[R-sig-Geo] maptools/SpatialPolygons2PolySet produces ascending instead of descending POS numbers for polygons with holes

[Daniel Rodolphe Schlaepfer]

Thank you for the suggestion. Unfortunately, this does not work for my application: I plot the polygon on top of other another plot where I need the holes of the polygon to be transparent. For instance, I don’t want the African countries in the following examples to be covered by white:

library(maps)
map()
plot(polySP, density=10, angle=45, pbg=“white")

However, this was not the point that I try to ask about. My point is that maptools/SpatialPolygons2PolySet does not translate into correct PolySet objects as illustrated in my previous example.

Thanks,
Daniel

On Aug 6, 2014, at 12:52 PM, Roger Bivand <Roger.Bivand at nhh.no> wrote:

> On Wed, 6 Aug 2014, Daniel Rodolphe Schlaepfer wrote:
> 
>> Hello all,
>> 
>> I try to convert a sp SpatialPolygons object with a hole to a PBS PolySet object - my ultimate goal is to plot the polygon with hatching considering the hole.
> 
> The problem is not that you need a PolySet representation, but that you need to set the polygon background explicitly to a value other than "transparent" using the pbg= argument:
> 
> plot(polySP, density=10, angle=45, pbg="white")
> 
> or adjust par("bg") to suit. When hatching is used, polypath is not used, so automatic handling of holes in the plot method is not available.
> 
> Roger
> 
>> 
>> My problem is that maptools/SpatialPolygons2PolySet only produces PolySet objects with increasing POS even for polygons with holes; however, the documentation of PolySet indicates that “We adopt the convention that POS goes from 1 to n along an outer boundary, but from n to 1 along an inner boundary, regardless of rotational direction.”
>> 
>> 
>> #Create simple doughnut-shaped polygon
>> library(sp)
>> library(maptools)
>> 
>> coords1 <- matrix(c(108, -54, -108, -54, -108, 54, 108, 54, 108, -54), ncol=2, byrow=TRUE)
>> coords2 <- matrix(c(36, -18, -36, -18, -36, 18, 36, 18, 36, -18), ncol=2, byrow=TRUE)
>> 
>> polySP <- SpatialPolygons(list(Polygons(list(Polygon(coords1, hole=FALSE), Polygon(coords2, hole=TRUE)), ID=1)), proj4string=CRS("+proj=longlat +datum=WGS84"))
>> 
>> #Convert sp-SpatialPolygons to PBS-PolySet with maptools function
>> polyPBS <- SpatialPolygons2PolySet(polySP)
>> 
>> #-> POS for SID == 2 are increasing and thus do not reflect PBS standards for a polygon with a hole
>>  PID SID POS    X   Y
>> 1    1   1   1  108 -54
>> 2    1   1   2 -108 -54
>> 3    1   1   3 -108  54
>> 4    1   1   4  108  54
>> 5    1   1   5  108 -54
>> 6    1   2   1   36 -18
>> 7    1   2   2   36  18
>> 8    1   2   3  -36  18
>> 9    1   2   4  -36 -18
>> 10   1   2   5   36 -18
>> 
>> I believe that it would require only a small change to maptools/SpatialPolygons2PolySet to produce PolySet objects that meet the PBS standards also for polygons with holes, i.e., replace the line
>> 	POS <- c(POS, 1:k)
>> with
>> 	POS <- if(slot(srs[[j]], "hole")) c(POS, k:1) else c(POS, 1:k)
>> inside the loops: for (i in 1:n) … for (j in 1:m) …
>> 
>> 
>> 
>> Sincerely,
>> Daniel Schlaepfer
>> 
>> 
>> My session infos:
>> sessionInfo()
>> 	R version 3.1.1 (2014-07-10)
>> 	Platform: x86_64-apple-darwin13.3.0 (64-bit)
>> 
>> 	locale:
>> 	[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
>> 
>> 	attached base packages:
>> 	[1] stats     graphics  grDevices
>> 	[4] utils     datasets  methods
>> 	[7] base
>> 
>> 	other attached packages:
>> 	[1] PBSmapping_2.67.60 maptools_0.8-30
>> 	[3] sp_1.0-15
>> 
>> 	loaded via a namespace (and not attached):
>> 	[1] foreign_0.8-61  grid_3.1.1
>> 	[3] lattice_0.20-29
>> 
>> 
>> -------------------------------------------------------
>> Daniel Schlaepfer, PhD
>> University of Wyoming
>> Laramie, WY 82071
>> 
>> _______________________________________________
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>> R-sig-Geo at r-project.org
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>> 
> 
> -- 
> 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