[R-sig-Geo] Merging two spatial polygon objects

[Roger Bivand]

On Thu, 5 Nov 2009, BRWIN338 at aol.com wrote:

> Good Evening
>
> I have two spatial polygon objects. The first  "StMaps50"  contains state
> level map data for the 50 US states.  The second  "CAMaps" contains
> provincial and territorial polygons  for Canada.  I would like to merge these two
> objects into a  single Canada-US spatial polygon object that I could then write
> to a  shapefile.  The data fields in the @data component have been
> constructed  with identical fields.  I have tried using the  maptools   "spRbind"
> function but get the following error  message:

With recent versions of sp, you can say:

rownames(StMaps50)
rownames(CAMaps)

By default, the IDs for each Polygons object, corresponding to each row in 
the data.frame object in the "data" slot, are the feature IDs read in form 
file. In the case of shapefiles, these usually run from "0" to say n-1. So 
they will be the same. Use the spChFIDs() method in maptools to change the 
IDs of at least one of your objects to disambiguate them (say paste "US" 
in front of the US ones and "CAN" in front of the Canadian ones). I'm also 
assuming the the coordinate reference systems are identical.

Hope this helps,

Roger

>
>> spRbind(StMaps50,CAMaps)
> Error in spRbind(as(obj,  "SpatialPolygons"), as(x, "SpatialPolygons")) :
> non-unique polygon  IDs
>
> I have attached additional information below.
> Any suggestions would be appreciated.
>
> Joe
>
> ##################################################
>>  class(StMaps50)
> [1] "SpatialPolygonsDataFrame"
> attr(,"package")
> [1]  "sp"
>
>> _StMaps50 at data[1:5_ (mailto:StMaps50 at data[1:5) ,]
> ctry        sname sabbr sfips  pfips
> 1   US   Washington     WA    53    NA
> 2    US      Montana    MT     30    NA
> 3    US        Maine     ME    23    NA
> 4   US North  Dakota    ND    38     NA
> 5   US South Dakota    SD     46    NA
>
>> class(CAMaps)
> [1]  "SpatialPolygonsDataFrame"
> attr(,"package")
> [1] "sp"
>
>> _CAMaps at data_ (mailto:CAMaps at data)
> ctry                  sname sabbr sfips pfips
> 0     CA                Alberta   ALB    61  CA01
> 1     CA      British Columbia    BCL    62  CA02
> 2     CA               Manitoba   MNT    63  CA03
> 3     CA         New Brunswick    NBR    64  CA04
> 4     CA           Nova  Scotia   NSC    65  CA07
> 5    CA  Northwest Territories   NTR    66   CA06
> 6     CA           Newfoundland   NWF    67   CA05
> 7     CA                Ontario   ONT    68  CA08
> 8     CA  Prince Edward Island   PEI    69   CA09
> 9     CA                 Quebec   QUE    70  CA10
> 10    CA           Saskatchewan   SSK    71  CA11
> 11    CA       Yukon Territory    YTR    72  CA12
>>
> ################################################
>
>
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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