Automating Zonal statistics in QGIS?

Question

How do I use zonal statistics in the python console to get CSV/dbf output?

I have used the code like this:

# Python code for Zonal statistics
from PyQt4.QtGui import *
from PyQt4.QtCore import *
from qgis.core import *
from qgis.utils import iface
from qgis.analysis import QgsZonalStatistics
#specify polygon shapefile vector
polygonLayer = QgsVectorLayer(' E:/Trash/BhagaP.shp', 'zonepolygons', "ogr") 
# specify raster filename
rasterFilePath = 'E:/Trash/Test48P.img'
# usage - QgsZonalStatistics (QgsVectorLayer *polygonLayer, const QString &rasterFile, const QString &attributePrefix="", int rasterBand=1)
zoneStat = QgsZonalStatistics (polygonLayer, rasterFilePath, 'pre-', 1, QgsZonalStatistics.Mean)
zoneStat.calculateStatistics(None)

Answer 1

Your code works perfectly but, you cannot visualize changes immediately because it's necessary to add polygonLayer to registry. However, it can be loaded manually.

A simplified version of your code (where is loaded vector layer to registry) is as follows (with my paths):

from qgis.analysis import QgsZonalStatistics

#specify polygon shapefile vector
polygonLayer = QgsVectorLayer('/home/zeito/pyqgis_data/polygon1.shp', 
                              'polygon1', 
                              'ogr')

QgsMapLayerRegistry.instance().addMapLayer(polygonLayer)

# specify raster filename
rasterFilePath = u'/home/zeito/pyqgis_data/utah_demUTM12.tif'

# usage - QgsZonalStatistics (QgsVectorLayer *polygonLayer, const QString &rasterFile, const QString &attributePrefix="", int rasterBand=1)
zoneStat = QgsZonalStatistics (polygonLayer, rasterFilePath, 'pre-', 1, QgsZonalStatistics.Mean)
zoneStat.calculateStatistics(None)

After running it at Python Console of QGIS:

it can be observed that 'pre-mean' field was correctly added.