Cut SpatialLines into two Lines when intersecting with SpatialPolyon

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

--
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