Dear Adrian, even this solution is pretty cool.
Again, thanks to all who suggested me how to do that. Best Paolo ________________________________ Da: Adrian Baddeley <adrian.badde...@curtin.edu.au> 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 polygon You can use the spatstat function 'edges' to extract the edges of a polygonal window. Example: W <- letterR #polygonal window E <- edges(W) X <- runifpointOnLines(20, E) plot(E) plot(X, add=TRUE) Prof Adrian Baddeley DSc FAA Department of Mathematics and Statistics Curtin University, Perth, Western Australia ________________________________ From: Paolo Piras <paolo.pi...@uniroma3.it> 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 polygon Thanks a lot Rolf! This is virtually exactly what I need; I'm very grateful for that All the best Paolo ________________________________ Da: Rolf Turner <r.tur...@auckland.ac.nz> Inviato: gioved� 13 ottobre 2016 23.12 A: Paolo Piras Cc: r-sig-geo; adrian.badde...@curtin.edu.au; Ege Rubak Oggetto: Re: [FORGED] [R-sig-Geo] uniformly sample points on a border of a polygon 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: require(spatstat) W <- owin(poly=mypol) m <- cbind(mypol[-nrow(mypol),],mypol[-1,]) m <- rbind(m,c(mypol[nrow(mypol),],mypol[1,])) m <- as.data.frame(m) names(m) <- c("x0","y0","x1","y1") L <- with(m,psp(x0,y0,x1,y1,window=boundingbox(W))) set.seed(42) #X <- runifpointOnLines(1000,L) X <- runifpointOnLines(100,L) plot(W,main="Piras's Polygon") plot(X,add=TRUE) 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. cheers, Rolf Turner -- Technical Editor ANZJS Department of Statistics University of Auckland Phone: +64-9-373-7599 ext. 88276 [[alternative HTML version deleted]]
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