[GRASS-user] Question about isnull() in map algebra

[Margherita Di Leo]

On Thu, Jun 6, 2019 at 6:28 PM Markus Metz <markus.metz.giswork at gmail.com>
wrote:

> Hi Madi,
>
> On Thu, Jun 6, 2019 at 4:13 PM Margherita Di Leo <diregola at gmail.com>
> wrote:
> >
> > Hi,
> >
> > reading the training material here:
> http://training.gismentors.eu/grass-gis-irsae-winter-course-2018/units/23.html
> > I have a doubt. When cloud mask is applied to calculate NDVI
> >
> > t.rast.mapcalc input=b4,b8,cloud output=ndvi_cloud \
> > expression="if(isnull(cloud), null(), float(b8 - b4) / ( b8 + b4 ))" \
> > basename=ndvi_cloud nproc=3
> >
> > what happens if there is no cloud mask at all for a certain map? In this
> case NDVI should be taken as is. Instead, to my understanding, what happens:
> >
> > if(isnull(cloud), null()
> >
> > If cloud map is not found (meaning there is no cloud detected),
> isnull(cloud)=True, so the NDVI resulting map is null too. Please correct
> me if I'm wrong.
>
> the cloud masks are apparently created with "sentinel-cloud-mask.py" which
> requires an AOI vector as input, and a cloud mask is always generated: if
> there are no clouds or no cloud mask, only the AOI is used as mask.
>
> from a quick glance at "sentinel-cloud-mask.py"
>
> Markus M
>
> Thank you Markus!

-- 
Margherita Di Leo
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