[R] Polygon-like interactive selection of plotted points

[Clint Bowman]

Roger,

Just for fun I tried your script--nothing wrong with the script, but I
created a seven sided polygon by clicking on seven points and letting
getpoly complete the figure.  At the end I notice that only three of the
seven vertices are coded as being inside the ploygon (the blue points.)

I'd send you a screen dump but I haven't gotten xwd to work with Exceed.
Also I haven't checked any docs to see whether this is a known problem but
suspect that Marc could be surprised by the behavior.

Clint

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On Wed, 26 Apr 2006, Roger Bivand wrote:

> On Wed, 26 Apr 2006, Marc Schwartz (via MN) wrote:
>
> > On Wed, 2006-04-26 at 18:13 +0100, Florian Nigsch wrote:
> > > [Please CC me for all replies, since I am not currently subscribed to
> > > the list.]
> > >
> > > Hi all,
> > >
> > > I have the following problem/question: Imagine you have a two-
> > > dimensional plot, and you want to select a number of points, around
> > > which you could draw a polygon. The points of the polygon are defined
> > > by clicking in the graphics window (locator()/identify()), all points
> > > inside the polygon are returned as an object.
> > >
> > > Is something like this already implemented?
> > >
> > > Thanks a lot in advance,
> > >
> > > Florian
> >
> > I don't know if anyone has created a single function do to this (though
> > it is always possible).
> >
> > However, using:
> >
> >   RSiteSearch("points inside polygon")
> >
> > brings up several function hits that, if put together with the above
> > interactive functions, could be used to do what you wish. That is, input
> > the matrix of x,y coords of the interactively selected polygon and the
> > x,y coords of the underlying points set to return the points inside or
> > outside the polygon boundaries.
>
> This sequence from the splancs package should do it:
>
> library(splancs)
> set.seed(20060426)
> xy <- cbind(x=runif(100), y=runif(100))
> plot(xy)
> poly <- getpoly() # this gets the polygon on-screen
> plot(xy)
> polygon(poly)
> io <- inout(xy, poly)
> # this returns a logical vector for points in the polygon
> points(xy[io,], pch=16, col="blue")
>
> Roger
>
> >
> > Just as an FYI, you might also want to look at ?chull, which is in the
> > base R distribution and returns the set of points on the convex hull of
> > the underlying point set. This is to some extent, the inverse of what
> > you wish to do.
> >
> > HTH,
> >
> > Marc Schwartz
> >
>
> --
> 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
>
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