[R-sig-Geo] reading tiff with jump?

[Roger Bivand]

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

>
> 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