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

[Otto Dassau]

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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