[GRASS-user] Null values in attribute table get converted to 0 (zero) during v.to.rast

[Veronica Andreo]

Hi Micha

El jue., 31 oct. 2019 a las 22:36, Micha Silver (<tsvibar at gmail.com>)
escribió:

>
> On 31/10/2019 22:20, Markus Metz wrote:
>
>
>
> On Tue, Oct 29, 2019 at 7:40 PM Veronica Andreo <veroandreo at gmail.com>
> wrote:
> >
> > Hi Daniel,
> >
> > I agree that if there's a NULL in the column, there should be NULL in
> the resulting raster. I suggest to open a ticket here:
> https://trac.osgeo.org/grass/
>
> easy solution: v.to.rast where="<attribute_column> is not null"
>
> or the new PR #173
> https://github.com/OSGeo/grass/pull/173
> if an attribute value is null, the corresponding vector features will not
> be rasterized
>
> Markus M
>
> I'm not sure I agree with the approach that a polygon with missing
> attribute should become NULL. In my view, NULL should be used only for
> pixels not covered at all by any polygon.
>
but in the resulting raster (in rasters in general), there's no difference,
it's rather NULL or it has a value... it does not matter if the null comes
from areas originally covered by a polygon or not... A different story
might be if we go from raster to polygon...

> Perhaps an additional input parameter "missing_attribute" so the user can
> choose a value to enter into the raster if the attribute is missing. If no
> parameter is supplied, and the chosen attribute column has missing values,
> I'd prefer that the script exit gracefully, with a message that a
> "missing_attribute" value is required.
>
yes, this could be a solution to keep track of where you had polygons, but
then you would need to use r.null anyway, no? I try to think of use cases
in which one uses a vector attribute to convert to raster, but then still
needs to know where the polygons were... because, for example, to query a
raster map with another raster map representing zones one would convert the
vector of polygons using cat and not an attribute, no?

cheers,
Vero

>
> > El jue., 24 oct. 2019 a las 14:40, Daniel Victoria (<
> daniel.victoria at gmail.com>) escribió:
> >>
> >> Hi list,
> >>
> >> I have a vector polygon map that I'm converting to raster. The
> attribute column that I process has some empty rows (no data / null). When
> I run v.to.rast, these empty rows become 0 (zero) on my resulting raster
> map.
> >>
> >> Shouldn't v.to.rast respect the empty attribute table and create null
> values for those polygons?
> >>
> >> For now I'll use r.null to fix this problem. But what if 0 is a valid
> value?
> >>
> >> Cheers
> >> Daniel
> >>
> >> PS - Running Grass 7.6.1 on Ubuntu
> >> _______________________________________________
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> >> grass-user at lists.osgeo.org
> >> https://lists.osgeo.org/mailman/listinfo/grass-user
> >
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> --
> Micha Silver
> Ben Gurion Univ.
> Sde Boker, Remote Sensing Lab
> cell: +972-523-665918
>
>
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