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@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user