All zero link when reading a modified shapefile into R

Hi all-

When I read a shapefile using read.shape into R, and used Map2poly ->
poly2nb to create my neighbor list, the summary of the newly created
neighbor list indicates that all the regions have zero link.

The shapefile is a modified census tract shapefile of New Jersey: a few
tracts that have no useful attributal information were deleted from the
map (hence left some holes) in ArcGIS （Start Edit -> select ->
delete).

The original shapefile (with all the tracts) was read fine and create
the correct neighbor list. But when using this modified one, I got all
zero link. The same happened when I used GeoDa. Is this because of the
holes created in the shapefile by deleting some polygons? If so, how can
I make it work? If not, can anyone please direct me to the right
solution?

Many thanks.

Best,
Danlin

On Thu, 31 May 2007, Danlin Yu wrote:

 

Hi all-

When I read a shapefile using read.shape into R, and used Map2poly ->
poly2nb to create my neighbor list, the summary of the newly created
neighbor list indicates that all the regions have zero link.

The shapefile is a modified census tract shapefile of New Jersey: a few
tracts that have no useful attributal information were deleted from the
map (hence left some holes) in ArcGIS （Start Edit -> select ->
delete).

The original shapefile (with all the tracts) was read fine and create
the correct neighbor list. But when using this modified one, I got all
zero link. The same happened when I used GeoDa. Is this because of the
holes created in the shapefile by deleting some polygons? If so, how can
I make it work? If not, can anyone please direct me to the right
solution?
   

Could you try with just one deleted polygon first? Is it possible that 
ArcGIS does some cleaning at the same time, and moves the shapes apart? 
If that is the case, might increasing the snap= argument in poly2nb() 
help?

Incidentally, poly2nb() does take an object extending SpatialPolygons too, 
so you can also do:

x <- readShapePoly("x.shp")
plot(x)
x_nb <- poly2nb(x) # maybe increase snap=

avoiding using Map and polylist objects. Then

plot(x, border="grey")
plot(x_nb, coordinates(x), pch=".")

plots the neighbour graph on the tract boundaries.

In the last resort, do the deletion in R:

x <- readShapePoly("x.shp")
keep <- x$attribute is useful, logical condition
table(keep)
plot(x, col=c("red", "green")[(keep+1)])
x_subset <- x[keep,]
writePolyShape(x_subset, fn="x_subset")

or the equivalent readOGR() and writeOGR() functions in rgdal.

Hope this helps,

Roger

 

Many thanks.

Best,
Danlin