[R-sig-Geo] unique spatial polygons

Roger Bivand Roger.Bivand at nhh.no
Wed Apr 29 08:42:35 CEST 2009

On Tue, 28 Apr 2009, Alina Sheyman wrote:

> I'm trying to combine two shapefiles using
> zipmaps5 <- spRbind(zipmaps1,zipmaps2)
> and getting the following error message
> Error in spRbind(as(obj, "SpatialPolygons"), as(x, "SpatialPolygons")) :
>  non-unique polygon IDs
> Does anyone know how I can get rid of duplicate polygons in a shapefile?

This isn't the problem. The real problem is that zipmaps1 and zipmaps2 
have non-unique polygon IDs, so that it isn't obvious to the function what 
you want to do. By default:

sapply(slot(zipmaps1, "polygons"), slot, "ID")

is set to the FID of the shapefile in readOGR() and equivalently in 
readShapeSpatial() in maptools. These typically run 0:(n-1). For two 
objects, they obviously overlap. Please use spChFIDs() methods in maptools 
to assign IDs that are unique for the output object; in the worst case:

zipmaps1 <- spChFIDs(zipmaps1, paste("map1", sapply(slot(zipmaps1,
   "polygons"), slot, "ID"), sep="_"))

and a different "map*" string for zipmaps2 will work, but a unique, 
meaningful ID is best.

This is covered in detail in the code examples for Chapter 5 in our book, 
and on the help page for ?"spChFIDs-methods".

Hope this helps,


> thank you!
> 	[[alternative HTML version deleted]]
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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

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