[R-sig-Geo] Single side buffer in R

[Arnaud Mosnier]

I solved part of my problem cutting my original line into segments each
time the line makes an angle larger than a defined value, thus, I have now
relatively straight segments on which I can use my previous methodology
(buffer cutting buffer).
But still, I would appreciate if someone can suggest a solution in order to
obtain directly (i.e without using the previous trick) a left or right
buffer based on a SpatialLine.

Thanks and Happy New Year !!!

Arnaud

2015-12-31 7:45 GMT-05:00 Barry Rowlingson <b.rowlingson at lancaster.ac.uk>:

> What's happened here is that part of the RHS of the buffer (from the
> acute angle segment) has wrapped round so far that it has overlapped
> with the LHS of the buffer from the first segment. So there's an area
> which is both on the LHS of the line and the RHS. What do you want it
> to be?
>
> You could just extend the ends of the last segments to infinity (and
> beyond!!) and use that to cut the buffer, but an odd little end line
> segment could mess this up. Fundamentally I don't think there's a
> clear definition of what to do...
>
> You probably have to decide more formally what constitutes the LHS and
> RHS of your line segment, but I'm not sure in what terms you can make
> this without ambiguity...
>
> On Tue, Dec 29, 2015 at 8:05 PM, Arnaud Mosnier <a.mosnier at gmail.com>
> wrote:
> > Dear spatial object users,
> >
> > Is there a way to make a single side buffer (left or right) based on
> > SpatialLines?
> >
> > Generally I use the trick consisting in cutting the buffer with the
> > expected width with a really small one, but in this case, it does not
> works
> > (see below).
> > I think that a single side buffer tool exists in Geos (but not via
> rgeos).
> > Is that kind of function available in another package?
> >
> > Thanks
> >
> > Arnaud
> >
> >
> > ### Example
> >
> > library(rgeos)
> > library(sp)
> > coords <- data.frame(matrix(c(rep(seq(0,10),each=2)), ncol=2, byrow=T))
> > colnames(coords) <- c("X","Y")
> > coordinates(coords) <- ~ X + Y
> > spL <- SpatialLines(list(as(coords,"Lines")))
> > plot(spL)
> > Buf <- gBuffer(spL, width=2, capStyle="FLAT")
> > Cut <- gBuffer(spL, width=0.001, capStyle="SQUARE")
> > out <- disaggregate(gDifference(Buf,Cut))
> > plot(out, col=c(1,2))
> > plot(spL, col="blue", add=T, lwd=2) # It works
> >
> > coords <- data.frame(matrix(c(0,0,1,1,1.5,0,3,3,4,4), ncol=2, byrow=T))
> > colnames(coords) <- c("X","Y")
> > coordinates(coords) <- ~ X + Y
> > spL <- SpatialLines(list(as(coords,"Lines")))
> > plot(spL)
> > Buf <- gBuffer(spL, width=2, capStyle="FLAT")
> > Cut <- gBuffer(spL, width=0.001, capStyle="SQUARE")
> > out <- disaggregate(gDifference(Buf,Cut))
> > plot(out, col=c(1,2))
> > plot(spL, col="blue", add=T, lwd=2) # Now it is broken because the small
> > buffer does not cut the larger on all its length
> >
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> >
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