[R-sig-Geo] Reassigning coordinates?

[Barry Rowlingson]

On Tue, Nov 2, 2010 at 10:00 AM, Brian Oney <zenlines at gmail.com> wrote:
> Hello List,
> I need to reassign coordinates of a few points to have coordinates of the
> nearest raster cell. I KNOW that this is BAD practice but for the time being
> I  assume that it does not affect my analysis (species distribution
> analysis). I think the datum of the point data is different than that of my
> raster data, but for now it is not important.
>
> I have already perused the "sp" and "raster" packages extensively.
> I guess I just need to be pointed in the right (WRONG) direction. I had a
> few thoughts with the distance function but I am now stuck. Maybe use
> "distance" then "which.min" then take the coordinates?
>

 If you have some points in a SpatialPointsDataFrame and a grid as a
SpatialPixelsDataFrame then you can do an overlay. So for example,
using 'm' from help(SpatialPixelsDataFrame) and 'pts' as some random
clicking points:

 > pts[1:10,]
        coordinates         zz
1  (179333, 330803) 0.76290682
2  (179600, 331026) 0.38275344
3  (180462, 331370) 0.75330357
...

 > overlay(m,pts)
 [1] 1930 1737 1434 1093 1115 1495 1705 2025 2146 2386 1780  728  487 1056 1256
[16] 1567 1778 1039  819 2465

 - gives the index in 'm' where each point in pts is, hence something like:

> cbind(coordinates(pts),coordinates(m)[overlay(m,pts),])
             x        y      x      y
 [1,] 179332.9 330802.6 179340 330820
 [2,] 179599.7 331025.5 179580 331020
 [3,] 180461.9 331369.9 180460 331380
 [4,] 180533.7 331785.2 180540 331780
 [5,] 180010.3 331734.6 180020 331740

 gives the original and grid cell coordinates.

 something similar might work with package:raster objects...

Barry