[R-sig-Geo] Overlay of SpatialGrid/Raster and polygons

[Zia Ahmed]

I  am trying to install "raster" package, but  it is not available  in 
this site.  http://R-Forge.R-project.org .

Zia

Robert J. Hijmans wrote:
> Hi Ned,
>
> Here is an approach to get values from a RasterStack to all cells in
> each polygon:
>
> library(raster)
> # a polygon
> data(meuse.riv)
> pol <- SpatialPolygons(list(Polygons(list(Polygon(meuse.riv)), "x")))
> # a raster
> r1 <- raster(system.file("external/test.ag", package="sp"))
> r2 <- sqrt(r1)
> # a stack
> s <- stack(r1, r2)
>
> # convert the polygon to a RasterLayer
> rr <- polygonsToRaster(pol, s)
>
> par(mfrow=c(1,2))
> plot(r1)
> plot(pol, add=TRUE)
> plot(rr)
>
> # extract points that are not NA
> pts <- rasterToPoints(rr) # see additional arguments to select a
> subset (useful for very large rasters)
> # perhaps subsample your points
> sampl <- sample(1:length(pts[,1]), min(100, length(pts[,1])))
> pts <- pts[sampl,]
>
> v <- xyValues(s, pts[,1:2])
> # if you have mutiple polygons, bind the polygon ID to the raster values:
> v <- cbind(pts[,3], v)
>
> # You could also sample points with spsample
> pts <- spsample(pol, 100,  "random")
> # remove duplicate cells
> cells <- unique(cellFromXY(s, pts))
> v2 <- cellValues(s, cells)
>
> # or sample from a SpGDF if you can create it from the RasterLayer (if
> it is not too big)
> spdf <- as(rr, 'SpatialGridDataFrame')
> pts <- spsample(spdf, 100,  "random")
> v3 <- xyValues(s, pts)
>
>
> For extremely large rasters, rasterToPoints could fail. If so, first
> create a RasterLayer with random values with calc and "fun=runif" and
> make values F for e.g x > 0.01, overlay that with rr, and try
> rasterToPoints again...
>
> Hth, Robert
>
>
>
>
> On Mon, Nov 23, 2009 at 12:16 PM, Ned Horning <horning at amnh.org> wrote:
>   
>> Greetings,
>>
>> I am looking for a way to use R to extract pixel values (currently in a
>> large RasterStack object) that fall under polygons (currently a
>> SpatialPolygonsDataFrame object). I seem to recall a discussion about using
>> overlay to do this but I can't find a method that would work.
>>
>> Any insight would be appreciated.
>>
>> Ned
>>
>> _______________________________________________
>> 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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>   
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