Hi,
I've felt that I've been making reasonable progress with interfacing GEOS
geometries and methods for the R language (cran.r-project.org), in a draft
contributed package rgeos:
https://r-forge.r-project.org/projects/rgeos/
However, I encountered a problem that I do not understand, and would be
very grateful if I could be put back on track. The specific problem is that
an R function uses the C API to dissolve polygon borders for adjacent
polygons sharing a category given as its second argument.
I've used the buffer technique from the JTS documentation, and all was well
until I tried to dissolve borders between touching squares when the
coordinate measures were small (square side 0.1). When the squares are 100
units, all is well, and GEOMTouches is TRUE. But for 0.1, GEOMTouches is
FALSE, and no dissolving takes place.
This can be reproduced (I'm on RHEL5, x86_64) by installing R, contributed
packages sp and maptools from CRAN, GEOS (3.1.1 or 3.2.0), and installing
rgeos from R-Forge. Start R, say
library(rgeos) example(unionSpatialPolygonsGEOS)
and look for undissolved borders in:
image(grd, axes=TRUE)
plot(spol1, add=TRUE)
but dissolved in:
image(grdx, axes=TRUE)
plot(spol1x, add=TRUE)
I started on an alternative implementation using GEOMTouches in
unionSpatialPolygonsGEOS(..., buffer=FALSE) output to console, where one
sees in the example output:
spol1F <- unionSpatialPolygonsGEOS(as(spol, "SpatialPolygons"),
+ as.character(spol$xvs), buffer=FALSE)
# 4 squares within (0,0), (0.2,0.2) NE, NW, SW share category, SE doesn't
npls: 3, nnpls: 3
type[0] Polygon
i: 0, j: 1, touches: 0
i: 0, j: 2, touches: 0
type[1] Polygon
i: 1, j: 2, touches: 0
type[2] Polygon
out of function
npls: 1, nnpls: 1
type[0] Polygon
out of function
spol1xF <- unionSpatialPolygonsGEOS(as(spolx, "SpatialPolygons"),
+ as.character(spolx$xvs), buffer=FALSE)
# 4 squares within (0,0), (200,200), same categories
npls: 3, nnpls: 3
type[0] Polygon
i: 0, j: 1, touches: 1
i: 0, j: 2, touches: 1
type[1] Polygon
i: 1, j: 2, touches: 1
type[2] Polygon
out of function
npls: 1, nnpls: 1
type[0] Polygon
out of function
The C code is in:
https://r-forge.r-project.org/plugins/scmsvn/viewcvs.php/pkg/src/rgeos_sp.c?rev=44&root=rgeos&view=log
around line 466 rgeos_SpatialPolygonsUnion(), calling rgeos_plspairUnion()
- desperate test framework, or rgeos_plsbufUnion() which I had thought
worked, but which clearly doesn't dissolve small squares.
Very grateful for any help, this is a show-stopper, and I had hoped to
release rgeos before February (small chance now!)
Roger
