Extract max.... es: PROBLEM writing geotiff

Thanks everybody. It works fine with following codes for extracting  max value from list from a huge data set (max pixel value of  NDVI from May to August  for CO, NM, WY and KS states) .  But I have a problem with writing a  geotiff file. This is first time I am working with rgdal. I was looking for   appropriate code in R-sig-Geo Archives<https://stat.ethz.ch/pipermail/r-sig-geo/> for solving this problem.  But I do not find any possible solution for doing this (may be it is due to my limiting   understanding of  rgdal working environment). Any suggestion will be appreciated.
Thanks again
Zia

rm(list=ls(all=TRUE))
# Get Max values
library(raster)
library(rgdal)

setwd("L:/MODIS_DATA/NDVI")
files <- list.files(pattern='.tif$')

s <- stack(files)
r <- max(s)
str(r)
plot(r)

# Writing tiff file:

tr <- "L://MODIS_DATA//NDVI/max.tif"
writeGDAL(r, tf, drivername="GTiff", options="TFW=YES")

?Error in function (classes, fdef, mtable)  :
 unable to find an inherited method for function ?gridded? for signature ?"RasterLayer"??





From: romunov at gmail.com<mailto:romunov at gmail.com> [mailto:romunov at gmail.com] On Behalf Of Roman Lu?trik
Sent: Friday, March 01, 2013 2:27 AM
To: Zia Uddin Ahmed
Cc: r-sig-geo at r-project.org<mailto:r-sig-geo at r-project.org>
Subject: Re: [R-sig-Geo] Extract max. pixel value from a set geoTif raster image and create a new geoTIF image with these max values

Yes. You will also want to install rgdal, if you haven't already. You can read and write files with readOGR() and writeOGR(), while raster functions are quite intuitive to work with. See raster package vignette and drop us a line if you have any more questions.

Cheers,
Roman


On Fri, Mar 1, 2013 at 3:24 AM, Zia Uddin Ahmed <zua3 at cornell.edu<mailto:zua3 at cornell.edu>> wrote:
      Dear list,
      I like to  extract  max. pixel value from a set  geoTif raster image and create a new geoTIf  image  with these max values. Is it possible to do this with R "raster" package?
      Thanks
      Zia