[GRASS-user] Downscaling rasters (resampling to LOWER resolution)

Glynn Clements glynn at gclements.plus.com
Thu Jun 19 13:03:23 EDT 2008

Paulo Marcondes wrote:

> What would be the recommended way to downsample a raster?
> my region resolution is currently 100m, say I want to resample my
> rasters at 300m.
> Why would I want to do that? Well, currently the data is too rough,
> but filtering may introduce some artifacts.

Ultimately, resampling is also filtering, and may also introduce
artifacts. Actually, downsampling without filtering (i.e. nearest
neighbour resampling) will probably produce worse results than the
typical filters.

> I know I'll loose some information with the downscaling, but that is
> precisely what I want, because I deem the roughness to be just noise.
> I am not sure if just filtering would do the same.
> what would be the best way to achieve this?

The "best" way is subjective, but the main options are
r.resamp.interp, r.resamp.stats, r.resamp.rst, r.mfilter.fp, or any
combination of those.

Another option is to use i.fft followed by i.ifft with a mask applied. 
However, that requires that your raster contains no null cells (so you
may need to use e.g. r.fillnulls first), and it also requires that you
can fit the entire raster in memory (so it isn't suitable for very
high resolution data).

Glynn Clements <glynn at gclements.plus.com>

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