[R-sig-Geo] thinning a SpatialPolygonsDataFrame
friendly at yorku.ca
Wed Nov 18 20:18:10 CET 2009
I think, for my application, I'd be happy with the D-P polyline
simplification algorithm, because that is what is used
in SAS (worked well), and I don't think there are unusual topologies in
my map of France that would be so severely
out of whack as to lead to *significant* visual artifacts. In fact, you
might well expect some artifacts from any visual
thinning, but it's a matter of the tradeoff in the way the thinned map
is used in a visualization. Mark Monmonier's
US Visibility Map might be an extremely thinned, but highly useful example.
For R spatial analysis, I think this is worth pursuing and integrating
into sp methods. In SAS, proc greduce works simply
by adding another variable, density, to the (x,y) coordinates of the
spatial polygons, density %in% 1:5, where density==5
is the full map. It is then a simple matter to subset the polygon
outlines by saying
proc gmap map=mymap(where=(density<4));
Meanwhile, I can't see easily how I could use shapefiles::dp() to thin
my Guerry::gfrance maps, because the documentation is,
shall we say, somewhat thin.
Roger Bivand wrote:
> On Wed, 18 Nov 2009, Pinaud David wrote:
>> Hi Michael,
>> maybe you should try the function dp() in the package shapefiles that
>> is an implementation of the Douglas-Peucker polyLine simplification
> Note that its help page does warn that it is not topology-preserving,
> that is that lines are generalised, but that coincident lines
> (boundaries of neighbouring polygons) may be generalised differently.
> GEOS offers a topology-preserving line generalisation facility, which
> ought to take longer but do better than dp(), because it will not lead
> to visual artefacts (overlapping polygons, interpolygon slivers, etc.).
>> Michael Friendly a écrit :
>>> The Guerry package contains two maps of france (gfrance, gfrance85)
>>> which are quite detailed and large in size (~900K).
>>> In writing a vignette for the package, there are quite a few figures
>>> that use the map multiple times in a layout, and
>>> consequently result in huge file sizes for the .PDF files created.
>>> For these purposes, the map need not be nearly
>>> so detailed.
>>> I'm wondering if there is a facility to "thin" the map by drawing it
>>> at a lower density of lines in the polygon regions.
>>> When I was working with SAS, there was a GREDUCE procedure that did
>>> this nicely.
Michael Friendly Email: friendly AT yorku DOT ca
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