[R-sig-Geo] invalid geometries from negative buffering in rgeos/sp

[Robert J. Hijmans]

A more general fix might be to support NULL geometries in Spatial objects.
Robert

On Thu, Oct 3, 2013 at 6:29 AM, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> 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
>>
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>
> --
> Roger Bivand
> Department of Economics, NHH Norwegian School of Economics,
> 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
>
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