gwt2nb - regions with zero links

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.

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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