[R-sig-Geo] Find incircle of a polygon

[Roger Bivand]

On Wed, 22 Apr 2009, Etienne Bellemare Racine wrote:

> Thanks Roger it helped me a lot.
> Now I would like to iteratively find the incircle in a polygon until a 
> certain radii (i.e. as if I was stuffing balls in my polygon, from the 
> biggest to the smallest). So I have to subtract that circle to my polygon.
> Should I subtract it from my polygon by adding an inside loop (setting hole 
> to TRUE, is it that simple) or can I substract it directly from the owin 
> object ?

I guess that others may have ideas here - this is uncharted territory. 
You'd be using gpclib through spatstat or maptools (or PBSmapping) to 
construct a new object with a hole, so trial and error will get you some 
of the way. Ideas, anyone?

Roger

>
> Thanks,
> Etienne
>
> Roger Bivand a écrit :
>> On Wed, 22 Apr 2009, Etienne Bellemare Racine wrote:
>> 
>>> Hi,
>>> I'm looking for a way to find the largest circle fitting in an irregular
>>> polygon. This is not the real definition of an incircle (as it has to be
>>> tangent to all edges, but it's a more synthetic title).
>>> Spatstat package has an incircle function, but designed for an owin
>>> class object. Before trying to adapt the function to my needs, so I can
>>> input a shapefile, I would be happy to get some advices (if any
>>> available), or another package to look at.
>> 
>> You can read (polygon) shapefiles into SpatialPolygon* objects using 
>> readShapeSpatial in maptools and readOGR in rgdal. Once there, coerce to an 
>> owin object with methods in maptools, something like:
>> 
>> x <- readOGR(".", "myshape")
>> ox <- as(as(x, "SpatialPolygons"), "owin")
>> 
>> should do it, but watch for topology checking in spatstat. Spatstat ships 
>> with a vignette on using shapefiles, see for an online link:
>> 
>> http://cran.r-project.org/web/packages/spatstat/vignettes/shapefiles.pdf
>> 
>> which is up to date, humourous, and should tell you what you need to know. 
>> Using the advice there should resolve any topology problems. If you are 
>> stuck after trying things from there, please follow up on this list.
>> 
>> Hope this helps,
>> 
>> Roger
>> 
>>> 
>>> Thanks,
>>> Etienne
>>>
>>>     [[alternative HTML version deleted]]
>>> 
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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