nb object in spdep from categorical attribute

Dear list,

Does anyone know how to make nb objects (to eventually make spatial
weights) with spdep using an attribute in a shapefile's data frame?

Consider, for example, all property parcels on the same street segment
should be defined as neighbors. In the parcel polygons I have the
associated street segment name.

KNN forces sparsely settled areas to become neighbors, which is undesirable
for the subsequent analyses.

Distances between parcels vary, so distance-based neighbors are also
undesirable in this case.

Contiguity-based methods do not connect parcels on opposite sites of the
same street segment.

Is there a way to use an attribute to define what constitutes a neighbor?

Yes, spdep::nb2blocknb() is a specific case where say observations without 
positional data are known to belong to an aggregate entity for which we 
have positional data. The output is an nb object with diagonal blocks (if 
the observations are sorted by block ID, not in the case below), as this 
example shows. You'll need the development version to run it, as released 
spdep::aggregate.nb() failed with empty aggregate sets - the nc median 
poish grid has such empty sets:

# devtools::install.github("r-spatial/spdep")
library(spdep)
data(nc.sids)
ncCC89.nb[sapply(ncCC89.nb, length) == 0L] <- 0L
image(as(nb2listw(ncCC89.nb, zero.policy=TRUE), "CsparseMatrix"))
anb <- aggregate(ncCC89.nb, interaction(nc.sids$M.id, nc.sids$L.id))
image(as(nb2listw(anb, style="B"), "CsparseMatrix"))
# when the neighbour object is not empty
bnb <- nb2blocknb(anb, as.character(interaction(nc.sids$M.id,
   nc.sids$L.id)))
image(as(nb2listw(bnb, style="B"), "CsparseMatrix"))
# and when the neighbour object is empty
anb1 <- lapply(anb, function(x) x <- 0L)
attributes(anb1) <- attributes(anb)
bnb1 <- nb2blocknb(anb1, as.character(interaction(nc.sids$M.id,
   nc.sids$L.id)))
image(as(nb2listw(bnb1, style="B"), "CsparseMatrix"))

This isn't your case, but crafting an nb object by hand (adding 
attributes after creating the list) then using spdep::union.nb or similar 
to combine with a positional nb (such as poly2nb) may work. For parcels 
across a stream or street, maybe look at the snap= argument if the streets 
are narrower than frontages. Could you provide a small reproducible 
example?

Hope this clarifies,

Roger

All ideas welcome,
Dexter

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Roger Bivand
Department of Economics, Norwegian School of Economics,
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