[R-sig-Geo] gIntersection returns error "TopologyException: no outgoing dirEdge found at"

[Jean-Luc Dupouey]

Why gIntersection returns the ""TopologyException: no outgoing dirEdge 
found at" error in the following very simple case:

 > #construct a spatial layer with two adjacent squares, each of 10 x 10 
units:
 >
 > Pol1=rbind(c(0,0),c(0,10),c(10,10),c(10,0),c(0,0))
 > Pol2=rbind(c(0,0),c(10,0),c(10,-10),c(0,-10),c(0,0))
 >
 > Pols1=Polygons(list(Polygon(Pol1)),"Pols1")
 > Pols2=Polygons(list(Polygon(Pol2)),"Pols2")
 > MyLay=SpatialPolygons(list(Pols1,Pols2))
 >
 > #construct the same layer, but lagged by 0.5 in x and y directions
 >
 > Pol1l=Pol1+0.5
 > Pol2l=Pol2+0.5
 >
 > Pols1l=Polygons(list(Polygon(Pol1l)),"Pols1l")
 > Pols2l=Polygons(list(Polygon(Pol2l)),"Pols2l")
 > MyLayl=SpatialPolygons(list(Pols1l,Pols2l))
 >
 > #view the resulting spatial layers:
 >
 > plot(MyLay)
 > plot(MyLayl,add=TRUE)
 >
 > #try to intersect:
 >
 > inter=gIntersection(MyLay,MyLayl)
Error in RGEOSBinTopoFunc(spgeom1, spgeom2, byid, id, drop_lower_td, 
"rgeos_intersection") :
   TopologyException: no outgoing dirEdge found at 0.5 0

It works with the option byid=TRUE.

But my question is why it does not work without? Is this behaviour 
predictable?

I went through some previous related posts to the list, but could not 
find the answer.

Thanks,

-- 
Jean-Luc Dupouey

INRA
Forest Ecology & Ecophysiology Unit
F-54280 Champenoux
France
mail: dupouey at nancy.inra.fr