The length of common boundaries

Dear all,

Any way to get the length of common boundaries shared by neighbouring
counties in a map in R?

The short answer is no. The R polygon objects do not have topology. 

The longer answer is that if you have e00 or ArcInfo binary vector layers,
then the polygons are built from lists of directed arcs, and the arcs have
lengths. So if your input data are in this format, it is feasible but is 
not implemented.

It looks as though the GRASS6 vector format also provides similar
information. But topology is a GIS thing rather than an R thing, so no
solution within R is available.

If you use GRASS, it would be possible to look at how this might be done
(read a shapefile into GRASS, output an ASCII dump of the arcs, which
polygons they separate, and how long they are, and do something with this,
not losing trace of which polygon(s)  belong to which observation - this
was where progress stopped the last time I looked). Arc lengths would be
nice for cartograms too.

Roger

Yilin

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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