On Mon, 6 Jun 2016, Kumar Mainali wrote:
Thank you, Roger. I cannot fix the upstream processes to eliminate
generation of these problems. So, I need to deal with the data I got.
I tried various online resources including
https://gist.github.com/mstrimas/1b4a4b93a9d4a158bce4 and tried:
setScales()
set_RGEOS_dropSlivers(TRUE)
set_RGEOS_polyThreshold()
before performing:
gUnaryUnion(p.ranges, id=NULL)
This eliminates the slivers but it also dissolves ALL the polygons to get a
single polygon. That is not something I wanted. I would like to dissolve
only the tiny polygons to the neighboring bigger ones.
Any help is very much appreciated!
Then you must decide which larger polygon (Multipolygon, Polygons object)
they are to belong to. They are Polygon objects within Polygons objects,
and are not entities you can "get at" easily. In your case, all the
entities are Polygons objects, and perhaps by buffering the first Polygons
object northwards, you could include all of the entities. They are far
from trivial in size, and it is not without consequences for downstream
analysis that you are shifting a boundary by tens of km. Really the
upstream data source should provide clean data.
Fixing this would be arbitrary and messy, with consequences for downstream
analysis. In addition, there are empty slivers too. I really advise you to
get back to the data providers, and I wouldn't trust Eliminate either -
it's just making a tidy picture, but one that isn't what the data
processing is saying. I could do it technically, but I wouldn't share that
script with anyone, because it isn't the way to go.
Roger
ᐧ
Postdoctoral Associate
Department of Biology
University of Maryland, College Park
On Mon, Jun 6, 2016 at 9:24 AM, Roger Bivand <roger.biv...@nhh.no> wrote:
On Mon, 6 Jun 2016, Kumar Mainali wrote:
I am trying to use unionSpatialPolygons() of maptools to eliminate sliver
in species range. I want to dissolve tiny sliver polygons in a shapefile
to
bigger polygons as "Eliminate (Data Management)" of ArcMap does. Whereas I
can dissolve polygons that have identical features in the argument "IDs",
I
cannot dissolve tiny polygons based on some threshold in area. In fact,
the
argument "threshold" has no effect in the output.
Indeed, threshold is not passed through, it was used when the function
used gpclib rather than rgeos; I'm minded to deprecate
maptools::unionSpatialPolygons anyway. Note that the data are in
geographical coordinates, which may very well not be appropriate for the
topological operations you are trying to do. Use rgeos::gUnaryUnion
instead, and refer to this thread:
https://stat.ethz.ch/pipermail/r-sig-geo/2015-November/023667.html
for using rgeos::set_RGEOS_polyThreshold() and friends. They do not,
however, try to guess which of the polygons neighbouring the sliver should
get the extra area, so you'll have to think that through yourself.
Googling for lists:R-sig-geo dissolve slivers gets a fair number of hits.
There is always also the upstream question of where the slivers came from,
and whether the resolution is not in the earlier process - generate a map
without slivers that says exactly what you mean, rather than fudging it
afterwards.
Roger
Input data is available here:
https://www.dropbox.com/sh/a0x5bbo9u60y7is/AAB6RjXHFQKZv-i-t4JclF3ba?dl=0
p.ranges <- shapefile{raster}
(IDs <- p.ranges$style_id)
library(maptools)
unionSpatialPolygons(p.ranges, IDs = IDs, threshold = 1.5)
-- Kumar Mainali
Postdoctoral Associate
Department of Biology
University of Maryland, College Park
ᐧ
--
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.biv...@nhh.no
http://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
http://depsy.org/person/434412
--
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.biv...@nhh.no
http://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
http://depsy.org/person/434412
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