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

[Ege Rubak]

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
> 
>                                  
> 
> 
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