[R-sig-Geo] (no subject)

[Steven J. Pierce]

Hi folks,

After using spdep to construct a neighbor list from a SpatialPolygons
object, I plotted the neighbor list and noted that I needed one more link
that wasn't caught by the poly2nb command. I found the edit.nb() function
and can successfully use the interactive graphical interface to add the
link, but would like to know if there is an alternative method for adding a
link between two regions. What would I need to do to directly edit the
neighbor list object to include an additional link between regions 23 and
31?


> # Create a SpatialPolygons object from a SpatialPolygonsDataFrame object,
this
> # helps set up for creating a neighbor list via commands from spdep
package
> 
> CITY.polys <- as.SpatialPolygons.PolygonsList(CITY.ZCTA.data at polygons, 
+
proj4string=CRS(projection.details))
> 
> # Create a neighbor list based on shared boundary. We need this to
facilitate
> # creating a weights matrix when we go to do spatial regression models.
> 
> CITY.nb <- poly2nb(CITY.polys, row.names = NULL,
snap=sqrt(.Machine$double.eps), queen=TRUE)
> 
> # Capture coordinates of ZCTA centroids into a matrix we can use in
plotting
> # the neighbor list.
> 
> coords.mat <- cbind(CITY.ZCTA.data$centroid.x, CITY.ZCTA.data$centroid.y)
> 
> # Plot the neighbor list (reveals we need to add 1 more link)
> 
> plot(CITY.ZCTA.data, col="grey", border="black", axes=FALSE, asp=1,
yaxs="i")
> title(main="Neighbor List Links", adj= 0, cex.sub=.75)
> plot(CITY.nb, coords.mat, add=TRUE, col="black", lwd=2)
> 
> # Add 1 link to neighbor list. This requires interaction with the graphics
> # on screen.
> 
> CITY.nb <- edit.nb(CITY.nb, coords.mat, polys=CITY.polys)
Identifying contiguity for deletion ...
No contiguity between chosen points
Add contiguity? (y/n) y
added contiguity between point 23 and 31 
Options: quit[q] refresh[r] continue[c] q
> 
> # Plot the revised neighbor list (now everything looks right)
> 
> plot(CITY.ZCTA.data, col="grey", border="black", axes=FALSE, asp=1,
yaxs="i")
> title(main="Neighbor List Links", adj= 0, cex.sub=.75)
> plot(CITY.nb, coords.mat, add=TRUE, col="black", lwd=2)


Steven J. Pierce
E-mail: pierces1 at msu.edu