[R-sig-Geo] Can I read only a portion of a raster? (With more code)

Ben Harrison harb at student.unimelb.edu.au
Fri Sep 27 08:13:40 CEST 2013

On 26/09/13 16:57, Michael Sumner wrote:
> Use crop() once it's been "read":
> r <- raster(system.file("external/test.grd", package="raster"))
> my.ext <- extent(179000, 181000, 330000, 333000)
> r <- crop(r, my.ext)
> There's no "ext" argument to raster() for reading from a file. There
> is that argument when you create a raster from scratch with no data,
> as an alternative to xmin/ymin/xmax/ymax.
> You can read a grid partially with rgdal, by either
> 1) using the index arguments to readGDAL (offset, region.dim) - but
> you have to determine those index values yourself
> 2) using GDAL.open to create a handle to the file that can be indexed
> with [,], but again you need to figure out the indexes
> But, raster is pretty good at opening a file and not attempting to
> read the whole thing. I would just try doing it as above with crop.
> Cheers, Mike.

Thanks Mike. I was wrong thinking raster read the entire file. So my 
problem is baseless.
I read about crop() too just after I sent my message, so that's the way 
to do it!

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