[R-sig-Geo] Splitting spatial lines at self intersections (line crossings)

[Michael Sumner]

I think rgeos::gNode will do it,  prior to decomposition, and perhaps
requiring a union beforehand -  there are other options, but I got side
tracked exploring them :)

On Sat, 27 Jan 2018, 09:05 Glenn Stauffer, <gestauffer at gmail.com> wrote:

> I think I have a solution now.
> To recap, I wanted to planarize a line feature (split every line at every
> line crossing), and I wanted an R solution so I could iterate over many
> feature objects. My solution obviously does not strictly use R, but it is
> carried out completely from within R. I used the v.clean algorithm from
> GRASS, passed as an argument to the run_qgis function from the RQGIS
> package. Seems to do exactly what I want.
> My initial problem with v.clean was that I was passing a SpatialLines
> feature instead of a SpatialLinesDataFrame feature to the algorithm.
>
> Glenn
>
> -----Original Message-----
> From: Glenn Stauffer [mailto:gestauffer at gmail.com]
> Sent: Friday, January 26, 2018 12:29 PM
> To: 'Ege Rubak' <rubak at math.aau.dk>
> Subject: RE: [R-sig-Geo] Splitting spatial lines at self intersections
> (line
> crossings)
>
> I think I am really close to a solution using the RQGIS package and the
> GRASS function v.clean. The v.clean function should do exactly what I want,
> but I have not been able to get it to work yet from R, although I could get
> another GRASS functions to work (see example from Jannes Muenchow at
>
> http://r-sig-geo.2731867.n2.nabble.com/v-split-length-GRASS-in-R-td7590611.h
> tml
> <http://r-sig-geo.2731867.n2.nabble.com/v-split-length-GRASS-in-R-td7590611.html>
> ). When I try v.clean, I keep getting an error " Error in
> file.exists(params[[i]]) : invalid 'file' argument", and have not been able
> to ferret out the cause. I am going to start a new thread about this, and
> report back here if I can get resolution.
>
> Glenn
>
> -----Original Message-----
> From: R-sig-Geo [mailto:r-sig-geo-bounces at r-project.org] On Behalf Of Ege
> Rubak
> Sent: Friday, January 26, 2018 12:49 AM
> To: r-sig-geo at r-project.org
> Subject: Re: [R-sig-Geo] Splitting spatial lines at self intersections
> (line
> crossings)
>
> The `psp` class in spatstat consists of individual line segments and
> `selfcrossing.psp` checks whether each individual line intersects one of
> the
> other lines, which happens at all the points in your plot.
>
> If instead you treat each of the two line sequences as a `psp` you can
> check
> find the crossings of the two `psp` objects. I.e., continuing your code
> (with `spatstat` and `maptools` loaded):
>
> L1 <- SpatialLines(list(Lines(list(Line(pts)), "X")))
> L2 <- SpatialLines(list(Lines(list(Line(pt2)), "X")))
> l1 <- as.psp(L1)
> l2 <- as.psp(L2)
> int <- crossing.psp(l1, l2)
>
> This gives you the four intersections between the lines. I don't know of a
> simple way to do the next task in `spatstat`. A useful function if you are
> going to try to write some code is `test.crossing.psp` which gives you a
> logical matrix indicating which segments cross.
>
> Cheers,
> Ege
>
> On 01/25/2018 11:26 PM, Glenn Stauffer wrote:
> > I have a Spatial Lines object I would like to split at every point
> > where the line self-intersects (crosses or touches itself), or
> > anywhere lines touch each other, if there are multiple lines.
> >
> > I've tried using spatstat to convert the SpatialLines to a psp object
> > and then use the selfcrossing.psp function to find the intersection
> > points (see example below). But that seems to identify all the nodes,
> > not just the points where the line crosses. Also, even if it
> > identified only the crossings (which is what I want), I am not sure
> > how to perform the next step and split the lines at those points.
> >
> > So, 1) I need to identify the crossings/touches, and 2) split the
> > lines at those points.
> >
> >
> >
> > Essentially, what I am looking for is an R equivalent to the planarize
> > function in ArgGIS. Is there a relatively easy way to do this in R?
> >
> >
> >
> > Thanks,
> >
> > Glenn
> >
> >
> >
> > Here is an example of what I have tried.
> >
> >
> >
> >
> >
> > pts <- cbind(c(120:123,121,125),c(100,100,104,102,99,98))
> >
> > pt2 <- cbind(c(124,124,123,118,124,125),c(100,97,100,104,106,110))
> >
> > projstr <- "+init=epsg:3071"         # make everything in meters
> >
> > L <-
> > SpatialLines(list(Lines(list(Line(pts),Line(pt2)),"X")),proj4string =
> > CRS(projstr))
> >
> > plot(L)
> >
> > PSP <- as.psp.SpatialLines(L)
> >
> > int <- selfcrossing.psp(PSP)
> >
> > plot(int,add=TRUE,col="red") # identifies more than just the crossings
> >
> >
> >
> >
> >       [[alternative HTML version deleted]]
> >
> > _______________________________________________
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> > R-sig-Geo at r-project.org
> > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >
>
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-- 
Dr. Michael Sumner
Software and Database Engineer
Australian Antarctic Division
203 Channel Highway
Kingston Tasmania 7050 Australia

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