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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