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

[Glenn Stauffer]

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 ). 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]]
> 
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>

_______________________________________________
R-sig-Geo mailing list
R-sig-Geo at r-project.org
https://stat.ethz.ch/mailman/listinfo/r-sig-geo