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

[Markus Metz]

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.

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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>> --
>> Micha Silver
>> Ben Gurion Univ.
>> Sde Boker, Remote Sensing Lab
>> cell: +972-523-665918
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