On Fri, Jul 13, 2012 at 1:25 AM, Lorenzo Cattarino
<l.cattar...@uq.edu.au> wrote:
> Hi R users,
>
> I would like to be able to calculate how many cells of a raster, and their
> values (1 or 0), lie at a distance r from a particular cell in the raster. In
> other words, I need to calculate the number and values of the cells lying on
> a ring with radius r and centred in x0 and y0.
>
> This is as far I got:
>
>
>
> require(spatstat)
stop there!
You might want to use the raster package, especially if you are using
geo-referenced data:
> library(raster);library(sp)
> z = raster(matrix(runif(10000),100,100))
> projection(z)="+init=epsg:4400"
> pt = cbind(.2,.6)
> zd = distanceFromPoints(z,pt)
> circ = zd>.4 & zd<.41
> plot(circ)
Note the circle is here computed as all grid cells within .4 and
.4+cell size distance. Then you can easily get the values in z that
correspond to circ==TRUE...
> summary(z[circ])
Min. 1st Qu. Median Mean 3rd Qu. Max.
0.002594 0.230600 0.479600 0.475500 0.690800 0.985400
Another way would be to rasterize a circular line:
> cline = cbind(.2+.4*sin(seq(0,2*pi,len=100)),.6+.4*cos(seq(0,2*pi,len=100)))
> L = SpatialLines(list(Lines(Line(cline),"C")))
> plot(z)
> plot(L,add=TRUE)
> LR = rasterize(L,z)
> plot(LR)
> plot(L,add=TRUE)
but this is a bit slower, and slightly different:
> summary(z[LR])
Min. 1st Qu. Median Mean 3rd Qu. Max.
0.003472 0.239600 0.474300 0.476800 0.692900 0.997500
Barry
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