[R-sig-Geo] rgeos::gDifference

[Edzer Pebesma]

On 01/05/2015 01:53 PM, Roger Bivand wrote:
> On Mon, 5 Jan 2015, Edzer Pebesma wrote:
> 
>> When running the following script:
>>
>> library(sp)
>> gt = GridTopology(cellcentre.offset = c(0,0), cellsize = c(1,1),
>>     cells.dim = c(3,3))
>> sgdf = SpatialGridDataFrame(SpatialGrid(gt),
>>    data.frame(r = round(runif(9, max = 10), 1)))
>>
>> gt = GridTopology(cellcentre.offset = c(.25,.25), cellsize = c(1,1),
>>     cells.dim  = c(1,1))
>> grd.red = SpatialGrid(gt)
>>
>> library(rgeos)
>> sp1 = as(sgdf, "SpatialPolygonsDataFrame")
>> sp2 = as(grd.red, "SpatialPolygons")
>>
>> plot(sp1, axes = TRUE)
>> plot(sp2, border = 'red', add = TRUE)
>>
>> dif = gDifference(sp2, gUnionCascaded(sp1))
>> dif = gDifference(sp2, sp1)
>>
>>
>> the last command gives the following error:
>>
>> Error in RGEOSBinTopoFunc(spgeom1, spgeom2, byid, id, drop_not_poly,
>> "rgeos_difference") :
>>  TopologyException: no outgoing dirEdge found at -0.25 -0.25
>> Calls: gDifference -> RGEOSBinTopoFunc -> .Call
>> Execution halted
>>
>> What does this mean? Why does gUnionCascaded(sp1) not do this?
>>
> 
> gUnionCascaded() is deprecated, use gUnaryUnion(). This isn't the
> problem, the argument order is probably the cause:
> 
> dif = gDifference(sp1, sp2, byid=c(TRUE, FALSE))
> length(dif)
> 
> works, preserving the input geometries, as does:
> 
> dif = gDifference(gUnaryUnion(sp1), sp2)
> length(dif)
> 
> if you want to aggregate.
> 
> This:
> 
> dif = gDifference(sp2, gUnionCascaded(sp1))
> 
> gives a NULL result, as sp2 is within sp1, as does:
> 
> dif = gDifference(sp2, gUnaryUnion(sp1))
> 
> but:
> 
> dif = gDifference(sp2, sp1, byid=c(FALSE, TRUE))
> 
> preserves the input geometries. If it was clear what you wanted as
> output from the operation, it would be easier to see what is going on.

Thanks!

I wanted to know if there was any part of sp2 that is not covered by
sp1, across the entire objects sp2 and sp1. Given the documentation, I
had expected

gDifference(sp2, sp1)

to give me that, and didn't understand the error message. I'm fine to
gUnaryUnion() sp1, but don't understand why.

Best regards,
-- 
Edzer Pebesma,  Co-Editor-in-Chief Computers & Geosciences
Institute for Geoinformatics (ifgi), University of Münster
Heisenbergstraße 2, 48149 Münster, Germany. Phone: +49 251
83 33081 http://ifgi.uni-muenster.de GPG key ID 0xAC227795

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