[R-sig-Geo] Extracting regions with no neighbors from Map2poly object

[Roger Bivand]

On Thu, 23 Jun 2005 Alexander.Herr at csiro.au wrote:

> Hi List,
> 
> I am trying to delete polygons with no neighbors in a Map2poly object. 
> 
> Region.id exists, however, I cannot find a way to identify/delete the
> list of polygons (from poly2nb()) with no neighbors. I have trouble
> subsetting the Map2poly object, so I can create a new object without
> these polygons.
> 

I'll try to replicate:

> library(spdep)
> try3 <- read.shape(system.file("shapes/sids.shp", package="maptools")[1])
> mappolys <- Map2poly(try3, as.character(try3$att.data$FIPSNO))
> cents <- do.call("rbind", lapply(lapply(mappolys, function(x) attr(x, 
+   "centroid")), unlist))
> str(cents)
 num [1:100, 1:2] -81.5 -81.1 -80.7 -76.1 -77.4 ...
 - attr(*, "dimnames")=List of 2
  ..$ : NULL
  ..$ : chr [1:2] "x" "y"
> nb <- dnearneigh(cents, 0, 0.3)
> nb
Neighbour list object:
Number of regions: 100 
Number of nonzero links: 78 
Percentage nonzero weights: 0.78 
Average number of links: 0.78 
44 regions with no links:
6 8 10 11 12 14 24 33 37 38 39 42 44 46 51 54 55 56 57 58 62 63 64 66 67 
68 70 71 75 78 79 80 82 84 85 86 88 89 92 94 95 96 98 100
> table(card(nb))

 0  1  2  3 
44 36 18  2 
> plot(mappolys)
> plot(nb, cents, add=TRUE, lwd=3, col="blue")

This generates a neighbour list with lots of no-neighbour counties, so 
subsetting is just a matter of dropping the counties with zero neighbour 
cardinality (subset= are the ones to retain):

> sub_mappolys <- subset(mappolys, subset=card(nb) > 0)
> sub_nb <- subset(nb, subset=card(nb) > 0)
> sub_cents <- subset(cents, subset=card(nb) > 0)
> sub_nb
Neighbour list object:
Number of regions: 56 
Number of nonzero links: 78 
Percentage nonzero weights: 2.487245 
Average number of links: 1.392857 
> plot(mappolys, col="lightgreen")
> plot(sub_mappolys, col="white", add=TRUE)
> plot(sub_nb, sub_cents, add=TRUE, lwd=3, col="blue")

Of course, this is just for this data set, and maybe there is something 
different about yours.

Hope this helps,

Roger


> Any help appreciated
> 
> Thanks HErry
> 
> --------------------------------------------
> Alexander Herr - Herry
> Northern Futures
> Davies Laboratory, CSIRO
> PMB, Aitkenvale, QLD 4814
> Phone (07) 4753 8510
> Fax   (07) 4753 8650
> Home: http://herry.ausbats.org.au
> Webadmin ABS: http://ausbats.org.au
> Sustainable Ecosystems: http://www.cse.csiro.au/
> --------------------------------------------
>  
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no