A small note regarding this issue. My problem is that I never get a mean value of 1 even if all pixels inside the geometry is one.
Could this be because: 6x6 pixels goes into one polygon when visually controlling. If each pixel has the value 1, then this will be calculated as 36 / 36 = 1. However, if it calculates the sum to be 36 and divide by a number higher than 36 pixels, then the result will always be below 1. What i am thinking is that while it sums up the pixel values correctly, it does not count only the 36 pixels, but also neighboring pixels. Therefore: 1+1+1+1...n36 / Number of pixels higher than 36 will always lead to a number lower than 1. Anyone who knows the functions well could probably answer this. Best regards, Andreas 2011/11/25 Andreas Forø Tollefsen <[email protected]> > Could this have to do with the tiling of the raster? > I will try to run the same query with a untiled mountain raster to see if > that changes anything. > > Btw. When loading a tiled postgis raster into qgis it shows up with many > artifacts and no data areas. The same raster untiled does not show up the > same way. > Qgis bug? > > Andreas > > 2011/11/25 Andreas Forø Tollefsen <[email protected]> > >> Hi, >> >> Thanks for all of the suggestions. I will do some more testing. However, >> as for suggestion 1 i think the pixel size should be the same as the >> original raster or am I wrong? >> >> Both the mean_mnt_bin raster and the priogrid_land shapefile can be >> downloaded as zip (2 mb) here: >> http://gisintersect.com/mean_mnt_bin.zip >> http://gisintersect.com/priogrid_land.zip >> >> Any help on getting the correct values would be very much appreciated. >> >> My query: >> DROP TABLE IF EXISTS mountain_cell; >> >> SELECT >> a.gid As id, >> (ST_SummaryStats((ST_Union(ST_MapAlgebraExpr(ST_AsRaster(a.cell, b.rast, >> '32BF'), b.rast, 'rast2', '32BF','INTERSECTION','0','0',0))).rast, >> false)).mean As avgmnt >> INTO mountain_cell >> FROM >> priogrid_land a LEFT JOIN >> mountain b >> ON ST_Intersects(a.cell, b.rast) >> GROUP BY a.gid >> ORDER BY a.gid; >> > >
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