invalid geometries from negative buffering in rgeos/sp

On Thu, 3 Oct 2013, Barry Rowlingson wrote:

Using scot_BNG from the rgdal package:

dsn <- system.file("vectors", package = "rgdal")[1]
scot_BNG <- readOGR(dsn=dsn, layer="scot_BNG")

and buffering with a sufficiently large negative width:

b=gBuffer(scot_BNG,width=-10000,byid=TRUE)

produces an object that gives an error on plot:

Yes, but ...

The user gets a SpatialPolygonsDataFrame object with the data slot intact,
but some of the polygons are clearly now invalid. Should we do book-keeping
to only retain the data slot rows for valid polygons, only return the subset
of valid polygons, or proceed as now, and have the user do:

plot(b[gArea(b, byid=TRUE) > 0,])

What makes more sense? I'm open to suggestions ...

Roger

plot(b)

Error in checkSlotAssignment(object, name, value) :
 assignment of an object of class ?NULL? is not valid for slot
"coords" in an object of class ?Line?; is(value, "matrix") is not TRUE

Which I think is because the negative buffer has shrunk a feature to
nothing.

b[1,] is fine but:

b[2,]

Error in SpatialPolygons(x at polygons[i], proj4string = CRS(proj4string(x)))
:
 INTEGER() can only be applied to a 'integer', not a 'NULL'

I'm not sure what the right thing to do here is. Either return a valid
zero-area polygon (if that's not a contradiction in terms) or drop the
degenerate cases?

Barry

--
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
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