[R-sig-Geo] from x,y plot to a raster with Raster package?

[Robert J. Hijmans]

Josep,

I take it you have three variables, and want to create a raster for
the third variable, based on the other two, that are also represented
as rasters. If so, you can

# make a three column data.frame
d <- data.frame( x=c(23,25,27), y=c(33,34,35) , z=c(0.1,0.2,0.3))

# your two rasters, representing x and y
library(raster)
x <- y <- raster ( nrows=10, ncols=10, crs=NA)
values(x) <- runif(n=100, min=20, max=28)
values(y) <- runif(n=100, min=31, max=36)

# combine them
s <- stack(x, y)

# round them to match to x and y in 'd'
s <- round(s)

# compute z, based on x and y using 'subs' (equivalent to base::merge)
z <- subs(s, d, by=1:2)

# test
x <- stack(s, z)
# should have the same values as 'd'
unique(na.omit(values(x)))

Robert

On Sun, Jul 19, 2015 at 10:10 AM, Josep M Serra diaz
<pep.bioalerts at gmail.com> wrote:
> Hi everyone,
>
> I am wondering if I can speed up the process of transforming from  a 'plot'
> (or matrix) to a raster.
>
> The idea is that I have a matrix with x,y,z values. and I have a raster
> with spatial data for x and for y, and I want to get a raster of z.
>
> I would know how to do it on a cell-by-cell basis, but I was hoping there
> was a more efficient way, since I have more than 14Milion Cells.
>
> I left an simple example below to see the not very efficient way in which I
> was dealing with it, hoping that someone will let me know that I have a
> better chance trying with a function in the raster package that can do that
> very easily. So far, I have tried with calc, but it is saying that 'I
> cannot use this function'.
>
>
>
>
> ###### SIMPLE EXAMPLE
>
> ##dimensions of the matrix
> mat <- list( x=c(23,25,27), y=c(33,34,35) , z=c(0.1,0.2,0.3))
> mat2 <- matrix (data = mat$z,nrow=3, ncol=3,dimnames = mat[c(1,2)])
>
> #input raster x axis
> r1 <- raster ( nrows=10, ncols=10, crs=NA)
> r1[] <- runif(n = 100,min=20,max=28)
>
> #input raster y axis
> r2 <- raster ( nrows=10, ncols=10, crs=NA)
> r2[] <- runif(n = 100,min=31,max=36)
>
>
> #AIM: I want a raster that has in the cell values the corresponding value
> of the z dimension in mat.
> #    That is r1 contains the values of the xaxis in mat, r2 contains inputs
> values of the y axis in mat
> #    I want the raster with the values of z in mat
>
>
> #on a cell by cell basis it is 'easy'
>
> cell.values <- lapply (1:ncell(r1),FUN = function (i){
>   pos.x.axis <- if (r1[i] < range(mat$x)[1] | r1[i] > range(mat$x)[2]) {NA}
> else{
>     which(abs(r1[i]-mat$x) == min(abs(r1[i]-mat$x))) #it gives me the
> closest postition in the y axis
>     }
>
>   pos.y.axis <- if (r2[i] < range(mat$y)[1] | r2[i] > range(mat$y)[2]) {NA}
> else{
>     which(abs(r2[i]-mat$x) == min(abs(r2[i]-mat$x))) #it gives me the
> closest postition i nthe y axis
>   }
>
>   if (is.na (pos.x.axis) ==T | is.na(pos.y.axis)==T) {out.value <- NA} else{
>     out.value <- mat2[pos.x.axis, pos.y.axis]
>   }
>
>   return (out.value)
>
> })
>
> raster.out <- r1 ; raster.out [] <- NA
> raster.out[]<- unlist(cell.values)
> plot (raster.out)
>
>
> any hint on how to do it for 14Milion cells?
>
>         [[alternative HTML version deleted]]
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo