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
data smallmap;
set mymap;
where density<4;
or
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.
-Michael
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
algorithm.
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.).
Roger
HTH
David
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.
thanks,
-Michael
--
Michael Friendly Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street http://www.math.yorku.ca/SCS/friendly.html
Toronto, ONT M3J 1P3 CANADA
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