[R-sig-Geo] clipping polygons, overlaying big polygon on polygons

[Roger Bivand]

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
>>>
>>>        [[alternative HTML version deleted]]
>>>
>>> _______________________________________________
>>> R-sig-Geo mailing list
>>> R-sig-Geo at stat.math.ethz.ch
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>
>>>
>> --
>> 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