clipping polygons, overlaying big polygon on polygons
On Sun, 21 Feb 2010, Markus Loecher wrote:
I should have been much more specific with my question and supplied an
example.
In the meantime I believe I found the solution in the PBSmapping package,
this time I will provide an example that illustrates what I am trying to
achieve:
#Get the boundaries for Manhattan:
library(maps);
library(PBSmapping);
Manhattan <- map('county', 'new york,new york', plot=F);
#define a few points in Manhattan and compute the corresponding Voronoi
regions:
XY <- cbind.data.frame( PID = 1, POS = 1:10,
X =
c(-73.97220,-73.95456,-73.97906,-73.98685,-73.99968,-73.96716,-73.97127,-73.96524,-73.98322,-74.00587),
Y =
c(40.74983,40.77402,40.75022,40.73952,40.72842,40.75677,40.75112,40.75513,40.76271,40.74026)
);
polys <- calcVoronoi(as.PolySet(XY, projection="LL") );
#plot the Voronoi regions:
plotMap(polys)
#It would be a much more meaningful plot to clip these Voronoi regions to
the Manhattan boundaries:
m <- as.PolySet (cbind.data.frame(PID = 1, POS = 1:length(Manhattan$x), X =
Manhattan$x, Y = Manhattan$y))
plotMap(joinPolys(m, polys, "INT" ), xlim = Manhattan$range[1:2], ylim =
Manhattan$range[3:4]);
So the key operation I was looking for was simply the intersection provided
by the function (joinPolys(..., "INT" ) from PBSmapping.
Yes, but sadly this does not solve the problem if you are going to use the output beyond hobby or education, as PBSmapping also uses the GPC code. Attempts were made to persuade the author to license his code in a more helpful way, but he has left his employing university, and they have retained the rights to his software. Although the PBSmapping maintainers claim to have permission to ship GPC under GPL, I would try to stay as far away from it as possible. Once code using it gets into the workflow, it is really hard to remove. The spatstat maintainers are working hard to make users aware of the problems, which will not go away, I'm afraid. If you'd like to try rgeos, please checkout or download from R-Forge. The gpc.poly-shadowing classes and methods now provide an intersection operation. Kicking the GPC habit is hard, but must be done, otherwise we risk spreading downstream dependencies. Before long CRAN may have colour-coded license dependencies, and organisations can already block the installation of CRAN packages that have non-free (as in speech) licenses or that depend upstream on non-free packages. The akima package is another worry, but has a much clearer license that does permit research under noncommercial use. GPC only permits "non-commercial use of GPC (for example: private / hobbyist / education)" while "commercial research" requires a commercial use license. Any contract research could fall under "commercial research", because it generates income. I would be very grateful for help with rgeos ... Roger
Thanks again, Markus On Sun, Feb 21, 2010 at 8:26 AM, Roger Bivand <Roger.Bivand at nhh.no> wrote:
On Sun, 21 Feb 2010, Markus Loecher wrote: Dear all,
I apologize if this is too simple a question as it does appear to be a basic GIS operation but I have not found the appropriate function that performs a clipping of polygons into boundaries given by a (larger "outer") polygon. I tried overlay() from package sp and clipPolys from package PBSmapping but none converted the "inner" polygons into new ones laying inside the boundaries given by the outer polygon.
Maybe a basic GIS operation should be done in a GIS? Otherwise, please feel free to consult threads on the license problems of gpclib, and about the R-forge rgeos package. If you'd like to contribute such an intersection (I think you want the intersection between an arbitrary SpatialPolygons object and a simple SpatialPolygons object for the outer polygon) function tool to rgeos, or at least a sketch of how it might work, that would be helpful. Roger
Any help would be greatly appreciated,
Thanks,
Markus
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-- Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: Roger.Bivand at nhh.no
Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: Roger.Bivand at nhh.no