[R-sig-Geo] Hole attribute in Polygon lost when made into Polygons object?

[Roger Bivand]

On Wed, 27 May 2015, MacQueen, Don wrote:

> I must be missing something basic
> (either in understanding, or possibly being blind to a typo!)
>
> Here is the example data:
>
> m1 <-structure(c(2.8, 8, 8.2, 3.9, 1.6,
>    9.2, 6.8, 3, 1.1, 4.2),
>   .Dim = c(5L, 2L))
>
>
> Pnh <- Polygon(m1, hole=FALSE)
> Ph  <-  Polygon(m1, hole=TRUE)
>
> Pnhs <- Polygons(list(Pnh), ID=1)
> Phs <- Polygons(list(Ph), ID=1)
>
>
>
> Then:
>> Pnh at hole
> [1] FALSE
>> Ph at hole
> [1] TRUE
>
>> Pnhs at Polygons[[1]]@hole
> [1] FALSE
>> Phs at Polygons[[1]]@hole
> [1] FALSE
>
> It appears that the hole attribute has been lost??
>

Yes, by design. A Polygons object is like an OGC/SFS MultiPolygon or 
Polygon object, and must have at least one exterior ring (non-hole 
sp:Polygon object). If the only Polygon object in a Polygons object has 
its hole slot set to TRUE, this is treated as a misunderstanding and 
silently changed to FALSE (and ring direction reversed if need be). Please 
see the note in ?"Polygons-class".

Hope this clarifies,

Roger

>
> sp_1.1-0
> R 3.1.2
>
> Thanks
> -Don
>
>

-- 
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