Dear Adrian,

even this solution is pretty cool.

Again, thanks to all who suggested me how to do that.



Da: Adrian Baddeley <>
Inviato: venerd� 14 ottobre 2016 02.14
A: Paolo Piras; Rolf Turner
Cc: r-sig-geo; Ege Rubak
Oggetto: Re: [FORGED] [R-sig-Geo] uniformly sample points on a border of a 

You can use the spatstat function 'edges' to extract the edges of a polygonal 


            W <- letterR    #polygonal window

            E <- edges(W)

            X <- runifpointOnLines(20, E)


            plot(X, add=TRUE)

Prof Adrian Baddeley DSc FAA

Department of Mathematics and Statistics

Curtin University, Perth, Western Australia

From: Paolo Piras <>
Sent: Friday, 14 October 2016 5:22 AM
To: Rolf Turner
Cc: r-sig-geo; Adrian Baddeley; Ege Rubak
Subject: Re: [FORGED] [R-sig-Geo] uniformly sample points on a border of a 

Thanks a lot Rolf!

This is virtually exactly what I need;

I'm very grateful for that

All the best


Da: Rolf Turner <>
Inviato: gioved� 13 ottobre 2016 23.12
A: Paolo Piras
Cc: r-sig-geo;; Ege Rubak
Oggetto: Re: [FORGED] [R-sig-Geo] uniformly sample points on a border of a 

On 14/10/16 07:03, Paolo Piras wrote:
> HI folks,
> I write for a (hopefully) relatively simple question:
> I would need to uniformly sample 1000 or more points **along the border** of 
> a polygon (not within the area enclosed) that is identified by ordered but 
> not equally spaced points; which is the fastest way?
> In a first moment I thought to sample between any pair of consecutive points 
> but, given that starting points are not uniformly distributed, the final 
> result would be very far from a uniform distribution.
> here my polygon:
>   mypol<-round(matrix(c(-13.8447497369687, -3.51439434200449, 
> 6.09494902836977, 6.83498916728338, 9.20403746769121, 15.3061452155498, 
> 18.4050681631565, 15.334153355932, 9.21809033073377, 6.90467983448734, 
> 6.17942233200763, -3.4864867866601, -13.8299219386242, -17.5237987124776, 
> -17.2262670680261, -17.5217563171495, -2.29667185082115, -7.72275721405543, 
> -9.77084968112857, -8.81725304021858, -8.32894043391822, -4.76080777897439, 
> -0.0600572363382094, 4.62779963258511, 8.20771806467615, 8.70484104396818, 
> 9.68531129857718, 7.67574865642846, 2.46081860449754, 1.31152149442131, 
> 0.0845735294613392, -1.11988475144136),ncol=2),digits=2)
>   plot(mypol,asp=1,cex=0)
>   text(mypol[,1],mypol[,2],c(1:nrow(mypol)))
> Thanks in advance for any hints

This can be done reasonably easily using the spatstat package, for some
value of the word "reasonably".  Here's how:

W <- owin(poly=mypol)
m <- cbind(mypol[-nrow(mypol),],mypol[-1,])
m <- rbind(m,c(mypol[nrow(mypol),],mypol[1,]))
m <-
names(m) <- c("x0","y0","x1","y1")
L <- with(m,psp(x0,y0,x1,y1,window=boundingbox(W)))
#X <- runifpointOnLines(1000,L)
X <- runifpointOnLines(100,L)
plot(W,main="Piras's Polygon")

Note that I have just generated 100 uniform points, r.t. 1000, so that
the resulting plot is a little less cluttered.

There may be a sexier way of accomplishing your desideratum; I have
cc-ed this email to my co-authors Adrian and Ege who may come up with
better ideas.


Rolf Turner

Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

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