# [R] x, y for point of intersection

MacQueen, Don macqueen1 at llnl.gov
Wed Nov 23 23:03:42 CET 2011

```The function crossing.psp() in the spatstat package might be of use.
Here's an excerpt from its help page:

crossing.psp             package:spatstat              R Documentation
Crossing Points of Two Line Segment PatternsDescription:
Finds any crossing points between two line segment patterns.
Usage:
crossing.psp(A,B)
-Don

--
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062

On 11/22/11 12:48 PM, "R. Michael Weylandt <michael.weylandt at gmail.com>"
<michael.weylandt at gmail.com> wrote:

>If it's a one off, the identify() function might be of help -- if you
>need something algorithmic it's harder due to floating point stuff and
>sampling frequencies. Let me know if that's the case.
>
>Michael
>
>On Nov 22, 2011, at 3:40 PM, Monica Pisica <pisicandru at hotmail.com> wrote:
>
>>
>>
>>
>> Hi everyone,
>>
>>
>>
>> I am trying to get a point of intersection between a
>> polyline and a straight line Š.. and get the x and y coordinates of
>>this point.
>> For exemplification consider this:
>>
>>
>>
>>
>>
>> set.seed(123)
>>
>>
>>
>> k1 <-rnorm(100, mean=1.77, sd=3.33)
>>
>>  k1 <- sort(k1)
>>
>> q1 <- rnorm(100, mean=2.37, sd=0.74)
>>
>> q1 <- sort(q1, decreasing = TRUE)
>>
>> plot(k1, q1, xlim <- c((min(k1)-5), (max(k1)+5)),
>> type="l")
>>
>>
>>
>> ya <- 2
>>
>> xa = -5
>>
>> yb=4
>>
>> xb=12
>>
>>
>>
>> lines(c(xa, xb), c(ya, yb), col = 2)
>>
>>
>>
>> # I want to get the x and y coordinates of the
>> intersection of the 2 lines Š.
>>
>>
>>
>> m <- (ya-yb)/(xa-xb)
>>
>> b <- ya-m*xa
>>
>> ln <- loess(q1~k1)
>>
>> lines(ln)
>>
>>
>>
>> It is clear that the x, y will satisfy both linear
>> equations, y = m*x + b and the ln polyline Š.. but while I can
>>visualize the
>> equation of the straight line ­ I have problems with the polyline. I
>>will appreciate
>> any ideas to solve this problem. I thought it a trivial solution but it
>>seems I
>> cannot see it.
>> Thanks,
>> Monica
>>
>>
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>>http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>______________________________________________
>R-help at r-project.org mailing list
>https://stat.ethz.ch/mailman/listinfo/r-help
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.

```