gUnion error -> TopologyException: side location conflict at...

Dear all,

I would like to join the polygons of two maps (with
SpatialPolygonsDataFrame format). I was trying to use gUnion funcition
(rgeos library) but I obtained the following error:
setwd("R:/CARTOGRAFIA/Barcelona")

BCN.SC<-readShapeSpatial("barcelona_SC2001.shp")
proj4string(BCN.SC) <- CRS("+proj=merc +datum=WGS84 +units=m")
BCN.SC<- spTransform(BCN.SC, CRS("+init=epsg:23031")) #ED50
BCN.CP<-readShapeSpatial("Dist_postals_BCN.shp") #ED50
proj4string(BCN.CP) <- CRS("+init=epsg:23031")

gIsValid(BCN.CP)
[1] TRUE
gIsValid(BCN.SC)
[1] TRUE

aux <- gUnion(BCN.SC,BCN.CP)
Error in RGEOSBinTopoFunc(spgeom1, spgeom2, byid, id, drop_lower_td,
"rgeos_union") :
  TopologyException: side location conflict at 425106.60792063997
4583876.4374835696

I can't understand what's happening, glsValid is true for both shp
files. Moreover, I've made a zoom to this point and I can see anything
wrong.

Thank you for your help,

Marc
I've found you can get around this error from some rgeos functions
(like gIntersection) by using the byid = TRUE argument, and in the
extreme you can iterate over one or both sets individually and collate
the results. I haven't been able to construct a clear example to
isolate the cause of the problem.

Since it's a union perhaps you can gUnaryUnion each object together
first. But is that the goal? One big union object, or a per-ID union
matching objects from the two sets? (I think I understand this, but
again I'd like to build a set of examples that make it clear).

Cheers, Mike.

On Wed, Oct 29, 2014 at 1:12 AM, Marc Mar? Dell'Olmo
Dear all,

I would like to join the polygons of two maps (with
SpatialPolygonsDataFrame format). I was trying to use gUnion funcition
(rgeos library) but I obtained the following error:

setwd("R:/CARTOGRAFIA/Barcelona")

BCN.SC<-readShapeSpatial("barcelona_SC2001.shp")
proj4string(BCN.SC) <- CRS("+proj=merc +datum=WGS84 +units=m")
BCN.SC<- spTransform(BCN.SC, CRS("+init=epsg:23031")) #ED50
BCN.CP<-readShapeSpatial("Dist_postals_BCN.shp") #ED50
proj4string(BCN.CP) <- CRS("+init=epsg:23031")

gIsValid(BCN.CP)
[1] TRUE
gIsValid(BCN.SC)
[1] TRUE

aux <- gUnion(BCN.SC,BCN.CP)
Error in RGEOSBinTopoFunc(spgeom1, spgeom2, byid, id, drop_lower_td,
"rgeos_union") :
  TopologyException: side location conflict at 425106.60792063997
4583876.4374835696

I can't understand what's happening, glsValid is true for both shp
files. Moreover, I've made a zoom to this point and I can see anything
wrong.

Thank you for your help,

Marc

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Michael Sumner
Software and Database Engineer
Australian Antarctic Division
Hobart, Australia
e-mail: mdsumner at gmail.com