[R-sig-Geo] combining polygons from shapefiles...

[Roger Bivand]

On Wed, 2 Dec 2009, Sean O'Riordain wrote:

> Good afternoon,
>
> I've been making good progress in the last few weeks thanks to this
> list, along with Roger and Adrians' books and some great geo-packages.
>
> I'm trying to combine some shapefiles...
>
> library(maptools)
> CA <- readShapePoly('ca.shp')
> LA <- readShapePoly('la.shp')
>
> Now I want to combine these two into a bigger object...
>
> I've searched the books and the internet and I've tried a number of
> things... including...
>
>> class(LA)
> [1] "SpatialPolygonsDataFrame"
> attr(,"package")
> [1] "sp"
>> class(CA)
> [1] "SpatialPolygonsDataFrame"
> attr(,"package")
> [1] "sp"
>>
>>     S1 <- unionSpatialPolygons(CA, LA)
> Loading required package: gpclib
> General Polygon Clipper Library for R (version 1.4-4)
>        Type 'class ? gpc.poly' for help
>
> Error in unionSpatialPolygons(CA, LA) : input lengths differ
>>
>>
>> S1 <- spRbind(CA, LA)
> Error in spRbind(as(obj, "SpatialPolygons"), as(x, "SpatialPolygons")) :
>  non-unique polygon IDs
>>

Correct. Look at row.names(CA) and row.names(LA). Do something of your 
choice to disambiguate them and re-assign with spChFIDs() - since you 
refer to the ASDAR book, there is an example in chapter 5, sections 5.4 & 
5.5, pp. 120-126. You'll also see that your reading of how to use 
unionSpatialPolygons() did not agree with that in the function, which is 
to dissolve Polygons objects within an existing SpatialPolygons object.

Hope this helps,

Roger

>
> Any ideas please?
>
> Thanks in advance!
> Sean
>
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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