[GIS] How to use QgsOverlayAnalyzer class in pyQGIS

intersectionpyqgispythonqgis

I am very new to Python scripting in QuantumGIS, so bear with me please.

I am trying to use the QgsOverlayAnalyzer class in QuantumGIS (http://www.qgis.org/api/classQgsOverlayAnalyzer.html#details) to automate the intersection between different layers (I have plenty of intersections to do).

However, I can't get it to work in the python console.. assume I want to intersect layer1 with layer2, and write the results in C:\shapefile.shp, I am trying something like this with no success:

QgsOverlayAnalyzer.intesection(layer1, layer2, "C:\shapefile.shp", false, 0)

The error I make must be quite serious, as the message I get is:

NameError: name 'QgsOverlayAnalyzer' is not defined

Any help on this is very much appreciated.

Dom

Best Answer

It seems that the correct answer is here.

You can access to that object importing the analysis module:

from qgis import analysis
qgis.analysis.QgsOverlayAnalyzer

Related Solutions

QGIS – Running a Python Script to Extract Raster Data

Yes you can do this in a stand alone script. To do this you efficiently, you will need SciPy. SciPy is a cousin of NumPy and together they extend Python with a lot of extremely powerful array handling capabilities. In SciPy you will use the 'ndimage' module.

Your procedure is to make a raster of your zones. Then import your original raster and the zones raster each into a NumPy array. Make a third numpy array of the unique values in your zones array to use as an index for the ndimage function. Finally use one of the ndimage statistics to extract your result, e.g.

myIndex = numpy.unique(zonesArray)    
minResult = ndimage.minimum(myRasterArray, labels=zonesArray, index=indexArray)

(obviously I have not included all the code to open a raster or vector file etc. but see below for help with that).

I often write my results to a a simple csv text file and can then do a table join to pull the statistics back into my zone polygons. If you have many rasters, then you can automate the process into a batch process.

While you can do this with GDAL and indeed, will need GDAL to read the original raster and polygon data, I would not recommend that you attempt it without SciPy because the process will have glacial alacrity! See this excellent series on coding GDAL with Python. Note tutorial 6 in the series as this is especially relevant. However, it doesn't mention SciPy as it is a bit old (and where it mentions 'Numeric' understand that this has been replaced by 'NumPy'). ndimage is very fast and much better than manually iterating over a raster in a bitwise fashion or even iterating over an array cell by cell. The speed difference of using SciPy vs. NumPy alone can be seconds compared to hours for the same data (from personal experience). As a bonus, it takes less code too :)

There is a lot packed into this message and it may appear daunting if you know nothing of Python. In which case, start with the tutorial series I mention above, which assumes zero coding experience. Then after the first couple of lessons, skip forward to the raster material. Once you know how to open a raster and get it into a Numpy (aka 'numeric') array, read the SciPy documentation on ndimage.

QGIS – Fixing QString Errors in Map Tools and Map Canvas Using PyQGIS

That documentation in the cookbook is a little out of date. You shouldn't need to use QString at all now, and the following code should work:

actionZoomIn = QAction("Zoom in", self)
actionZoomOut = QAction("Zoom out", self)
actionPan = QAction("Pan", self)

Basically, just remove all references to QString - they'll be converted automatically behind the scenes and you don't need to do it manually yourself anymore.

Best Answer

Related Solutions

QGIS – Running a Python Script to Extract Raster Data

QGIS – Fixing QString Errors in Map Tools and Map Canvas Using PyQGIS

Related Question