[R-sig-Geo] gwt2nb - regions with zero links

[Roger Bivand]

On Mon, 11 Oct 2010, Dennis Guignet wrote:

> Dear All,
>
> My end goal is to run a spatial lag model, where the weight matrix will 
> be nonsymmetric and contain observations with zero neighbors.  I am 
> having some difficulties in creating the neighbor list in R.  Any advice 
> or pointers on where to find more information would be greatly 
> appreciated.

Maybe try some other format, such as the MATLAB-style sparse matrix in 
read.dat2listw(). However then you cannot use your codes, just the 
sequence 1:n. If this doesn't help, put the file(s) on a website and post 
the link - debugging without the input data is hard. I can't follow how a 
region with only zero links has non-zero links (below).

Roger

>
> I needed to account for both time and space when defining a list of 
> neighbors, and did so first using ArcInfo and then SAS, and saved the 
> final list as a dbf.  It seems I next want to convert this dbf into a 
> neighbor list in R, and was told the best thing to do is save the dbf as 
> a gwt file and change the header line.  A brief portion is pasted below:
>
> 0 34451 nfred3 DGID
> 105450   105578   1
> 105451   105763   1
> 105451   105627   1
> 105451   105899   1
> 105451   105898   1
> 105451   105915   1
> 105452   105763   1
> 105452   105841   1
>
> nfred3 is the name of the gwt file, and DGID is the unique id for each 
> observation.  There are a total of 34,451 unique observations, some of 
> which do not have any neighbors and are thus not included in the above 
> gwt file.  If I run gwt2nb accounting for these zero link observations I 
> get the following warning messages:
>
>> DGID<-sales$dgid
>> length(DGID)
> [1] 34451
>> n2n<-read.gwt2nb("nfred3.gwt", region.id=DGID)
> Warning messages:
> 1: In read.gwt2nb("nfred3.gwt", region.id = DGID) :
>  region.id not named DGID
> 2: In read.gwt2nb("nfred3.gwt", region.id = DGID) :
>  105462, 105463, 105466, 105468, 105472, ...  [... truncated]
> 3: In read.gwt2nb("nfred3.gwt", region.id = DGID) :
>  105450, 105454, 105455, 105462, 105468, ... [... truncated]
>
> The issue is that there are a total of 1,555 regions with no links, but 
> 1,090 of these regions should have a positive number of links .  The 
> truncated list in the warning message contains id numbers that should 
> have zero links, as well as some that should have nonzero links.
>
> If I run similar code but only account for observations with nonzero 
> links, then I have the correct number of nonzero links. So this neighbor 
> list at first glance seems correct, but I still need to account for 
> observations with zero links when later forming the weights, and I am 
> concerned about the below warning messages.
>
>> #create vector of unique sale ids with nonzero spatial link (i.e. has neighbors)
>> n2n0<-read.table("nfred4.gwt",header=FALSE,skip=1)
>> DGID<-unique(n2n0$V1)
>> length(DGID)
> [1] 33986
>> n2n<-read.gwt2nb("nfred4.gwt", region.id=DGID)
> Warning messages:
> 1: In read.gwt2nb("nfred4.gwt", region.id = DGID) :
>  region.id not named DGID
> 2: In read.gwt2nb("nfred4.gwt", region.id = DGID) :
>  105450, 105454, 105455, 105470, ... [... truncated]
>
> Any advice or pointers of where I can find some helpful info would be 
> greatly appreciated.
>
> Thank you,
> Denny
>
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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