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

GRASS GRASS user
1

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.

Thanks

···

Margherita Di Leo

2

Hi Madi,

On Thu, Jun 6, 2019 at 4:13 PM Margherita Di Leo <diregola@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

Thanks

–
Margherita Di Leo

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3

Thank you Markus!

···

Margherita Di Leo