[R-sig-Geo] col,row coordinates from utmx,utmy coordinates

[Roger Bivand]

On Fri, 19 Jun 2009, Agustin Lobo wrote:

> Hi!
> Given a jpg + jpgw files I would like
> to get the col,row coordinates of the jpg file
> that correspond to a set of projected
> utmx, utmy coordinates, best
> not having to import the jpg file into R because of its size.

Try this:

library(sp)
getRowCol <- function(SG, pts) {
  o <- overlay(SG, pts)
  cd <- slot(slot(SG, "grid"), "cells.dim")
  cols <- o %% cd[1]
  rows <- cd[2] - o %/% cd[1]
  list(cols=cols, rows=rows)
}
GT <- GridTopology(c(1,1), c(1,1), c(20000, 10000))
SG <- SpatialGrid(GT)
pts <- spsample(SG, n=20, type="random")
pts
getRowCol(SG, pts)

which just depends on the definition of the raster. Watch the row numbers 
- here they follow the coordinates (lowest number at south), but maybe 
ought to have lowest number at north. This is not memory intensive.

Hope this helps,

Roger

> Does this exist within R (in raster package or any other)?
> Or should I use spgrass and/or rsaga?
>
> Thanks
>
> Agus
>
>
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no