unionSpatialPolygons from the command-line

Hello,

I am trying to call "maptools::unionSpatialPolygons()" from the command-line. To illustrate the problem, I can call example(unionSpatialPolygons) from an interactive session but running an R script (testmaptools.R) containing:

`library(maptools)`
`example(unionSpatialPolygons)`

With R --vanilla < testmaptools.R

and adding print(sessionInfo()) to the script, we see:

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

and it runs correctly. With

Rscript testmaptools.R

we see:

attached base packages:
[1] stats     graphics  grDevices utils     datasets  base

The script runs using Rscript if library(methods) is added as the first 
line. This issue has been seen elsewhere recently in another context, we 
don't know why.

Hope this clarifies,

Roger
yields:

`$ Rscript testmaptools.R`
WARNING: ignoring environment value of R_HOME
Loading required package: sp
Checking rgeos availability: TRUE

unnSpP> if (rgeosStatus()) {
unnSpP+ nc1 <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1],
unnSpP+  proj4string=CRS("+proj=longlat +datum=NAD27"))
unnSpP+ lps <- coordinates(nc1)
unnSpP+ ID <- cut(lps[,1], quantile(lps[,1]), include.lowest=TRUE)
unnSpP+ reg4 <- unionSpatialPolygons(nc1, ID)
unnSpP+ row.names(reg4)
unnSpP+ }
Error in rgeos::gUnaryUnion(spgeom = SpP, id = IDs) :
  Invalid geometry, may only be applied to polygons
Calls: example ... eval -> eval -> unionSpatialPolygons -> <Anonymous>
Execution halted
Is this not possible?

Thank you,

Joseph Stachelek

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Roger Bivand
Department of Economics, Norwegian School of Economics,
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unionSpatialPolygons from the command-line

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