Dear Mao, If r is a RasterLayer, you can also do
library(dismo) xy <- randomPoints(r, n=100) plot(r) points(xy) Best, Robert On Tue, Aug 3, 2010 at 2:51 AM, Paul Hiemstra <p.hiems...@geo.uu.nl> wrote: > On 08/03/2010 11:15 AM, caspar hallmann wrote: >> >> Dear Mao, >> >> You can use function rpoint from spatstat, after converting your >> raster object into a pixel image. >> >> consider the following: >> >> library(raster) >> library(spatstat) >> library(maptools) >> library(sp) >> >> # An arbitrary raster >> r<- raster(system.file("external/test.grd", package="raster")) >> # plot it >> image(r) >> >> # convert to SpatialGridDataFrame >> r.spgrd<-as(r,"SpatialGridDataFrame") >> > > I would consider converting it to SpatialPointsDF instead. > > r.spgrd<-as(r,"SpatialPointsDataFrame") > > Now you can eliminate the NA value: > > r.spgrd = r.spgrd[!is.na(r.spgrd[[1]]),] > > In stead of rpoint you can also use sample: > > selectedPoints = sample(1:length(r.spgrd[[1]]), 1000) > r.sampled = r.spgrd[selectedPoints,] > > cheers, > Paul >> >> r.spgrd$constant<-ifelse(is.na(r.spgrd[[1]]),NA,1) >> # ...this to ensure an equal weight to each non-NA cell >> >> # convert to im >> r.im<-as.im(r.spgrd["constant"]) >> >> # sample points according to constant >> r.points<-rpoint(100,r.im) >> >> # plot the random points >> points(r.points) >> >> #..to get the coordinates >> as.data.frame(r.points) >> >> Good Luck! >> Caspar >> >> >> On Tue, Aug 3, 2010 at 10:43 AM, Mao Jianfeng<jianfeng....@gmail.com> >> wrote: >> >>> >>> Dear r-sig-geoers, >>> >>> I want to randomly sample n points from regions of a raster layers, >>> the cells denoted as "NA" is not >>> included in this sampling process. And, I want to got the longitude >>> and latitude of the sampled points. >>> >>> I checked the manual of raster package, I found several functions is >>> relative to my purpose. I tried them all, but I failed. >>> >>> Can it can be done by raster functionalities. Could you please refer >>> me to the right direction? >>> >>> I expect to hearing from you. Your helps are very valuable for a >>> Chinese who can not reach helps nearby. >>> >>> Best, >>> >>> Sincerely, >>> Mao Jian-Feng >>> >>> _______________________________________________ >>> R-sig-Geo mailing list >>> R-sig-Geo@stat.math.ethz.ch >>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo >>> >>> >> >> _______________________________________________ >> R-sig-Geo mailing list >> R-sig-Geo@stat.math.ethz.ch >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo >> > > > -- > Drs. Paul Hiemstra > Department of Physical Geography > Faculty of Geosciences > University of Utrecht > Heidelberglaan 2 > P.O. Box 80.115 > 3508 TC Utrecht > Phone: +3130 253 5773 > http://intamap.geo.uu.nl/~paul > http://nl.linkedin.com/pub/paul-hiemstra/20/30b/770 > > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@stat.math.ethz.ch > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo