[GRASS-user] problem on vectorizing a float point raster

[Roger Bivand]

I think that r.to.vect wants to build a line structure, like river channels,
but sees all the raster cells occupied, so no linear structure, and the
advice to thin first is then appropriate. If this is connected to your
question about calculating a measure of spatial autocorrelation for the
raster data, then I suspect that you do not need polygons but rather points,
where r.out.xyz may be helpful, followed by v.in.ascii if you will be using
GRASS downstream, or if you want to emit a shapefile for GeoDa (another
posting).

If you really want to calculate a measure of spatial autocorrelation for
your raster, I suggest copying the raster to R with readRAST6, creating the
neighbour list with dnearneigh() with max. distance the greater of ewres and
nsres, and proceeding from there in the usual way. But please consider the
inevitable fact that unless the resolution of your raster matches the
"natural" support of the phenomenon of interest, the observed
autocorrelation will certainly be driven by your having multiple
neighbouring "observations" of each "entity", in addition to not having
demeaned (detrended) the data. This means that any results will almost
certainly be spurious.

Hope this helps,

Roger


Milton Cezar Ribeiro wrote:
> 
> Dear Grass-Gurus,
> 
> I have a 2400x2200 raster image with values ranging
> from 0.1 to 42, in float format. Now I need
> vetororize the image, on the way that each pixel
> come to be a polygon and the pixel value be stored as
> attribute.
> 
> I am trying to do this using:
> r.to.vect input=temp71.img output=temp71_img_integer_vect_200m --o -b
> 
> but grass return the following error messages:
> 
> GRASS 6.4.0svn (newLocation):C:/GRASS-6-SVN/msys/home/mjfortin > r.to.vect
> inpu
> t=temp71.img output=temp71_img_integer_vect_200m --o -b
> WARNING: Vector map <temp71_img_integer_vect_200m> already exists and will
>          be overwritten
> WARNING: Table <temp71_img_integer_vect_200m> linked to vector map
>          <temp71_img_integer_vect_200m> does not exist
> Extracting lines...
> ERROR: Raster map is not thinned properly.
>        Please run r.thin.
> By the way, I am running grass under a WinXp 64bit.
> Any help are welcome.
> 
> Cheers
> 
> milton
> brazil=toronto
> 
> _______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user
> 
> 


-----
Roger Bivand
Economic Geography Section
Department of Economics
Norwegian School of Economics and Business Administration
Helleveien 30
N-5045 Bergen, Norway

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