Find incircle of a polygon

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.

Thanks,
Etienne

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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

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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 ?

Thanks,
Etienne

Roger Bivand a ?crit :

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

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