[Qgis-user] Zonal statistics behaviour

[Marco Hugentobler]

Hi Jason

Can you share sample data (or send it to me privately if it is not public)?

>(in v1.8.0)

I've recently fixed a bug in the zonal statistics code, but I think it 
was something different (out of raster bounds problem). Also, the 
performance for large data set has improved considerably with that 
commit, so you might try a recent nightly build.

Regards,
Marco

On 17.11.2012 03:20, jasperfs wrote:
> Hi there, (first-time poster here - please excuse me if this is the wrong
> place for this topic)
>
> Your posting might be an indirect confirmation of (similar?) problems we
> have discovered in the past few days - our first experience using Zonal
> Statistics tool (in v1.8.0).
>
> Our goal is to compute mean areal values over polygons from a raster layer.
> In this first attempt with the tool, the pixels of our raster layer are
> larger than the polygon areas (e.g, 5-10 polygons fit within a pixel).  When
> spot checking the mean output values, most of them *seem* to be correct
> except for occasional erroneous values that are easy to spot - that is,
> polygons that fall within in an area of all-zero pixels but still report a
> non-zero mean value (means such as 0.02, 0.3, 2.19, etc.).  It is not easy
> to detect if similar errors are occurring in the non-zero areas as well.
>
> There is no obvious pattern to the occurrence based our initial experiments
> except that they seem to occur primarily within basins that fall on or very
> near the corner-intersection of four pixels.  (In case that helps someone's
> diagnostics.)
>
> Because of these errors, we cannot use the tool for our intended purpose
> (compute mean areal precipitation over subcatchments) - but it would be a
> **HUGE** benefit to us, both in our training and operational purposes to
> incorporate additional forecasting data sets this way; it is such a simple
> solution, if only it produced correct results.
>
> We very much would like to see this tool work.
>
> Anyway... this report might be another "hit" to help identify the problem
> (if there actually is one) and get it fixed.  (please, please)
>
> Thanks for your post, it helps to find some sort of confirmation to the
> issue.
>
> Best regards,
>
> Jason
>
> <http://osgeo-org.1560.n6.nabble.com/file/n5017039/zonal_statistics_error_example.png>
>
>
>
>
>
>
> --
> View this message in context: http://osgeo-org.1560.n6.nabble.com/Zonal-statistics-behaviour-tp4993381p5017039.html
> Sent from the Quantum GIS - User mailing list archive at Nabble.com.
> _______________________________________________
> Qgis-user mailing list
> Qgis-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/qgis-user


-- 
Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee