[Qgis-user] Zonal statistics behaviour

[jasperfs]

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> 






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