I appreciate the "simple" explanation, which does make since.  I am  
still unable, however, to access the "coords" matrices inside of the  
SpatialPolyons object.  I'm sure that there is a simple way to do  
this, but I can't figure it out.  If I open a shapefile using  
readOGR() what code to I use to access the node coordinates?  Thanks  
again for your help.

Jon Thayn


On Oct 13, 2008, at 2:33 PM, Roger Bivand wrote:

> On Mon, 13 Oct 2008, Jonathan Boyd Thayn wrote:
>
>> I want to know the length of the shared border of neighboring  
>> polygons
>> in a shapefile.  I can use the slots () function to access the the
>> "polyons" slot of the file, but I can't seem to create a list of the
>> node coordinates of the polyon.  Is there a way to do this easily?
>
> I imagine that you mean the end nodes of arcs with known left and  
> right neighbours? The actual ring coordinates are simply the  
> "coords" slots of each "Polygon" object in the list in the  
> "Polygons" slot of each "Polygons" object in the list in the  
> "polygons" slot of the SpatialPolygons object (for a very special  
> definition of "simple"). But they wouldn't help much, because you  
> need to build the topology first. If you have access to ArcGIS, you  
> can push the polygons out for Arc as a shapefile, convert them into  
> a coverage (if they build cleanly) and get the coverage back into R  
> using readOGR as both polygons and arcs:
>
> http://www.asdar-book.org/exercises.php?excode=3
>
> and link therein. Getting the lengths from the arcs could use  
> SpatialLinesLengths() in sp, and add the values to the  
> SpatialLinesDataFrame; this function assumes planar coordinates at  
> present. The coverage2nb() would need gentle editing to return a  
> list of lengths in the same format as the nb object.
>
> If GRASS is OK, look at vect2neigh() in the spgrass6 package by  
> Markus Neteler - it does what you want if the topology builds  
> cleanly, and possibly not on all platforms.
>
> Hope this helps,
>
> Roger
>
>
>> Thanks.
>>
>>
>> Jonathan B. Thayn
>> Ph.D. Candidate – Geography and Remote Sensing
>> University of Kansas
>> 2101 Constant Avenue, Higuchi Hall, Rm 121
>> Lawrence, Kansas 66047-3759
>> jonthayn@ku.edu
>> http://people.ku.edu/~jonthayn
>>
>>
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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@nhh.no


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