[R-sig-Geo] gUnaryUnion Not Dissolving Correctly

[Roger Bivand]

On Tue, 3 Nov 2015, Matt Strimas-Mackey wrote:

> I'm working with a regular hexagonal grid stored as SPDF. At some
> point I subset this SPDF, then want to combine all adjacent hexagons
> together so that each contiguous set of hexagons is a single polygon.
> I'm doing this last step using gUnaryUnion (or gUnionCascaded, not
> clear what the different is actually). The problem is that in some
> cases boundaries between clearly adjacent polygons are not dissolved.
>
> Example:
>
> ## Create three adjacent hexagons
> library(sp)
> library(rgeos)
> p1 <- Polygon(cbind(
>      c(1276503.26781119, 1281876.11747031, 1287248.96712942,
>        1287248.96712942, 1281876.11747031, 1276503.26781119, 1276503.26781119),
>      c(204391.40834643, 207493.42454344, 204391.40834643, 198187.37595242,
>        195085.35975541, 198187.37595242, 204391.40834643)))
> p2 <- Polygon(cbind(
>      c(1287248.96712943, 1292621.81678854, 1297994.66644766,
>        1297994.66644766, 1292621.81678854, 1287248.96712943, 1287248.96712943),
>      c(204391.40834643, 207493.42454344, 204391.40834643, 198187.37595242,
>        195085.35975541, 198187.37595242, 204391.40834643)))
> p3 <- Polygon(cbind(
>      c(1281876.11747031, 1287248.96712943, 1292621.81678854,
>        1292621.81678854, 1287248.96712943, 1281876.11747031, 1281876.11747031),
>      c(213697.45693745, 216799.47313446, 213697.45693745, 207493.42454344,
>        204391.40834643, 207493.42454344, 213697.45693745)))
> spoly <- SpatialPolygons(list(Polygons(list(p1, p2, p3), 's1')))
> plot(gUnaryUnion(spoly))

No, this is just numerical fuzz:

plot(spoly)
plot(gUnaryUnion(spoly), border="green", lty=3, lwd=2, add=TRUE)
oS <- getScale()
# default 1e+8
setScale(1e+4)
plot(gUnaryUnion(spoly), border="orange", lwd=2, add=TRUE)
setScale(oS)

JTS, GEOS, and consequently rgeos by default shift all coordinates to an 
integer grid after multiplying by a scale factor (finding integer matches 
is much easier than real matches). If the scaling is too detailed (in some 
cases), the operations do not give the expected outcomes.

There is work in progress in GEOS and JTS to provide other scaling options 
and models, and to permit iteration over scaling values until a "clean" 
result is obtained (for some meanings of clean).

gUnionCascaded() was the only possible function for GEOS < 3.3.0, from 
GEOS 3.3.0 gUnaryUnion() is available and the prefered and more efficient 
route. This is explained on the help page.

Hope this clarifies,

Roger

>
> Note that p2 and p3 are dissolved together, but p1 is separate. The
> shared edge of p1 and p2 is:
> p1:
> [2,] 1281876 207493.4
> [3,] 1287249 204391.4
> p2:
> [5,] 1287249 204391.4
> [6,] 1281876 207493.4
>
> So, exactly the same apart from the order. I originally thought this
> difference in order might be the problem, but this doesn't seem to be
> an issue with in this example, where order is also flipped:
> sss <- rasterToPolygons(raster(nrow=2, ncol=2, xmn=0, xmx=2, ymn=0,
> ymx=2, vals=1:4))
> lapply(sss at polygons, function(x) slot(x, 'Polygons')[[1]]@coords)
> plot(sss)
> plot(gUnaryUnion(sss))
>
> Session Info:
> R version 3.2.2 (2015-08-14)
> Platform: x86_64-apple-darwin13.4.0 (64-bit)
> Running under: OS X 10.10.5 (Yosemite)
>
> Message when rgeos is loaded:
>
> rgeos version: 0.3-14, (SVN revision 511)
> GEOS runtime version: 3.5.0-CAPI-1.9.0 r0
> Linking to sp version: 1.2-0
> Polygon checking: TRUE
>
> Any help on how to get these polygons to dissolve is appreciated.
>
> M
>
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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