col,row coordinates from utmx,utmy coordinates

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