Thanks to your input,
this is what I'm doing and I think it's ok:
require(rgdal)
fname <-
"/media/SWISNIFE1/GEODATABOLIVIAmini/geotif/p233_r070_ETM_20010825.tif"
GDALinfo(fname=fname)
output of GDALinfo:
rows 3736
columns 5108
bands 6
ll.x 694431
ll.y 8410014
res.x 28.5
res.y 28.5
oblique.x 0
oblique.y 0
driver GTiff
projection +proj=utm +zone=19 +south +ellps=WGS84 +datum=WGS84 +units=m
+no_defs
file
/media/SWISNIFE1/GEODATABOLIVIAmini/geotif/p233_r070_ETM_20010825.tif
I go on with reading band 4 with subsampling:
ima <- readGDAL(fname=fname, offset=c(0,0),band=4,output.dim=c(373,510))
str(ima)
image(ima)
imamat <- t(as.matrix(ima)) #put Northernmost line as first row
imamat[imamat==0]<-NA
Then use the following to plot the matrix with correct orientation:
mirot <- function(x) t(x[nrow(x):1,])
x11()
image(mirot(imamat))
Regards,
Agus
Ashton Shortridge wrote:
On Tuesday 05 August 2008, Roger Bivand wrote:
On Tue, 5 Aug 2008, Agustin Lobo wrote:
Hi!
is it possible to read a tiff file with readGDAL but just keeping
one pixel every p columns and q lines? That is,
a systematic sampling at reading. This is good to save memory
and still be able of displaying and performing
statistics. As images are large,
reading in the entire image and then sampling is not
an alternative.
If this is not contemplated in the readGDAL function,
is there any way to just read in one line, perform
the sampling and then go on with the next line?
I think I did this with binary images few years
ago, but using tiff would be much better for
practical reasons.
Look at the offset=, band= and region.dim= arguments to readGDAL() - you
could capture the GDALinfo() output to set the frame if need be. It may be
more efficient to use GDAL.open()/GDAL.close() and getRasterData() at a
lower level if you are taking many samples - loop over different values of
offset= with region.dim=c(1,1). Note that offset= may be from the NW
corner, not the SW corner, and that the arguments are (often) ordered
(y, x).
Roger
I'll just add to that: opening raster files is fairly expensive, so if you are
sampling many locations over many tiffs you will see great speed improvements
if you order your sampling by tiff to minimize the opens.
Yours,
Ashton
--
Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
LLuis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
email: [EMAIL PROTECTED]
http://www.ija.csic.es/gt/obster
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