[R-sig-Geo] different samples size for each polygon and raster question

[Arien Lam]

On Wed, April 8, 2009 21:33, Michael Denslow wrote:
>
> Dear r-sig-geo,
>
> I would like to generate random points within each of several polygons. I
> would like for the sample size of each point sample to be different for
> each polygon. I would then like to overlay a grid or raster to visualize
> these point over the entire study area. In other words have the cell value
> be based on the number of points that fall into each of the grid cells. I
> have included a reproducible example below to illustrate my problem.
>
> library(sp)
> Sr1 = Polygon(cbind(c(2, 4, 4, 1, 2), c(2, 3, 5, 4, 2)))
> Sr2 = Polygon(cbind(c(5, 4, 2, 5), c(2, 3, 2, 2)))
> Srs1 = Polygons(list(Sr1), "s1")
> Srs2 = Polygons(list(Sr2), "s2")
> SpP = SpatialPolygons(list(Srs1, Srs2), 1:2)
>
> attr = data.frame(a = 1:2, b = 2:1, row.names = c("s1",
> 	 "s2"))
> SrDf = SpatialPolygonsDataFrame(SpP, attr)
>
> plot(SpP)
> ## try to get same number in each polygon
> # this seems to generate 10 total not 10 for each
> # the help file says 'use subsetting to vary sampling intensity'
> # I am not sure how to do this
> points(spsample(SrDf,n=10,type="random"), pch=3, cex=.5)

I don't know how to do it directly, but the following works (though I
would have expected that type='stratify' would also do something like
this)

ptsin1 <- spsample(SrDf[1,],n=10,type="random")
ptsin2 <- spsample(SrDf[2,],n=10,type="random")

plot(SrDf)
points(ptsin1,col='blue')
points(ptsin2,col='red')

allpts <- SpatialPoints(rbind(coordinates(ptsin1),coordinates(ptsin2)))
points(allpts,pch=3)

> ## 3 try to get unique sample size for each based on attribute data
> # this does not work
> points(spsample(SrDf,n=SrDf[[2]],type="random"), pch=3, cex=.5)

You could work the subsetting to a function, e.g. (not optimized):

stratsample <- function(spPolDf, neach=c(10,100)) {
   k = length(spPolDf at polygons)
   allcoors=c(0,0)
   for(i in 1:k) {
     ni = neach[i]
     ptsi <- spsample(spPolDf[i,],n=ni, type="random")
     allcoors = rbind(allcoors,coordinates(ptsi))
     }
   SpatialPoints(allcoors[-1,])
   }

Cheers, Arien (skipping the grid question)

>
> ## generate a grid file?
> ## use overlay to combine grid with points?
>
>
> Thanks in advance for any help you can provide,
> Michael
>
> Michael Denslow
>
> Graduate Student
> I.W. Carpenter Jr. Herbarium [BOON]
> Department of Biology
> Appalachian State University
> Boone, North Carolina U.S.A.
>
> -- AND --
>
> Communications Manager
> Southeast Regional Network of Expertise and Collections
> sernec.org
>
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>


-- 
drs. H.A. (Arien) Lam (Ph.D. student)
Department of Physical Geography
Faculty of Geosciences
Utrecht University, The Netherlands