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

[Paul Hiemstra]

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