrix of point coordinates with two columns
`row.names`	character vector of region ids to be added to the neighbours list as attribute `region.id`, default `seq(1, nrow(x))`

If coordinates are duplicated, this function cannot be used. If the coordinates are from a grid, then they need to be ordered such that the first three are not collinear, so that the first triangle can be constructed. This can be achieved by randomising the order of the coordinates (possibly several times), and then re-ordering the order of the data to match the new order of the neighbour list - if this fix is used, remember to re-order the row.names argument as well as the coordinates! Please also note that triangulation of grid points will give arbitrary diagonal neighbours, which may not be a sensible outcome, and dnearneigh() may serve better where tri2nb() cannot be used.

The function returns an object of class nb with a list of integer vectors containing neighbour region number ids.

Roger Bivand Roger.Bivand@nhh.no

knn2nb, dnearneigh, cell2nb

example(columbus)
coords <- coordinates(columbus)
ind <- sapply(slot(columbus, "polygons"), function(x) slot(x, "ID"))
col.tri.nb <- tri2nb(coords, row.names=ind)
W <- as(nb2listw(col.tri.nb, style="B"), "CsparseMatrix")
plot(columbus, border="grey")
plot(col.tri.nb, coords, add=TRUE)
title(main="Raw triangulation links")
x <- seq(0,1,0.1)
y <- seq(0,2,0.2)
xy <- expand.grid(x, y)
try(xy.nb <- tri2nb(xy))
seed <- 1234
xid <- sample(1:nrow(xy))
xy.nb <- tri2nb(xy[xid,])
plot(xy.nb, xy[xid,])

Loading required package: sp
Loading required package: Matrix

colmbs> require(maptools)
Loading required package: maptools
Checking rgeos availability: TRUE

colmbs> columbus <- readShapePoly(system.file("etc/shapes/columbus.shp",
colmbs+  package="spdep")[1])

colmbs> col.gal.nb <- read.gal(system.file("etc/weights/columbus.gal",
colmbs+  package="spdep")[1])
Warning message:
use rgdal::readOGR or sf::st_read 

     PLEASE NOTE:  The components "delsgs" and "summary" of the
 object returned by deldir() are now DATA FRAMES rather than
 matrices (as they were prior to release 0.0-18).
 See help("deldir").
 
     PLEASE NOTE: The process that deldir() uses for determining
 duplicated points has changed from that used in version
 0.0-9 of this package (and previously). See help("deldir").