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

[Daniel Victoria]

I was traveling so could not respond. So sorry to chime in a bit late. For
me it was a bit counter intuitive that polygons with null attributes became
zero in the finished raster. What if I had attributes with zero value in it?

I liked the idea of issuing a warning and using the `where` parameter.

Anyway, thanks for the help and fix

Cheers
Daniel

On Fri, Nov 1, 2019 at 12:33 PM Micha Silver <tsvibar at gmail.com> wrote:

>
> On 01/11/2019 9:38, Markus Metz wrote:
>
>
>
> On Fri, Nov 1, 2019 at 12:11 AM Veronica Andreo <veroandreo at gmail.com>
> wrote:
> >
> > 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...
>
> I think it depends on the particular use case if it matters where a NULL
> raster value comes from: a polygon with empty attribute or no polygon at
> all for that cell.
>
> >>
> >> 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.
>
> This can easily be done with existing tools:
> - convert only those polygons with a valid attribute value: v.to.rast
> where="attribute is not null"
> - replace missing attribute values with a valid value: v.db.update
> where="attribute is null", then v.to.rast
>
> I'm changing my PR to issue a warning if empty attribute values are found
> and replaced with zero.
>
>
> IMHO, that's probably the best way to deal with this.
>
>
>
> Markus M
>
> >
> > 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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> >>
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> >>
> >> --
> >> Micha Silver
> >> Ben Gurion Univ.
> >> Sde Boker, Remote Sensing Lab
> >> cell: +972-523-665918
>
> --
> Micha Silver
> Ben Gurion Univ.
> Sde Boker, Remote Sensing Lab
> cell: +972-523-665918
>
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