Using examples in Chapter 5, I am trying to get rid of non-unique polygons.
I'm working with a Massachusetts shapefile by zips. The file consists the
following fields
 ZCTA  NAME LSAD   LSAD_TRANS

I've done the following  to get rid of duplicate records

> zipmaps1_df <- as(zipmaps1, "data.frame")
[!duplicated(zipmaps1$LSAT_TRANS)]
> row.names(zipmaps1_df) <- zipmaps1_df$LSAD_TRANS
> zipmaps1a <- SpatialPolygonsDataFrame(zipmaps1, zipmaps1_df) ,

but get the following error message
Error in SpatialPolygonsDataFrame(zipmaps1, zipmaps1_df) :
  row.names of data and Polygons IDs do not match


How do I find out what field R is treating as Polygon ID, so that I can set
my row.names to that?


On Wed, Apr 29, 2009 at 2:42 AM, Roger Bivand <Roger.Bivand@nhh.no> wrote:

> 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,
>
> Roger
>
>
>> thank you!
>>
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>>
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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@nhh.no
>
>

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