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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple style='word-wrap: break-word;-webkit-nbsp-mode: space;-webkit-line-break: after-white-space'><div class=WordSection1><p class=MsoNormal>>> there is a gdal method to extract a single overview level from a tif but I can never <o:p></o:p></p><p class=MsoNormal>>> remember it maybe someone will chime in...<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>If so, that would be really helpful. Does anyone know how to extract a single overview from a pyramid’d image such as made from a command like:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>gdaladdo XXXX.tif 2 4 8 16 32 64 128 256<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> Norman Vine [mailto:nhv@cape.com] <br><b>Sent:</b> Tuesday, February 22, 2011 4:09 AM<br><b>To:</b> Jay Jennings<br><b>Cc:</b> gdal-dev@lists.osgeo.org<br><b>Subject:</b> Re: [gdal-dev] Downsample with averaging?<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Feb 21, 2011, at 3:26 PM, Jay Jennings wrote:<o:p></o:p></p></div><p class=MsoNormal><br><br><o:p></o:p></p><div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>Hello list,<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>(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.<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>I also looked at gdaladdo, which does have the “-r average” resampling option… the guidance at<span class=apple-converted-space> </span><a href="http://gdal.org">gdal.org</a><span class=apple-converted-space> </span>seems to suggest that it can produce a 32:1 GeoTIFF external overview with a command like this:<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>gdaladdo -r average -ro XXXXX.tif 32<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>But that produces a surprising error message, namely:<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>ERROR 4: `XXXXX.tif.ovr' does not exist in the file system, and is not recognised as a supported dataset name.<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>Am I barking up the wrong tree with gdaladdo for this purpose ? Anybody have any suggestions for highest-quality down-sampling ? Thanks in advance.<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Jay<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I think you need to build your overlays incrementally in order to get the average<o:p></o:p></p></div><div><p class=MsoNormal>of the 'block' at each overview level as gdaladdo has a very small sampling window<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>gdaladdo -r average my.tif 2 4 8 16 32<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>better to use gauss sampling<o:p></o:p></p></div><div><p class=MsoNormal>see <a href="http://en.wikipedia.org/wiki/Pyramid_%28image_processing%29">http://en.wikipedia.org/wiki/Pyramid_%28image_processing%29</a><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>gdaladdo -r gauss my.tiff 2 4 8 16 32<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>note you may want to convert your image to a floating point form first <o:p></o:p></p></div><div><p class=MsoNormal>so as not to possibly lose precision due to the multiple passes<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>there is a gdal method to extract a single overview level from a tif but I can never <o:p></o:p></p></div><div><p class=MsoNormal>remember it maybe someone will chime in if not you can use your gdal_translate<o:p></o:p></p></div><div><p class=MsoNormal>trick.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>HTH<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Norman<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div></body></html>