The length of common boundaries
On Tue, Oct 11, 2005 at 03:50:05PM +0200, Roger Bivand wrote:
On Tue, 11 Oct 2005, Yilin Liu wrote:
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.
In GRASS 6 you can use v.to.db [1] to generate this information (should be the combination of the options 'sides' and 'length'. Markus [1] http://grass.itc.it/grass60/manuals/html60_user/v.to.db.html
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