[R-sig-Geo] no boundaries between countries by using unionSpatialPolygons

[Roger Bivand]

On Wed, 7 Jan 2009, Hans-Jörg Bibiko wrote:

> Hi,
>
> I have a tiny problem. I'm using Roger Bivand's ( 
> http://spatial.nhh.no/R/etc/TM_WORLD_BORDERS_SIMPL-0.2.RData ) to create a 
> plot showing two countries without boundaries. I did this:
>
>
>
> library("foreign")
> library("sp")
> library("maptools")
> library("gpclib")
>
> load("TM_WORLD_BORDERS_SIMPL-0.2.RData")
> pe_bo_data <- wrld_simpl[wrld_simpl$NAME %in% c("Bolivia","Peru"),]
>
> lps <- coordinates(pe_bo_data)
> ID <- cut(lps[,1], quantile(lps[,1]), include.lowest=TRUE)
>
> pe_bo <- unionSpatialPolygons(pe_bo_data, ID, threshold = NULL)
> plot(pe_bo)
>

Excellent question - easy to reproduce!

Your ID has two unique values:

> ID
[1] (-67.1,-64.6] [-74.4,-71.9]
Levels: [-74.4,-71.9] (-71.9,-69.5] (-69.5,-67.1] (-67.1,-64.6]

with two country polygons:

> length(slot(pe_bo_data, "polygons"))
[1] 2

so you get two output polygons. If you do:

ID2 <- rep("pe_bo", length(slot(pe_bo_data, "polygons")))
pe_bo <- unionSpatialPolygons(pe_bo_data, ID2)

gives one output polygon:

> length(slot(pe_bo, "polygons"))
[1] 1

and no boundary.

Roger

>
>
>
> but the plot still shows the boundary between Peru and Bolivia. I also tried 
> to set a proper threshold but unfortunately no success.
>
> What's wrong with my approach? I'd be glad for any hint!
>
> I'm using R version 2.8.0 (2008-10-20) on Mac OSX 10.5.6 (Intel)
>
>
> Thanks in advance,
>
> --Hans
>
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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