Roger,
The technique of using buffer(0) to union polygons is now deprecated in
favour of using Unary Union (Geometry.union() - not sure what the exact
GEOS signature is). Unary Union is usually faster and more robust than
the previous technique. You might want to check this out.
Martin
Roger Bivand wrote:
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
--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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