I'm trying to write a Python script performing a geoalgorithm. What is surprising me is the following:

  1. I first test the algorithm by QGIS (2.8) interface. In my case, the GRASS interpolator v.surf.idw
  2. I see that the solution is sufficiently good using a certain setting of parameters.
  3. Then, I run the same algorithm with the same parameters, froma Python script. In my case:

    out_ras = processing.runalg("grass:v.surf.idw", vl,12,2,"field_3" ,False, "%f , %f, %f, %f "% (xmin , xmax , ymin , ymax), 0.5, -1, 0.001, fileoutput)


  • vl is the point vector layer
  • field_3 is the filed where vlaues to be interpolated
  • fileoutput is the raster file in output
  • (xmin, , xmax , ymin , ymax) are the Extent of my layer

This setting (perfectly working when launched from QGis interface) produices a Nodata value Raster (only 1 cell). It seems that the algorithm does not recognize the vector in input. I've also checked the CRS of the layer (with vl.crs().authid() ) and everything sounds good.

Any help? Any experience in detecting different behaviour of the SAME algorithm run by Python through processing instead of from QGIS UI ?

  • 1
    Did you import the required QGIS modules? Is your PYTHONPATH set to the same python that QGIS uses or another installed version? See stackoverflow.com/questions/8922568/error-no-module-named-qgis and gis.stackexchange.com/questions/22886/… Jul 15, 2015 at 13:45
  • Welcome to GIS:SE @iaborsi! Could you please edit your question to include a snippet of code instead of a one-liner, such as a couple of terms which you needed to define and the imports you used as this could help potential answerers :)
    – Joseph
    Jul 15, 2015 at 13:59
  • 1
    I'd suggest you to validate that your point layer is correctly loaded, this way: print vl.isValid(), which should print True. Please tell us if that check is OK. Jul 15, 2015 at 14:43
  • Thanks to all for your suggestion. However, the problem is not in loading module or layer. I paste below a snippet of the code.
    – iaborsi
    Jul 17, 2015 at 8:00
  • from PyQt4.QtGui import * from qgis.core import * from qgis.gui import * from PyQt4.QtCore import * import processing # Define the data source vl = iface.activeLayer() if vl.isValid() : print 'Layer is valid!' fileoutput = "C:\Users\iacopo.borsi\Desktop\rastout.tif" ext = vl.extent() xmin = ext.xMinimum() ymin = ext.yMinimum() xmax = ext.xMaximum() ymax = ext.yMaximum() myfield = 'Z'
    – iaborsi
    Jul 17, 2015 at 8:08

1 Answer 1


it seems that my problem was in the GRASS algorithm I was using. Now, I've moved to GDAL algorithm named "Grid - InverseDistance To a Power" and it works. I don't really know what was wrong: my only suspect is that after running grass algorithm I would need to "convert" the grass raster map created to a "readble" raster, to be loaded in QGis correctly. I realized such a guess after reading the processing log file and comparing the log obtained by python-based run with the one obtained after launching the process by the GUI. But this is only a guess... hoping this could help some body else playing with GRASS and processing.

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