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

[MacQueen, Don]

That makes sense, i.e., I did not realize that an  individual Polygon object can be a hole or not, as specified when it is created, but within a Polygons object there can be no hole Polygon unless there is also another non-hole Polygon.

Thanks
-Don

--
Don MacQueen
Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062


From: Michael Sumner <mdsumner at gmail.com<mailto:mdsumner at gmail.com>>
Date: Tuesday, May 26, 2015 at 4:10 PM
To: dh m <macqueen1 at llnl.gov<mailto:macqueen1 at llnl.gov>>, "R-sig-Geo at r-project.org<mailto:R-sig-Geo at r-project.org>" <R-sig-Geo at r-project.org<mailto:R-sig-Geo at r-project.org>>
Subject: Re: [R-sig-Geo] Hole attribute in Polygon lost when made into Polygons object?



On Wed, 27 May 2015 at 08:26 MacQueen, Don <macqueen1 at llnl.gov<mailto:macqueen1 at llnl.gov>> 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??





I believe that the hole attribute only makes sense when it's relative to at least one other Polygon object. If you combine your Pnh and Ph in the same Polygons object, the hole status is preserved (despite this being a broken object, see below):

 sapply(Polygons(list(Pnh, Ph), ID = 1)@Polygons, slot, "hole")
[1] FALSE  TRUE

If you create a Polygons object with just one Polygon that is a 'hole', there's nothing for it to be a hole of (there's no 'background'  in sp).  I presume this happens internally when a new Polygons is initialized with just one ring, it makes no sense for the hole attribute  to be TRUE (the sp source is here src/sp_xports.c but I'm not au fait with that).

Note that both of your Polygon objects are identical, so you get a nonsense object:


rgeos::gIsValid(SpatialPolygons(list(Polygons(list(Pnh, Ph), ID = 1))))
[1] FALSE
Warning message:
In RGEOSUnaryPredFunc(spgeom, byid, "rgeos_isvalid") :
  Self-intersection at or near point 1.6000000000000001 4.2000000000000002

Also, you can see the different ways the topology is treated by creating a hole, and then offset that to incorrectly overlap
it's background. With usePolypath the topology of the situation is more clearly represented
x1 <- Polygon(m1, hole=FALSE)
x2  <-  Polygon((m1 + 3) / 1.5, hole=TRUE)
p1 <- SpatialPolygons(list(Polygons(list(x1, x2), ID = 1)))

op <- par(mfrow = c(2, 2))
plot(p1, col = "grey", usePolypath = TRUE, main = "ok, polypath TRUE")
plot(p1, col = "grey", usePolypath = FALSE, main = "ok, polypath FALSE")

## now with a hole, but incorrectly overlapping

y1 <- Polygon(m1, hole=FALSE)
y2  <-  Polygon((m1 + 4) / 1.5, hole=TRUE)
p2 <- SpatialPolygons(list(Polygons(list(y1, y2), ID = 1)))

plot(p2, col = "grey", usePolypath = TRUE, main = "broken, polypath TRUE")
plot(p2, col = "grey", usePolypath = FALSE, main = "broken, polypath FALSE")
par(op)

Does that help?

Cheers, Mike.


sp_1.1-0
R 3.1.2

Thanks
-Don

--
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062

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