[R-sig-Geo] Detect islands (of more than 1 areas or polygons) into a cartography

[Roger Bivand]

On Fri, 30 Dec 2011, Marc Marí Dell'Olmo wrote:

> Dear all,
>
> Anyone knows a function to detect if there are islands into a
> cartography. I refer to islands with more than one area (or polygons).
> In other words, I would like to detect how many connected components I
> have in the graph.
>
> On the other hand, I would like to know the “id” of the areas (or
> polygons) for each island.
>

This wasn't easy to grasp. By graph, do you mean the neighbour graph of 
the Polygons objects in a SpatialPolygons object? Could you use the North 
Carolina counties data set included in maptools to give an example of your 
question? You'll find additional information in spdep, see the poly2nb() 
and n.comp.nb() functions.

Roger

> Note: I usually use the "readShapePoly " to open the cartography.
>
> Thank you very much!
> Marc
>
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-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
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