[R-sig-Geo] unionSpatialPolygons

[Roger Bivand]

On Thu, 18 Mar 2010, rajkumar at mail.riaex.com wrote:

> Hi,
>
> I have a Raster and converted to Polygons  using pol<-rasterToPolygons(r2)
> I have 1429 polygons and I would like to combine the attached polygon into
> a bigger polygon. what should I do to create index to use
> unionSpatialPolygons?

Create a vector (should be a factor) with the output unit IDs, as in ?by 
(INDICES=) or ?aggregate (by=) in base R. unionSpatialPolygons() only 
takes a single vector, so the extension to list arguments doesn't work (if 
anyone needs it, use interaction() to create a single factor from more 
than one). In your case either subset the polygons you wish to dissolve, 
create the ID variable, dissolve, then add back to the dataset, or run the 
operation on the whole data set, where union is a no-op for singleton IDs 
(it only runs for ID counts > 1). Play with by() first to see how the 
index works.

Roger

PS. An affiliation seems appropriate - is this for education, research, 
what?

>
> Thanks
> Rajkumar
>
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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