I am trying to process some raster files using python scripts in Qgis 2.20, and I have been having trouble finding any documentation on using raster calculator in scripts being run in the processing toolbox.

I am trying to use scripts to process weather data. I will need to recalculate values of cells in order to represent projected wind speeds at different heights and probable gusting speed. I am trying to use the raster calculator for this but have been having trouble. I was not able to easily find a function for the script, and after some digging through the python console I was able to find the saga:rastercalculator function. Documentation in the python console (obtained through alhelp()), say that the parameters are as follows. algorithm: raster calculator Grids formula Raster

I am trying to run a basic formula through the raster calculator to get it working, but am having no luck.

Is this even the right approach to recalculating grid values in a python script?

Here is the code:

#load and symbolize tiff

##[User scripts]=group

from qgis.core import *
from PyQt4.QtGui import *
from PyQt4.QtCore import *
from qgis.analysis import  *
import processing
from processing.core.Processing import Processing

#import Tiff

fileName = input_raster
fileInfo = QFileInfo(fileName)
baseName = fileInfo.baseName()
raster = QgsRasterLayer(fileName, baseName)
# add map to display

#recalculate values
processing.Saga:RasterCalculator(raster, "(raster)*2/2",outraster)

Here is the error:

Uncaught error while executing algorithm
Traceback (most recent call last):
Traceback (most recent call last):
  File "C:/PROGRA~2/QGISVA~1/apps/qgis/./python/plugins\processing  \core\GeoAlgorithm.py", line 202, in execute
  File "C:/PROGRA~2/QGISVA~1/apps/qgis/./python/plugins\processing\script\ScriptAlgorithm.py", line 252, in processAlgorithm
    exec script in ns
  File "<string>", line 65
    processing.Saga:RasterCalculator(raster, "(raster)*2/2",outraster)
SyntaxError: invalid syntax

1 Answer 1


First of all, to actually run a saga processing scrip use

processing.runalg("saga:algorithmname", parameter1, parameter2) 

Basically Qgis can do two things:

(1) Perform mathematical operations with rasters -> use saga raster calculator from processing toolbox

A thing that can help you is that you can refer to the grids as a,b,c etc. a+b will add two grids.

(2) perform logical tests e.g. raster "value" > 10 -> use QgsRasterCalculator from Qgis.analysis

The script below will execute a simple test. I believe you can use SQL Query within the Python code.

from qgis.analysis import QgsRasterCalculator, QgsRasterCalculatorEntry

# Add raster 
rasterpath  = "/Desktop/werkmap/CHIRPS/chirps-v1.8.2012.01.tif"

bohLayer  = QgsRasterLayer(rasterpath, "rasterlayername")
if not bohLayer.isValid():
    print "Layer failed to load!"

# Load raster layer into canvas (not required for script but hey it looks awesome) 

entries = []
# Define band1
boh1 = QgsRasterCalculatorEntry()
boh1.ref = 'boh@1'
boh1.raster = bohLayer
boh1.bandNumber = 1
entries.append( boh1 )

# repeat step above if multiple bands

# Process calculation with input extent and resolution
calc = QgsRasterCalculator( 'boh@1 > 10', '/Desktop/werkmap/output/outputfile.tif', 'GTiff', bohLayer.extent(), bohLayer.width(), bohLayer.height(), entries )

print "finish"

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