[GRASS-user] [Bulk] Re: Two simple questions

[Micha Silver]

Hi Luciano:
(Returning to the maillist - maybe this will help others...)

On 05/11/2012 18:29, Luciano La Sala wrote:
> Two simple questions
>
> Thank you Micha, I found the icons right under my nose.
>
> To tell you the truth I am kind of lost here. I am trying to do the 
> interpolation from within QGIS using the raster map of risk I did 
> following your instructions (JPEG attached).
>
> Pixel values, as calculated using the “r.mapcalculator” module in 
> GRASS, are (from the lightest yellow to the darkest red):
>
> 0.0406
>
> 0.1422
>
> 0.4268
>
> 0.5284
>
> 0.7154
>
> 0.8983
>
> I first used the module “r.surf.idw” module (Screen_01) which runs 
> successfully but produces an entirely black raster map as output 
> (Screen_02). All the pixels of this raster contain a value of 1.
>
> There is obviously something wrong here. Any tips? Should I use 
> “v.surf.idw” instead?
>

I found on an old maillist post that r.surf.idw works only with integer 
values. That's why you're getting everything "rounded" to 1. To work 
around this, you can try two options:

* revert back to vectors, as you suggest. (v.surf.idw accepts decimal 
values)
     r.to.vect -z in=risk_raster out=risk_vector feature=point
     v.surf.idw in=risk_vector out=risk_idw layer=0


OR


* multiply your risk raster by some constant, say 1000, to get integer 
values. Then run r.surf.idw on that, and divide the result by the same 
1000 again to get back to your normalized values.
     r.mapcalc risk_integer=risk_raster*1000
     r.surf.idw in=risk_integer out=risk_idw_tmp
     r.mapcalc risk_idw=risk_idw_tmp/1000.0
     # Use the decimal value 1000.0 to insure that the result is 
floating point.


Regards,

Micha

> Thank you so very much for your time Micha.
>
> Luciano
>
> *De:*Micha Silver [mailto:micha at arava.co.il <mailto:micha at arava.co.il>]
> *Enviado el:* Monday, November 05, 2012 4:01 AM
> *Para:* Luciano La Sala
> *Asunto:* [Bulk] Re: Two simple questions
>
> On 05/11/2012 04:42, Luciano La Sala wrote:
>
>     Hello Micha,
>
>     Where do you access the
>     modules///v.surf.idw/and///r.surf.idw/from?I just can’t seem to
>     find them!
>
> If you run GRASS on its own (not the QGIS plugin), these modules are 
> under the "Raster->Interpolate Surfaces"menu. In addition, all GRASS 
> commands can be run straight from the command line.
>
>     Thanks!
>
>     Luciano
>
>
>
>     This mail was received via Mail-SeCure System.
>
>
>
>
> -- 
> Micha Silver
> GIS Consulting
> 052-3665918
> http://www.surfaces.co.il
>
>
> This mail was received via Mail-SeCure System.


-- 
Micha Silver
GIS Consulting
052-3665918
http://www.surfaces.co.il