Loading raster layer in a standalone PyQGIS script

Question

I need to load a raster file from the provided file path. I wish to use it further in the Raster Calculator. I wrote a function which converts string (I assume it is a file path) to a raster object.

from qgis.core import QgsRasterLayer
from PyQt4.QtCore import QFileInfo
def StringToRaster(raster):
    # Check if string is provided
    if isinstance(raster,basestring):
        fileInfo = QFileInfo(raster)
        baseName = fileInfo.baseName()
        path = fileInfo.filePath()
        if (baseName and path):
            raster = QgsRasterLayer(path, baseName)
            if not raster.isValid():
                print "Layer failed to load!"
                return
        else:
            print "Unable to read basename and file path - Your string is probably invalid"
            return
    return raster

I have no idea why, but raster.isValid() always returns False. When I take exactly the same path which I provided to my function, I am able to add this layer to QGIS from the interface (from the menu Layer --> Add layer ---> Add raster layer).

My layer is a Float32 GeoTIFF with one band only.

Example path: /home/mateusz/many times I tried/data/TTT/TRR___niskorozdzielczy.tif — matandked, Apr 26 '15 at 09:33

score 7 · Accepted Answer · answered May 08 '15 at 02:12

7

If you keep getting non valid layers, even defining them correctly, you're probably missing the QGIS prefix definition in your script (in case that you work out of QGIS).

qgis_prefix="/usr"    
QgsApplication.setPrefixPath(qgis_prefix, True) 
QgsApplication.initQgis()

If you work on Windows, your qgis_prefix should be something like this:

qgis_prefix="C:\\Program Files\\QGIS Wiena\\apps\\qgis"

Now your raster layer should be valid.

answered May 08 '15 at 02:12

Germán Carrillo

36,307
5
123
178

Thanks, it seems that you need to import additional modules? Error message: name 'QgsApplication' is not defined. – matandked May 12 '15 at 04:51
Right, you would need (at least) this line: from qgis.core import QgsApplication – Germán Carrillo May 12 '15 at 15:42
1

Great, now it works perfectly. – matandked May 12 '15 at 18:53

score 6 · Answer 2 · edited Oct 29 '19 at 19:08

This code works in my Python Console:

from qgis.core import QgsRasterLayer
from PyQt4.QtCore import QFileInfo

def StringToRaster(raster):
    # Check if string is provided

    fileInfo = QFileInfo(raster)
    path = fileInfo.filePath()
    baseName = fileInfo.baseName()

    layer = QgsRasterLayer(path, baseName)
    QgsMapLayerRegistry.instance().addMapLayer(layer)

    if layer.isValid() is True:
        print "Layer was loaded successfully!"

    else:
        print "Unable to read basename and file path - Your string is probably invalid"

raster = '/home/zeito/Desktop/output2.tif'

StringToRaster(raster)

After running the code in the Python Console of QGIS:

result

For those who will work with QGIS 3:

from qgis.core import QgsRasterLayer
from PyQt5.QtCore import QFileInfo

def StringToRaster(raster):
    # Check if string is provided

    fileInfo = QFileInfo(raster)
    path = fileInfo.filePath()
    baseName = fileInfo.baseName()

    layer = QgsRasterLayer(path, baseName)
    QgsProject.instance().addMapLayer(layer)

    if layer.isValid() is True:
        print ("Layer was loaded successfully!")

    else:
        print ("Unable to read basename and file path - Your string is probably invalid")


raster = '/home/zeito/Desktop/output2.tif'

StringToRaster(raster)

Thanks, but my code also works in Python console within QGIS. For some reason, when I give exactly the same path, isValid() returns False outside Python QGIS console (for example in iPython) and True in QGIS Python console. — matandked, Apr 28 '15 at 04:13

score 3 · Answer 3 · answered Jul 13 '18 at 14:40

3

The QGIS documentation also proposes a second way, I found very helpful:

[R]aster layers can be loaded using the addRasterLayer function of the QgisInterface:

iface.addRasterLayer("/path/to/raster/file.tif", "layer name you like")

answered Jul 13 '18 at 14:40

Jeremy

366
1
3

Loading raster layer in a standalone PyQGIS script

3 Answers3

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