Errors while creating a neighbourhood matrix using poly2nb

Thanks Roger,
Changing the snap distance already did the trick. I didn't know Arcmap was
so inaccurate!
Oscar



                                                                           
             Roger Bivand                                                  
             <Roger.Bivand at nhh                                             
             .no>                                                       To 
             Sent by:                  Oscar Breugelmans                   
             r-sig-geo-bounces         <Oscar.Breugelmans at rivm.nl>         
             @stat.math.ethz.c                                          cc 
             h                         r-sig-geo at stat.math.ethz.ch         
                                                                   Subject 
                                       Re: [R-sig-Geo] Errors while        
             25-03-2008 10:29          creating a neighbourhood matrix     
                                       using poly2nb                       
                                                                           
             Please respond to                                             
             Roger.Bivand at nhh.                                             
                    no

Hi everybody,

I am trying to create a neigbourhood matrix based on a shapefile of

postal

code areas that was created in ArcMap. Normally this works fine using the
'readShapePoly' and 'poly2nb' commands. However, the postal code areas
change over the years or are made up of 2 or mory polygons because they

are

situated on both sides of a large waterway. I correct these difficulties

by

merging postal code areas in ArcMap, so that I end up with a shapefile in
which each postal code area is represented by just 1 polygon with an

unique

area ID.
I again import the updated shapefile in R using readShapePoly. But then

the

problems start. When I create a neighbourhood matrix using poly2nb, the
areas that have been merged behave very strange: they don't recognize

their

neighbours. Sometimes they recognize none, sometimes a few.
When I couldn't find a solution I tried to create the nb using GeoDa.

This

gave exactly the same problems. Then I turned to GeoBugs and - surprise -
the nb was created without any problems.

Almost certainly a topologically unclean set of boundaries. If you do the
merging in R using unionSpatialPolygons() in maptools, the output
SpatialPolygons ought to be OK. Use the post codes as the ID union key,
and they will be used beith as the Polygons objects IDs, and the region.id
values of the neighbour list. You could also try increasing the snap
distance in poly2nb(), as this may relieve problems of remaining slivers.

Hope this helps,

Roger