You can't specify it in rthin, which I can understand for the simple reason that this violates the independence of the sampling. If you put a fixed limit on your resampling, the chance of being selected depends on when some point is selected.
You could do it by hacking the object : rthin.exact <- function(x,n){ l <- x$n i <- sample(1:l,n,replace=F) x$n <- n x$x <- x$x[i] x$y <- x$y[i] return(x) } data(redwood) plot(redwood, main="thinning") # keep 30 points Y <- rthin.exact(redwood, 30) points(Y, col="green", cex=1.4) Cheers Joris On Sun, Jun 6, 2010 at 12:26 AM, Tom_R <tom.richard...@bristol.ac.uk> wrote: > > Fellow Users, > > In Spatstat, if you have a ppp object with, say 100 points, how do you > create another ppp containing a random selection, N, of the original? > > You can tell the function 'rthin' to make a sub-selection of points, i.e. > "every point is removed with probability 0.5" but you CAN'T set N exactly... > > I'm sure this should be easy... > > Ta! > > Tom > -- > View this message in context: > http://r.789695.n4.nabble.com/spatstat-sub-selection-tp2244619p2244619.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Ghent University Faculty of Bioscience Engineering Department of Applied mathematics, biometrics and process control tel : +32 9 264 59 87 joris.m...@ugent.be ------------------------------- Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.