SV: [GRASS-user] Raster addition

[Martin Album Ytre-Eide]

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.

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