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

[Otto Dassau]

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
> > >
> 
> 


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