How to impute missing neighbours of a spatial weight matrix (queen contiguity)

Question

I have a big shape file with approx 180.000 250m^2 polygons. I want to create a spatial weight matrix (queen contiguity). So a 1 if its a neighbour, 0 otherwise. But, there are several polygons without any neighbours (islands).

How can I impute the nearest neighbour for those units that do not have any direct neighbour?

(working with sf or sp package - in R)

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using a sample of the data and plotting it, it would look like:

sample shape file

I can then create a W-Matrix with the following code:

Create NB List
sample_queen_nb <- poly2nb(sp.sample, row.names=sp.sample$ID, queen=T)

# Create Weights/list
sample_W.list.queen <- nb2listw(sample_queen_nb, style="W", zero.policy = TRUE) 

# Create Matrix
sample_W.queen <- listw2mat(sample_W.list.queen)

and check its connectivity with plotting it:

# Plot ShpPolygons
plot(sp.sample)
# plot weightsmatrix to see connectivity
plot(sample_queen_nb, coords, add=TRUE, col="green", cex=0.5) 

getting this:

Connectivity Matrix

As you can see - there is an island that's not connected. I would like to impute the neared neighbouring unit for this island-grid-cell. How can I do that?

Answer 1

This is what I meant by make a sample data set:

> library(spdep);example(columbus)
> sp.sample = columbus[c(1:5,10,23),]
> plot(sp.sample)

First lets check we have disconnected regions:

> coords = coordinates(sp.sample)
> queen_nb = poly2nb(sp.sample, row.names=sp.sample$ID, queen=TRUE)
> plot(sp.sample);plot(queen_nb, coords, add=TRUE)

And here's a function that takes a set of region, computes the same adjacency, and then if there's any regions with no neighbours it finds the nearest neighbour and adds it to the adjacency object:

addnbs <- function(sp.sample){

    queen_nb <- poly2nb(sp.sample, row.names=sp.sample$ID, queen=TRUE)

    count = card(queen_nb)
    if(!any(count==0)){
        return(queen_nb)
    }

    ## get nearest neighbour index, use centroids:
    nnbs = knearneigh(coordinates(sp.sample))$nn

    no_edges_from = which(count==0)
    for(i in no_edges_from){
        queen_nb[[i]] = nnbs[i]
    }
    return(queen_nb)
}

Let's test that.

> n2 = addnbs(sp.sample)
> plot(sp.sample);plot(n2, coords, add=TRUE)