[R-sig-Geo] Cut SpatialLines into two Lines when intersecting with SpatialPolyon

[Wouter Steenbeek]

I'm sorry I didn't provide a toy example. I figured it was a simple question and I just couldn't find some well-known function.

I am not very familiar with readWKT(), but I used the sp_intro text to generate the following example:

library(sp)

Sr1 = Polygon(cbind(c(2,4,4,1,2),c(2,3,5,4,2)))
Sr2 = Polygon(cbind(c(5,4,2,5),c(2,3,2,2)))
Sr3 = Polygon(cbind(c(4,4,5,6,6,4),c(5,3,2,2,5,5)))
Sr4 = Polygon(cbind(c(6,6,8,8,6),c(5,2,2,5,5)))
Srs1 = Polygons(list(Sr1), "s1")
Srs2 = Polygons(list(Sr2), "s2")
Srs3 = Polygons(list(Sr3), "s3")
Srs4 = Polygons(list(Sr4), "s4")
SpP = SpatialPolygons(list(Srs1,Srs2,Srs3, Srs4), 1:4)
plot(SpP, col = 5:8) # these are three example neighborhoods

l1 = cbind(c(2,5),c(4,4))
l2 = cbind(c(5,7),c(3,3))
Sl1 = Line(l1)
Sl2 = Line(l2)
S1 = Lines(list(Sl1), ID="a")
S2 = Lines(list(Sl2), ID="b")
Sl = SpatialLines(list(S1,S2))
plot(Sl, add=TRUE) # two streets, which cross neighborhood boundaries

I am looking for a way to recode the lines into separate lines, when they cross a polygon boundary. So the resulting SpatialLines object should, for SpatialLine #1, have two new lines: one from (2,4) to (4,4), and another line from (4,4) to (5,4). For SpatialLine #2 there should also be two new lines created: one from (5,3) to (6,3) and another one from (6,3) to (7,3). Ideally these new lines should rename the IDs of the original SpatialLines in a convenient way.

Outcome would be a new SpatialLines object:

l1_1 = cbind(c(2,4),c(4,4))
l1_2 = cbind(c(4,5),c(4,4))
l2_1 = cbind(c(5,6),c(3,3))
l2_2 = cbind(c(6,7),c(3,3))
Sl1_1 = Line(l1_1)
Sl1_2 = Line(l1_2)
Sl2_1 = Line(l2_1)
Sl2_2 = Line(l2_2)
S1_1 = Lines(list(Sl1_1), ID="a_1")
S1_2 = Lines(list(Sl1_2), ID="a_2")
S2_1 = Lines(list(Sl2_1), ID="b_1")
S2_2 = Lines(list(Sl2_2), ID="b_2")
Sl2 = SpatialLines(list(S1_1,S1_2,S2_1,S2_2))
plot(Sl2, add=TRUE, col=1:4)

So I would like to convert Sl into Sl2 in some automatic manner. I'm thinking I should use "over" (from sp) to get the intersections of lines and polygons and go from there. But I don't quite see how to proceed from there (ie how to automatically "cut" a line into two lines).

By the way, if this is important to know: I'm actually working with SpatialPolygonsDataFrame and SpatialLinesDataFrame. So it would be nice if the two new lines (for example, S1_1 and S1_2) simply inherit the entire dataframe of the parent (in this example, the df attached to S1). My apologies that I don't provide that in the toy example, but I thought I'd keep the example as simple as possible.

Cheers,
Wouter



-----Original Message-----
From: Roger Bivand [mailto:Roger.Bivand at nhh.no] 
Sent: Tuesday, June 03, 2014 11:23 AM
To: Wouter Steenbeek
Cc: 'r-sig-geo at r-project.org'
Subject: Re: [R-sig-Geo] Cut SpatialLines into two Lines when intersecting with SpatialPolyon

On Tue, 3 Jun 2014, Wouter Steenbeek wrote:

> [My apologies if this is a double post. I wrote the first post in 
> HTML, and I think it may not have gotten through.]

Thanks for respecting our guidelines - it is much harder to infect plain text, which is why HTML is not welcome.

>
> I have a SpatialLines(DataFrame) (e.g. streets) and
> SpatialPolygons(DataFrame) (e.g. neighborhoods). When a Line 
> intersects with a polygon boundary, I want to split this Line into two new Lines.
> So: if a street begins in one neighborhood and ends in another 
> neighborhood, I want to remove the original SpatialLine and replace 
> that by two SpatialLines (streets): one street from the starting point 
> until the intersection with the polygon boundary, and another street 
> from the intersection with the polygon boundary to the original end point.
>
> Can someone give me advice on how to do this?

As in most cases, you will increase your chances of a reply if you provide a toy example, for instance using readWKT() in rgeos to generate the objects. Probably all you need is in rgeos, but without a toy example, it is hard to give advice that isn't more like hand-waving.

Roger

>
> Cheers,
> Wouter
>
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--
Roger Bivand
Department of Economics, Norwegian School of Economics, Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: Roger.Bivand at nhh.no