Hi Robert,
Thank you very much for the reply. Both datasets are in txt format. Lat and long are in WGS84. So I might need to do something like:
library(raster)
p <- read.table('points.txt')
d <- read.table('gridcells.txt')
r <- rasterFromPoints(d)
values<- extract(r,  p)

Is anything else I should add or modify?  rasterFromPoints seems not available in raster 2.0-05.

What I intend to do is extract and add lat/long of corresponding grid cells to my points dataset as below:
> points
   lat.sample  long.sample   lat.gridcell   long.gridcell
1  -21.258000  110.502000
2  -42.609683  144.412700
3  -36.733000  132.050000
4  -21.330000  110.482000
5  -13.156840  136.757080
6  -14.633000  117.967000
7  -23.502580  111.083450
8  -15.674870  138.058400
9  -12.650000  136.950000
10  -9.955000  143.806667

All the best,
Jin
________________________________
From: Robert J. Hijmans [mailto:r.hijmans@gmail.com]
Sent: Thursday, 4 October 2012 1:13 PM
To: Li Jin
Cc: r-sig-geo@r-project.org
Subject: Re: [R-sig-Geo] Extract location information from grid cells for sample points [SEC=UNCLASSIFIED]

Hi Jin,

>From your description it is not entirely clear what your input data looks like. But you can something like this

library(raster)
p <- read.table('points.txt')
r <- raster('file.asc')
values<- extract(r,  p)

Perhaps you need to subset "p" to get the appropriate columns representing longitude and latitude (in that order). Or perhaps your grid data is not really in a 'raster' format? I am not sure from your description. In that case you may need to to

d <- read.table('file.asc')
r <- rasterFromPoints(d)

You may also want to check out functions cellFromXY to link coordinates to cells in a raster.

All the best, Robert

On Wed, Oct 3, 2012 at 5:38 PM, <Jin.Li@ga.gov.au<mailto:Jin.Li@ga.gov.au>> wrote:
Dear list,

I have two datasets: one is for point samples with lat and long information at accuracy of 0.001 degrees. Another is grid cells at a resolution of 0.0025 degrees. Both datasets are in txt format. Now I need to extract the lat and long information from the grid cell dataset for the point samples by assigning the lat and long of a grid cell to a sample point that falls in the grid cell. I usually get this done in ArcGIS. I was wondering if it is possible in R. I searched but failed to find a right function to implement this in R.

Many thanks for any suggestions.

Regards,

Jin Li
Spatial Modeller/Computational Statistician  |  Coastal, Marine and Climate Change Group
Environmental Geoscience Division  |  GEOSCIENCE AUSTRALIA
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