[Qgis-user] QGIS modeler error message : "Unable to evaluate the expression for the extra parameter : no root node!"

[celati Laurent]

Dear all,

I'm working with QGIS 3.4. This is my first QGIS model. I'm facing to an
error message during the running of the modeler. If I understand the error
message, the error seems to occur during the step "D" on my little
screenshot (below) showing the modeler. For information, I've defined a
conditional branch. In addition to the screenshot of the modeler, I paste
also the error message.

Modeler explanation :

   - The output of the algorithm A is a raster clipped with a polygon (an
   area of interest)
   - For the algorithm B (difference vector processing between 2 input
   layers), I've setted a dependency so that it only triggers if a condition
   is met (we can see the conditional branch 'C' on the screenshot).
   - Algorithm D (clip raster with a polygon) takes as input the raster
   output/result of the algorithm A. And as a mask layer, the vector result of
   algorithm B (difference). For this algorithm D, I've still enable the same
   dependency setted for the algorithm B.
   - For algorithm E (raster polygonization),we can see there are 2
   possible raster inputs. The output of the algorithm A and the output of the
   algorithm D. Thus, as input to algorithm E, I set the following expression
   (pre-calculated value):

 if (@exclude_forest_zones ,
@cut_raster_segmentation_by_mask_vector_NO_foret_OUTPUT ,
@cut_raster_segmentation_by_study_area_vector_OUTPUT)


[image: image.png]


When I run the modeler, I get an error message that seems to be related to
algorithm D. Below is the error message:

"Unable to evaluate the expression for the extra parameter to clip the
raster segmentation by the NO forest mask: no root node! Execution
failed after 8 seconds."


[image: image.png]
For information, I paste below the overview of the algorithm D :

[image: image.png]

And I paste below for information the overview of the algorithm E : As you
can see on the screenshot, I haven't defined any dependencies for algorithm
this algo E.

[image: image.png]

Thanks so much for your valuable help.
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