# [R] problem with pip2d() from ptinpoly

Boris Steipe boris.steipe at utoronto.ca
Sat Apr 19 00:48:45 CEST 2014

```Apparently it matters whether your polygon is defined clockwise or counterclockwise.

A point outside your triangle is recognized ...
> q2 <- matrix(c(594893.0,115435.0), ncol=2, byrow=T)
> pip2d(Vertices = verts, Queries = q2)
[1] 1

... and defining the triangle in counterclockwise sense gives the expected behaviour.
> v2 <- matrix(c(594891,115309,594891,117201,594444,117201), ncol=2, byrow=T)
> pip2d(Vertices = v2, Queries = query)
[1] 1

Cheers,
B.

On 2014-04-18, at 6:00 PM, Waichler, Scott R wrote:

> Hi,
>
> pip2d() doesn't seem to work correctly for me.  I have a plot of a triangle that a query point fits inside, but the point is defined as outside the polygon by pip2d.
>
> library(ptinpoly)
> verts <- matrix(c(594891,115309,594444,117201,594891,117201), ncol=2, byrow=T)
> query <- matrix(c(594885.0,115435.0), ncol=2, byrow=T)
> pip2d(Vertices = verts, Queries = query)  # result = -1
> # contrary to -1 output of pip2d, plot shows point lies within triangle
> plot(c(594400, 595000), c(115000, 117500), type="n")
> polygon(verts, border="red")
> points(x=query[,1], y=query[,2], col="blue")
>
> Scott Waichler
> Pacific Northwest National Laboratory
> Richland, WA, USA
>
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