[R-sig-Geo] problem with edit.nb

[Roger Bivand]

On Thu, 26 Aug 2010, Michał Kwieciński wrote:

> 2010/8/26 Roger Bivand <Roger.Bivand at nhh.no>:
>> On Thu, 26 Aug 2010, Michał Kwieciński wrote:
>>
>>> Hi all,
>>>
>>> I am just about to finish my thesis. The spatial model I want to use
>>> there is an extension of some work I did back in April. I used R 2.9.2
>>> then and in order to include 3 additional administrative areas for
>>> Poland, I edited the shp files (the borders aren't perfectly aligned).
>>> Then in R I created the nb class object and edited it with edit.nb
>>> adding three new connections. Everything worked perfect, I had no
>>> regions with no links and I generated weight matrices with no
>>> problems.
>>>
>>> However, I'd been doing exactly the same thing entire night in R 2.11
>>> and it did not work (I use the same code I did 4 months ago) and I
>>> have no idea what is the reason for it. I've been looking for some
>>> other way to do it, I tried nb2mat and editing the matrix, but I
>>> surrendered having no idea where and what values I should use.
>>>
>>> Before editing nb object R claims that regions 377 and 378 have no
>>> links. However in edit.nb the 378 and 379 are visible as having no
>>> links (378 and 379 are cities added on top of bigger shapes, whereas
>>> 377 was just split from a bigger shape into two smaller ones and only
>>> the link between these two parts is missing). I connect the circles,
>>> quit and in the new object there are some new links - the overall
>>> number has increased - but 377 and 378 are still listed as having no
>>> links. Editing nb again shows the links, so they have been saved for
>>> sure.
>>>
>>> I am not an advanced R user and most of my code was based on my
>>> professor's book. However, I think I have spent enough time with
>>> spatial models and those matrices in order to call this problem really
>>> weird. Especially since it worked perfectly last time...
>>>
>>> I can attach shp files and my code if it will be of any help in order
>>> to properly investigate this problem. I would really appreciate some
>>> help, I need to finish the project over the weekend.
>>
>> Maybe you are using the wrong indices, as FIDs are 0-base but nb objects are
>> 1-base. So you may be editing the wrong ones. If this doesn't resolve the
>> problem, zip the shapefile and post a link to it, don't attach the
>> shapefile, as it would be sent to 1700 people.
>
>
> I must admit I did not understand your hint (I do not know what "base"
> is, assuming FID is Field ID - header in shp file). How is it possible
> I edited some other layer of information by function edit.nb? Could
> you please clarify what should I do to check it?

Google "0-based" gets you to Wikipedia:

"0 (zero-based indexing)
     The first element of the array is indexed by subscript of 0.
1 (one-based indexing)
     The first element of the array is indexed by subscript of 1."

So the FIDs in the shapefile are 0, ..., (n-1), and identify the 
observations, so are set in the region.id attribute of the nb object. Then 
if print(nb) says that "377" and "378" have no neighbours, and the 
region.id values are from the shapefile:

which(card(nb) == 0)

will likely say 378 379, and

attr(nb, "region.id")[which(card(nb) == 0)]

will say "377" "378".

The indices used internally in edit.nb are the 1-based indices. They 
probably should be the ones stored in the region.id attribute, but this 
would involve an extra level of indexing. If you don't understand, put the 
shapefile on a website and post the link.

Roger

>
> Thank you for prompt reply,
> Michal
>

-- 
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