Output file not generated by r.viewshed in PyQGIS

Question

I'm trying to use r.viewshed in PyQGIS to perform Viewshed analysis on a point placed on a raster layer. The algorithm seems to run correctly, as no errors come up, but the output .tif file is not created, and I've tried it using multiple different locations, using absolute and relative paths, both with no luck.

Here is a snippet of my python code below.

rasterLayer is the QgsRasterLayer object, the main raster of the project for the viewshed to be done upon.

pointPath =  r"C:/Users/me/Documents/ZVIs/zvi.tif"
maxRange = 500
pointElevationFromDTM = 100
coordStr = "230049.90500026295, 2841692.311715042"
result = processing.run("grass7:r.viewshed",
                           {'input' : rasterLayer,
                            'coordinates': coordStr,
                            'observer_elevation': pointElevationFromDTM,
                            'max_distance': maxRange,
                            'memory': 300,
                            'output': pointPath})
if result['output']:
    print("Viewshed analysis completed. Output saved to:", pointPath)
else:
    print("Viewshed analysis failed")
if os.path.isfile(pointPath):
    print(f"ZVI file was created.")
else:
    print(f"ZVI file was not created")

I added in some checks at the end so you can see what I mean. It seems that an output is being created, just not saved for whatever reason.

---

I've been able to print out the processing feedback after the answer by @Taras. The main errors I can see are below:

ami_single_temp_name: G_mkstemp() failed: : No such file or directory
Assertion failed: 0, file ami_stream.cpp, line 91
ERROR: Raster map <outputb422948942aa4ef6a24c5ce880e5fe23> not found
ERROR: Raster map or group <outputb422948942aa4ef6a24c5ce880e5fe23> not found

But when using isValid() on the raster layer it is returning True.

Where do I go next?

I've googled the errors but haven't had any luck yet.

Answer 1

There are several suggestions regarding your code:

• check firstly alg input and output by means of the processing.algorithmHelp("grass7:r.viewshed")

• as was suggested by @bugmenot123, check the logs. One can access them via putting the QgsProcessingFeedback() class into processing.run(...) and after applying the textLog()-method:

  feedback = QgsProcessingFeedback()
  processing.run("grass7:r.viewshed", params, feedback=feedback)
  print(feedback.textLog())
  

• use the Python Try Except blocks to handle errors and exceptions, e.g. QgsProcessingException.

  try:
     params = {
        'input': rlayer,
        'coordinates': coordStr,
        'observer_elevation': pntElevationFromDTM,
        'max_distance': maxRange,
        'output': outputPath
     }
     processing.run("grass7:r.viewshed", params, feedback=feedback)
  except QgsProcessingException as e:
     print(e)
  

• apply some useful functions e.g. basename, splitext, normpath, isfile, dirname, and join from the os.path library

• use the isValid()-method to check the status of the QgsRasterLayer

So, in the end, your code may look like this (Note: it was tested within a standalone application):

# imports
from sys import path
from qgis.core import (QgsApplication, QgsRasterLayer, QgsProcessingFeedback, QgsProcessingException)
from os.path import (basename, splitext, normpath, isfile, dirname, join)
See https://gis.stackexchange.com/a/155852/4972 for details about the prefix
QgsApplication.setPrefixPath('C:\OSGeo4W\apps\qgis-ltr\plugins', True)
qgs = QgsApplication([], False)
qgs.initQgis()
Append the path where processing plugin can be found
path.append('C:\OSGeo4W\apps\qgis-ltr\python\plugins')
import processing
from processing.core.Processing import Processing
Processing.initialize()
raster_path = normpath("D:\qgis_test\qgis_sample_data\raster\SR_50M_alaska_nad.tif")
if isfile(raster_path):
rlayer = QgsRasterLayer(raster_path, splitext(basename(raster_path))[0])
coordStr = "3178941, 3504031"
pntElevationFromDTM = 65  # draped from the raster
maxRange = 50000
outputPath = join(dirname(raster_path), "AlaskaViewshed.tif")

if rlayer.isValid():
    feedback = QgsProcessingFeedback()

    try:
        params = {
            'input': rlayer,
            'coordinates': coordStr,
            'observer_elevation': pntElevationFromDTM,
            'max_distance': maxRange,
            'output': outputPath
        }
        processing.run("grass7:r.viewshed", params, feedback=feedback)
        print(feedback.textLog())  # read the logs

    except QgsProcessingException as e:
        print(e)

With the above code, I could get the output for the SR_50M_alaska_nad.tif raster on QGIS version 3.28.10-Firenze.

---

References:

• r.series - no output 'ERROR 6: SetColorTable() only supported for Byte or UInt16 bands in TIFF format.!
• QGIS: Getting Concurrent mapset locking error using V.transform
• Returning error text from QgsProcessingFeedback