[R-sig-Geo] how to do randomly sampling in raster layer

[Robert J. Hijmans]

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.hiemstra at 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.mao at 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 at stat.math.ethz.ch
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>
>>>
>>
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>
>
> --
> 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
>
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