[Qgis-developer] get mean of pixel values within defined area around current mouse position

[Otto Dassau]

Hi,

I have a question to update the code I sent in a former below to QGIS 2.0.
Again I need the mean value, currently use:

vpoly = QgsVectorLayer("Polygon", "pointbuffer", "memory")
feature = QgsFeature()
feature.setGeometry(QgsGeometry.fromPoint(QgsPoint(3517601,5406205)).buffer(10000,5))
provider = vpoly.dataProvider()
provider.addFeatures( [feature] )
vpoly.commitChanges()
stats = QgsZonalStatistics(vpoly,"/path/to/corine2006_z0_test_gk3.tif")
stats.calculateStatistics(None)
allAttrs = provider.attributeIndexes()       
for feature in vpoly.getFeatures():
   for attr in feature.attributes():
      return attr

attr in this case is probably the sum as I can see from the results, but I
need the mean again, as below. 

For QGIS 1.8 k==0 is number of pixels, k==1 is sum, k==2 is mean, see below.
But something like return attr[2] doesn't work. It says TypeError: 'float'
object has no attribute 'getitem'. Maybe someone has an idea how to solve
this?

Thanks a lot

Otto


Am Wed, 17 Jul 2013 09:21:55 +0200
schrieb Otto Dassau <dassau at gbd-consult.de>:

> Hi,
> 
> I wanted to give a short feedback. Finally my solution for qgis 1.8 with
> some help from Anita was to create a buffer polygon around the point as a
> memory layer and calculated the mean value with QgsZonalStatistics. Here
> is the code I used, maybe useful for others:
> 
> from qgis.analysis import QgsZonalStatistics
> from PyQt4.QtGui import *
> 
> vpoly = QgsVectorLayer("Polygon", "pointbuffer", "memory")
> feature = QgsFeature()
> feature.setGeometry( QgsGeometry.fromPoint(QgsPoint(3517601,5406205)).buffer(10000,5))
> provider = vpoly.dataProvider()
> vpoly.startEditing()
> provider.addFeatures( [feature] ) 
> vpoly.commitChanges()
> stats=qgis.analysis.QgsZonalStatistics(vpoly,"/path/to/rasterlayer.tif")
> stats.calculateStatistics(None)
> allAttrs = provider.attributeIndexes()
> provider.select(allAttrs)
> provider.nextFeature(feature)
> attrs = feature.attributeMap()
> for (k,attr) in attrs.iteritems():
>   if k==2:
>     QMessageBox.information(qgis.utils.iface.mainWindow(),'Info',attr.toString())
> 
> Regards
> Otto
> 
> Am Wed, 3 Jul 2013 09:36:45 +0200
> schrieb Otto Dassau <dassau at gbd-consult.de>:
> 
> > Hi Fred,
> > 
> > thanks for the hints, A and B sound interesting - PostGIS is not an
> > option here. 
> > 
> > One more question, because you already used these workarounds. What
> > solution A or B should be more usable (faster) from your opinion, if I
> > need to calculate the mean while moving the mouse over the map canvas?
> > 
> > Regards
> > Otto
> > 
> > Am Tue, 2 Jul 2013 18:05:10 +0100
> > schrieb Fred Lehodey <lehodey at gmail.com>:
> > 
> > > Hi,
> > > 
> > > Not sure it's the best way but I did something similar with these
> > > workarounds:
> > > 
> > > A - with gdal:
> > > 1 - the user click and you get your buffer. (already done)
> > > 2 - use gdalwarp with "-cutline" and "*-*crop_to_cutline" options to
> > > extract a new raster. (python subprocess)
> > > 3 - read the created raster in a numpy array to do all the statistics
> > > you need.
> > > 
> > > B - with matplotlib:
> > > 1 - with the buffer: create an array of the vertices
> > > 2 - read the raster into another numpy array
> > > 3 - use the "points_inside_poly" function of "matplotlib" lib to
> > > create a numpy mask.
> > > 
> > > Not tried but postgis raster should do the job.
> > > 
> > > Hth,
> > > Fred
> > > 
> > > 
> > > On Tue, Jul 2, 2013 at 1:12 PM, Otto Dassau <dassau at gbd-consult.de>
> > > wrote:
> > > 
> > > > Hi,
> > > >
> > > > for a qgis 1.8 plugin I would like to query the mean of pixel values
> > > > of a raster layer within a defined buffer for the current mouse
> > > > position (not for
> > > > an existing point layer).
> > > >
> > > > For the buffer it works:
> > > >
> > > > ...
> > > > feat.geometry().asPoint()
> > > > (3.5176e+06,5.4062e+06)
> > > > buff_geom = feat.geometry().buffer(1000, 5)
> > > > buff_geom.asPolygon() [[(3.5186e+06,5.4062e+06),
> > > > (3.51855e+06,5.4059e+06), (3.51841e+06,5.40562e+06),
> > > > (3.51819e+06,5.4054e+06), (3.51791e+06,5.40525e+06),
> > > > (3.5176e+06,5.4052e+06), (3.51729e+06,5.40525e+06),
> > > > (3.51701e+06,5.4054e+06),...]]
> > > >
> > > > but now I try to find a solution to query the raster pixels within
> > > > the buffer zone and calculate the mean, but I don't know how and if
> > > > this is possible in QGIS 1.8 at all. I found QgsZonalStatistics, but
> > > > that doesn't work for my task.
> > > >
> > > > Does anybody have a solution for this or a better idea, how to solve
> > > > this?
> > > >
> > > > Thanks
> > > >
> > > > Otto
> > > > _______________________________________________
> > > > Qgis-developer mailing list
> > > > Qgis-developer at lists.osgeo.org
> > > > http://lists.osgeo.org/mailman/listinfo/qgis-developer
> > > >
> > 
> > 
> 
> 


-- 
Geoinformatik Büro Dassau - http://www.gbd-consult.de
FOSSGIS consulting , training , support  and analysis
Ackerstrasse 144c  ,  D - 40233 Düsseldorf  , Germany
Tel: +49-(0)211-47468178 , Mobil: +49-(0)171-4687540

--
Community Advisor - QGIS Project Steering Committee