SV: [GRASS-user] Raster addition

[Johannes Radinger]

-------- Original-Nachricht --------
> Datum: Thu, 14 Apr 2011 14:31:32 +0200
> Von: Martin Album Ytre-Eide <Martin.Album.Ytre-Eide at nrpa.no>
> An: \'Moritz Lennert\' <mlennert at club.worldonline.be>
> CC: "grass-user at lists.osgeo.org" <grass-user at lists.osgeo.org>
> Betreff: SV: [GRASS-user] Raster addition

> I would like the NULL values that overlap with none NULL values to be
> treated as 0, and still have NULL values where both maps have NULL values. The
> combined map would be a large map with NULL values and added values.

that is possible with the r.mapcalc and a if condition like:

if(map A &&& map B, 0, null()) 

something like that (untested) might work. the null () provides NULL values where the condition is false. The triple &&& is the logic and (treats NULL values different from double &&).

see: http://grass.fbk.eu/gdp/html_grass64/r.mapcalc.html for more information.

/j

> 
> Maybe the best way is to use r.null and set NULL values to 0. Add the maps
> and then convert the 0 values to NULL values?
> Or could one set the value in the if statement to be a the sum of maps
> somehow? I guess my biggest problem is summing with NULL values - maybe there
> is a way to treat NULL values as 0? I'll have a look at the man page
> 
> Thanks,
> 
> Martin 
> 
> -----Opprinnelig melding-----
> Fra: Moritz Lennert [mailto:mlennert at club.worldonline.be] 
> Sendt: 14. april 2011 14:15
> Til: Martin Album Ytre-Eide
> Kopi: grass-user at lists.osgeo.org
> Emne: Re: [GRASS-user] Raster addition
> 
> On 14/04/11 13:54, Martin Album Ytre-Eide wrote:
> > Hello.
> >
> > I am trying to add some raster maps. I thought I could do this in 
> > r.mapcalc, but "new_map= old_1 + old_2" seems to produce a map where 
> > the maps overlap and ignoring areas where only on of the maps have 
> > values.
> 
> This is due to the way r.mapcalc handles NULL values. See the man page,
> section NULL support for an explanation.
> 
> > I would like to do something similar to "v.overlay operator=or".
> >
> > Anyone who knows how to do this?
> 
> It depends on what you want the new map to contain as values. If you want
> to add the value of old_1 to old_2 in places where they overlap, one way
> would be to transform your null values to 0 using r.null null= and then
> applying your formula.
> 
> If you just want a (any) value where one of the maps is non-null, then you
> can use if. Something like this (untested):
> 
> new_map = if(!isnull(old_1) || !isnull(old_2), 1, null())
> 
> which should give you 1 where any of the two contains a value and a NULL
> value elsewhere.
> 
> Moritz
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