[GRASSLIST:3582] Re: How to resample and get a max value?

[Glynn Clements]

Terry Duell wrote:

> > > I have an interesting problem which one of you more experienced GRASS
> > > users may have the answer to.
> > > I want to resample a raster map to a lower resolution (eg 100m down to
> > > 1000m), but in doing so I want to use the max value in the 100m raster
> > > within the boundaries of each 1000m cell.
> > > I have a suspicion that r.mapcalc might be the only way...but the 'how
> > 
> > Maybe r.neighbors is what you're after?
> 
>  Ah. Indeed, it does look like it might do the trick. The man page does
> seem to frown upon generating a raster which doesn't have the same
> resolution as the input. I guess I'll have to try and see.

Actually, I doubt that r.neighbors will do what you require. Each
cell in the output map is based upon a "window" of cells surrounding
the corresponding cell in the input.

Note that the resolution of input and output are always the same. The
presence or absence of the "-a" flag simply controls whether the
resolution is taken from the input map (without "-a"), or the current
region (with "-a").

IOW, omitting "-a" just causes the resolution to be changed to that of
the input map for the duration of the command. Without "-a", the input
will be resampled according to the current resolution (this is how
most r.* commands normally behave).

AFAICT, what you need is a cross between r.resample/r.resamp.rst and
r.neighbors, i.e. a resampling program with more options as to how to
compute the resulting pixel. As there isn't such a program at present,
you would either need to modify an existing program, or use r.mapcalc
followed by r.resample.

-- 
Glynn Clements <glynn.clements at virgin.net>