[R-sig-Geo] gIntersection with drop_lower_td=T drops entire intersection

[Roger Bivand]

I didn't see this when you posted, sorry. I can replicate the problem 
- thanks for a useful report. I'll try to see where drop_lower_td= is 
being too aggresive.

I'd be very obliged if others (not necessarily the poster!) would try to 
answer questions like this, the code is open, my time is limited, and 
sometimes my day-job has priority ...

Roger

On Thu, 1 Oct 2015, Pius Korner wrote:

> Dear all
>
>
>
> I intersect a SpatialPolygons-Object with two polygons 1a and 1b with a
> polygon 2. Polygon 1a intersects with polygon 2 in one area but also only
> touches polygon 2 at another position.
>
> Here a schematic case for illustration:
>
>
>
> poly1 <-
> SpatialPolygons(list(Polygons(list(Polygon(coords=matrix(c(0,0,2,2,0,1,1,0),
> ncol=2,byrow=F))),ID=c("a")),
>
>
> Polygons(list(Polygon(coords=matrix(c(0,0,2,2,2,3,3,2),ncol=2,byrow=F))),ID=
> c("b"))))
>
> poly2 <-
> SpatialPolygons(list(Polygons(list(Polygon(coords=matrix(c(0,0,2,2,1,1,1,3,3
> ,0,0,2),ncol=2,byrow=F))),ID=c("c"))))
>
> plot(poly1,border="orange")
>
> plot(poly2,border="blue",add=T,lty=2,density=8,angle=30,col="blue")
>
>
>
> I'm interested in the area of overlap of polygons 1a and 1b with polygon 2.
>
> gIntersection(poly1,poly2,byid=T)
>
> does not work because of the different subgeometries that result from the
> intersection of 1a with 2; they cannot be collected.
>
> gIntersection(poly1,poly2,byid=T,drop_lower_td=T)
>
> drops the entire intersection between 1a with 2, hence I get no value for
> 1a, even though it does overlap with polygon 2.
>
>
>
> Is there a way so that only the Point-subgeometries (with area=0) are
> dropped, but the Polygon-subgeometry is retained and returned for the
> intersection of 1a and 2?
>
>
>
> my temporary workaround was to very slightly buffer poly1 (width=-0.0001),
> to prevent the point-subgeometry.
>
> I could create a loop and intersect each polygon in poly1 separately, but
> using byid=T would be nicer (faster..)
>
>
>
> many thanks
>
> Pius
>
>
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>
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-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: Roger.Bivand at nhh.no