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

[Robert J. Hijmans]

On Mon, Oct 7, 2013 at 5:51 AM, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> On Mon, 7 Oct 2013, Robert J. Hijmans wrote:
>
>> A more general fix might be to support NULL geometries in Spatial objects.
>
>
> Maybe, but we do not have in sp a class for mixed types of geometries
> either,

True, but that is a different issue.

> so we would have to define NULL of everything. OGC SFS has NULL, but
> I would argue that NULL geometries have no analytical meaning, like a point
> with no coordinate values - so we don't know where it is. It doesn't fit the
> Spatial class, unless we open for a bounding box with NA corners.

There have been cases where I thought it would have been handy, for
example when geometries (e.g. point locations) are known for some, but
not for all records in a data set.
However, addition this type of support would add a significant amount
of overhead to all functions (check for / remove NULL values; NA
bounding box vaues).

> In rgeos,
> the typical output if there is no positional data is the R NULL value. One
> might go towards objects with mixed geometries (GDAL is permitting them for
> the drivers that have them in the development version), but I don't see an
> analytical need.

Me neither

> How often does, for example, newdata for predicting from a
> geostatistical model include both points and lines?
>
> Roger
>
>
>> 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
>>>
>>> _______________________________________________
>>> R-sig-Geo mailing list
>>> R-sig-Geo at r-project.org
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>
>>
>
> --
> 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