[R-sig-Geo] Holes in polygons disappear after performing checkPolygonsHoles from maptools

[Grigory Alexandrovich]

Am 02.04.2015 um 22:24 schrieb Roger Bivand:
> On Thu, 2 Apr 2015, Roger Bivand wrote:
>
>> On Thu, 2 Apr 2015, Edzer Pebesma wrote:
>>
>>>
>>>
>>>  On 04/01/2015 08:03 PM, Grigory Alexandrovich wrote:
>>> >  Hello all,
>>> > >  I encountered the following unexpected result using 
>>> checkPolygonsHoles:
>>> > >  # attach the worldmap as SpatialPolygonsDataFrame from the 
>>> package >  maptools
>>> >  library(sp)
>>> >  library(maptools)
>>> >  data(wrld_simpl)
>>> > >  # get a polygon with a hole
>>> >  shape_with_hole <- wrld_simpl[5,]
>>> > >  # plot it (hole is left white, surrounded by blue color)
>>> >  plot(shape_with_hole, col = "blue")
>>> > >  # perform checkPolygonsHoles
>>> >  shape_with_hole at polygons <- lapply(shape_with_hole at polygons,
>>> >  checkPolygonsHoles)
>>> > >  # plot again, now holes aren't recognized as such
>>> >  plot(shape_with_hole, col = "blue")
>>> > >  # and even the original SpatialPolygonsDataFrame object is 
>>> changed !?
>>> >  plot(wrld_simpl[5,], col = "blue")
>>> > >  One irritating side effect here is that the original object 
>>> wrld_simpl
>>> >  is also changed. This result looks to me like a bug, or have I 
>>> missed
>>> >  something?
>>> >
>>>  It looks as if it is related to rgeos, as
>>>
>>>  avoidGEOS = FALSE
>>>  library(sp)
>>>  library(maptools)
>>>  data(wrld_simpl)
>>>  x = wrld_simpl
>>>
>>>  shape_with_hole <- wrld_simpl[5,]
>>>
>>>  if (avoidGEOS)
>>>      gpclibPermit()
>>>  shape_with_hole at polygons <- lapply(shape_with_hole at polygons,
>>>  checkPolygonsHoles,
>>>   avoidGEOS = avoidGEOS)
>>>
>>>  data(wrld_simpl)
>>>  all.equal(x, wrld_simpl)
>>
>> No, but:
>>
>> library(sp)
>> library(maptools)
>> data(wrld_simpl)
>> x = wrld_simpl
>> shape_with_hole <- wrld_simpl[5,]
>> pls <- slot(shape_with_hole, "polygons")
>> pls1 <- lapply(pls, checkPolygonsHoles, avoidGEOS = FALSE)
>> isTRUE(all.equal(pls, pls1))
>> # TRUE
>> gpclibPermit()
>> pls1 <- lapply(pls, checkPolygonsHoles, avoidGEOS = TRUE)
>> isTRUE(all.equal(pls, pls1))
>> # FALSE
>>
>> where the Polygon objects are re-ordered and plotted out of order, so 
>> the deprecated gpclib code may be broken here. However, this isn't 
>> the problem,
>
> The gpclib code is OK, it re-orders the Polygon objects, but the plot 
> order is correct. The problem is as described below.
>
> Roger
>
>> but rather (perhaps) graphics::polypath. If it is used (by default it 
>> is), and with the default path fill mode:
>>
>> plot(x[5,], col="blue", usePolypath=TRUE, rule="winding")
>>
>> does not whiten the holes. However:
>>
>> plot(x[5,], col="blue", usePolypath=TRUE, rule="evenodd")
>>
>> does. Using:
>>
>> plot(x[5,], col="blue", pbg="white", usePolypath=FALSE)
>>
>> also works by overpainting the island with the holes in white.
>>
>> Please summarise the your SO thread.
>>
>> By the way, in R it is unusual for changes in one object to propagate 
>> in an unintended way to another object, and this is not happening 
>> here. The visual impression is being driven by polypath doing 
>> different things depending on its rule setting. Look at ?polypath for 
>> details.
>>
>> Well, this doesn't clarify, but it explains what is happeing, I hope ...
>>
>> Roger
>>
>>>
>>>  returns a difference, but TRUE if avoidGEOS = TRUE.
>>>
>>

|Thank you for your answers guys. But I am still not fully convinced 
that it is an issue of the plot function,
because the object shape_with_hole edited with checkPolygonsHoles 
before, continues to behave in a strange way:

# we check which polygons are flagged as holes.
# The flags are still set properly,
# although the plot() function didn't recognize them:
sapply(shape_with_hole at polygons[[1]]@Polygons, slot, "hole")

[1] FALSE  TRUE  TRUE  TRUE

# load library rgdal for reprojection
library(rgdal)

# reproject with spTransform, just for testing
shape_with_hole <- spTransform(shape_with_hole, CRS("+proj=longlat 
+ellps=WGS84 +datum=WGS84"))

# after reprojection all flags are set to FALSE
sapply(shape_with_hole at polygons[[1]]@Polygons, slot, "hole")

[1] FALSE FALSE FALSE FALSE


So it is still something wrong with shape_with_hole, or?

Best

Grigory|


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