[Qgis-user] Zonal statistics behaviour

[jasperfs]

Hi Marco,

 

Thanks for such a quick reply.  I can certainly send along a copy of the raster – it is a public source – but let me get approval from management before sending a vector layer to you (not a public source).

 

I’ll hope to hear back from them tomorrow and perhaps send both layers together; if not, I’ll aim for Monday to get you some samples of some sort.  I’ll also work on getting an updated build to try (hadn’t thought of that yet – good to know there are improvements in the tool already).

 

Thanks for your help!


Jason

 

From: Marco Hugentobler-4 [via OSGeo.org] [mailto:ml-node+s1560n5017057h17 at n6.nabble.com] 
Sent: Friday, November 16, 2012 11:39 PM
To: jasperfs
Subject: Re: Zonal statistics behaviour

 

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> 
> 
> 
> 
> 
> 
> 
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-- 
Dr. Marco Hugentobler 
Sourcepole -  Linux & Open Source Solutions 
Weberstrasse 5, CH-8004 Zürich, Switzerland 
[hidden email] http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee 

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