[R-sig-Geo] rgeos::gBuffer(, byid=TRUE) fails

[Roger Bivand]

On Mon, 8 Feb 2016, Roger Bivand wrote:

> On Mon, 8 Feb 2016, Josh O'Brien wrote:
>
>>  Oops. The post's title should have read "rgeos::gBuffer( , byid=TRUE)
>>  fails when...". (I inadvertently left out the `byid=TRUE` bit).
>>
>>  My apologies.
>>
>>  On Mon, Feb 8, 2016 at 9:59 AM, Josh O'Brien <joshmobrien at gmail.com>
>>  wrote:
>> >  Hello,
>> > 
>> >  Not sure if this would be considered a bug, but it did recently bite
>> >  me (and might bite others using gBuffer(..., byid=TRUE)
>> >  programatically).
>> > 
>
> Thanks for a reproducible example - for rgeos 0.3-14 this gives a segfault, 
> in 0.3-16 an error as the data.frame has only one row, but three Polygons 
> objects are returned. As you note, the issue is triggered by byid=TRUE but 
> there is only one input object; only one buffer is created, around the NE 
> island.
>
> I'll look into this - it'll need debugging in C, close to recent bug-fixes 
> (then concerned with pre-sp-MultiPoint object signalling - multiple objects 
> making up features).

I have committed a fix to R-Forge (checking whether the length of spgeom 
== 1 and byid == TRUE, if so set byid=FALSE and warn). The problem was 
visible in the C code, but there we don't have an sp-class generic 
"length" measure of the # features, so the modification is at the R level.

Which OS would you like to test under?

Best wishes,

Roger

>
> And yes, this is a reasonable place to report, it should give users a 
> realistic view of how many corner cases actually exist and require someone's 
> attention.
>
> Roger
>
>> > 
>> >      library(sp)
>> >      library(rgdal)
>> >      library(rgeos)
>> > 
>> >      ## Example SpatialPolygonsDataFrame
>> >      x <- readOGR(system.file("etc/shapes", package="spdep"), 
>> >      "sids")[c(89,28),]
>> >      plot(x, col=c('dodgerblue', 'wheat'))
>> > 
>> >      ## Works fine on object with multiple @Polygons in @polygons, even
>> >      ## if one of the @Polygons is multi-part
>> >      gBuffer(x, width=0.01, byid=TRUE)
>> > 
>> >      ## Works with a single single-part @Polygons object in @polygons
>> >      gBuffer(x[1,], width=0.01, byid=TRUE)
>> > 
>> >      ## Fails with a single _multi-part_ @Polygons object in @polygons
>> >      gBuffer(x[2,], width=0.01, byid=TRUE)
>> > 
>> > 
>> >  Thanks (and please let me know an rgeos-specific note like this would
>> >  be better posted elsewhere),
>> > 
>> >  Josh O'Brien
>>
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>> 
>
>

-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: Roger.Bivand at nhh.no
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