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.
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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