handling circles

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Dear All

I need to handly some circular-shaped polygons on R. First I adapted the
code from Barry Rowlingson to create a function that create the list of X-Y
points which create a circle with a specified radii. After that I need to
save each circle as shapefile with writeOGR. My last task is to identify the
region that are overlaped by three circles.

I tryed to convert my points to Polygon, and after to SpatialPolygons, but
got error. Can anyone help me on this task?

Kind regards,

miltinho

createCircle <- function(x,y,r,start=0,end=2*pi,nsteps=20,...){
## adapted from Barry Rowlingson (2004)
  rs <- seq(start,end,len=nsteps)
  xc <- x+r*cos(rs)
  yc <- y+r*sin(rs)
  polygon(xc,yc,...)

  my.pol<-cbind(xc,yc)
  return<-my.pol
}

plot(1:10, type="n")
my.circle1<-createCircle(5,5,2, col="blue")
my.circle2<-createCircle(7,6,1,col="transparent")
my.circle3<-createCircle(7,4.5,1.5,col="transparent")

my.circle1.Sr <- Polygon(my.circle1)
The error message here was:

Error in validityMethod(object) : ring not closed

so you need a line closing it in your function:

createCircle <- function(x,y,r,start=0,end=2*pi,nsteps=20,...){
    rs <- seq(start,end,len=nsteps)
    xc <- x+r*cos(rs)
    yc <- y+r*sin(rs)
    my.pol<-cbind(xc,yc)
    my.pol <- rbind(my.pol, my.pol[1,])
    my.pol
}
From there:
my.circle1.Sr <- Polygons(list(Polygon(my.circle1)), ID="1")
...
my.circle1.Sr.SpatPol <- SpatialPolygons(list(my.circle1.Sr))

where the list would contain your collection of circles as a Polygon 
object inside a Polygons object, before assembling them in a 
SpatialPolygons object.

rn <- sapply(slot(my.circle1.Sr.SpatPol, "polygons"), function(x) slot(x,
   "ID"))
df <- data.frame(rn=rn, row.names=rn)
all.circles <- SpatialPolygonsDataFrame(my.circle1.Sr.SpatPol, data=df)

To output it as a shapefile, it needs a data frame too, so you will need 
to make something like all.circles.

Thanks for a well-structured question, it makes answering so much more 
straight-forward.

Roger
my.circle1.Sr.SpatPol <- SpatialPolygons(my.circle1.Sr)

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Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
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e-mail: Roger.Bivand at nhh.no