no boundaries between countries by using unionSpatialPolygons

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

no boundaries between countries by using unionSpatialPolygons

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