On Feb 21, 2011, at 3:26 PM, Jay Jennings wrote: > Hello list, > (using GDAL 1.8.0) I am trying to create a 32:1 down-sampled overview of a > GeoTiff satellite image. My first thought was gdal_translate, with args such > as “-outsize 3.125% 3.125%”… which produces surprisingly high quality given > the absence of a resampling option. However I’m looking for a downsampling > scheme that creates a result pixel by averaging all relevant source pixels (I > know, for 32:1 downsample, that means 1024 source pixels for each result > pixel !) with the hope of an output that is not “speckled” or “grainy” > insofar as possible. > > I also looked at gdaladdo, which does have the “-r average” resampling > option… the guidance at gdal.org seems to suggest that it can produce a 32:1 > GeoTIFF external overview with a command like this: > > gdaladdo -r average -ro XXXXX.tif 32 > > But that produces a surprising error message, namely: > ERROR 4: `XXXXX.tif.ovr' does not exist in the file system, and is not > recognised as a supported dataset name. > > Am I barking up the wrong tree with gdaladdo for this purpose ? Anybody have > any suggestions for highest-quality down-sampling ? Thanks in advance. >
Jay I think you need to build your overlays incrementally in order to get the average of the 'block' at each overview level as gdaladdo has a very small sampling window gdaladdo -r average my.tif 2 4 8 16 32 better to use gauss sampling see http://en.wikipedia.org/wiki/Pyramid_%28image_processing%29 gdaladdo -r gauss my.tiff 2 4 8 16 32 note you may want to convert your image to a floating point form first so as not to possibly lose precision due to the multiple passes there is a gdal method to extract a single overview level from a tif but I can never remember it maybe someone will chime in if not you can use your gdal_translate trick. HTH Norman
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