[R-sig-Geo] project2segment -> connecting SpatialLines: some connect, some not

[Mathieu Rajerison]

Hi,


This post could have followed the one about shortest path but I preferred
to make a separate post to make things clearer.

Given a set of points and a set of lines, I tried to make a graph of it.
The operation consisted, firstly, in getting the lines that connect each
point to the nearest point on the segment pattern

For this, I used spatstat::project2segment. spatstat is really a wonderful
package. I think it can be useful for snapping line operations.

I'm stuck because I manage to create some lines between the points and the
segments but some lines connect to the network while others don't.

When they don't, the distance between the end connecting segment point and
the network is extremely small. I put an image of it as attached file.

Here is the code, for those who'd like to give it a try and, eventually,
give an issue. Maybe I did it in a wrong way. It's based on Barry's initial
code.

library(spatstat)
library(rgeos)
library(maptools)

makeNest <- function(n) {
    ### generate a bunch of SpatialLines on (0,1) square
    xy = cbind(runif(n), runif(n), runif(n), runif(n))
    lines = Lines(apply(xy, 1, function(z) {
        Line(rbind(z[1:2], z[3:4]))
    }), ID = 1)
    SpatialLines(list(lines))
}

connection2Lines <- function(lines, pts){

  sourcePts <- as(pts,"ppp")
  projectedPts <- project2segment(sourcePts, as(lines, "psp"))$Xproj
  bb <- union.owin(bounding.box(sourcePts), bounding.box(projectedPts))
  cnnLines <- as(psp(sourcePts$x, sourcePts$y, projectedPts$x,
projectedPts$y, window=bb), "SpatialLines")
  cnnLines <- spChFIDs(cnnLines, sapply(cnnLines at lines, function(x)
paste("connect", x at ID, sep="_")))
  proj4string(cnnLines) <- proj4string(lines)

  return(cnnLines)
}

n <- 5
pts <- SpatialPoints(cbind(runif(n), runif(n)))

gg = makeNest(10)

cnnLines <- connection2Lines(gg, pts)

plot(gg)
plot(cnnLines, col="red", add=T)

any(gIntersects(cnnLines, gg, byid=TRUE)==FALSE)
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