[R-sig-Geo] merging polygons by indices

[Roger Bivand]

On Tue, 16 Aug 2011, majesty wrote:

> still I am not quiet sure how to achieve this by manipulation of ID. Could
> you give an example, say I have 5 polygons in my shapefile shape.shp but
> would like to have 4 polygons, where shape[2] is merged with shape[3]. I am
> somewhat clueless because ID has to be of equal in length to polygons slot
> (in my case 5)

So maybe try:

shape1 <- unionSpatialPolygons(shape, IDs=as.character(shape$output_area))

which will copy across the unchanged entities, and join the ones with 
multiple output_area IDs. Note that the only way to retrieve the IDs will 
be row.names(shape1), as the function doesn't aggregate other data in 
shape.

Hope this helps,

Roger

>
> Thanks,
>
> Juta Kawalerowicz
>
>
> MPhil Candidate in Sociology
> Oxford University
>
>
> --
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>
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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