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

Micha Silver micha at arava.co.il
Wed Jun 10 03:31:17 EDT 2009

Hello Milton

Roger's answer was, as always, both complete and to the point.

As a further exercise, I took a small region of 540000 cells with 
floating point values, and using the "feature=area" option to r.to.vect 
I created a vector map of polygons. It produced over 360000 polygons. 
Most will be polygons of 1 cell in size (from the original raster). I 
assume that's *not* what you're looking for...



Roger Bivand wrote:

> 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
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> -----
> 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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