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<p class="MsoNormal">Currently the scenes do not include any overviews. This forces applications to read the imagery at full resolution even if accessing at a small scale.<o:p></o:p></p>
<p class="MsoNormal">It’s common to include overviews/pyramids/reduced resolution datasets with such imagery.<o:p></o:p></p>
<p class="MsoNormal">I would like to recommend that such overviews are included with the imagery<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Typically this would increase the file size by approx 33%, which is quite considerable.<o:p></o:p></p>
<p class="MsoNormal">The size of the overviews can be reduced by skipping a level (eg use level 4 8 16 32) vs 2 4 8 16 32, alternatively using a factor of 3 ie 3 9 27 81<o:p></o:p></p>
<p class="MsoNormal">I tested this out on some images using Deflate with predictor 2<o:p></o:p></p>
<p class="MsoNormal">2 4 8 16 32 adds about 35%<o:p></o:p></p>
<p class="MsoNormal">4 8 16 32 adds about 9%<o:p></o:p></p>
<p class="MsoNormal">3 9 27 81 adds about 13%<o:p></o:p></p>
<p class="MsoNormal">My recommendation is to go with the 3 9 27 81 factors. This keeps the factor constant between all levels.<o:p></o:p></p>
<p class="MsoNormal">Average sampling would be used, except for the BQA band for which Nearest (else Majority) should be used.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Implementation would only require adding GDALADDO to the existing script<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We would also need to determine if we use internal or external overviews.<o:p></o:p></p>
<p class="MsoNormal">Internal overviews would increase the size of the files, but not change the directory structure.<o:p></o:p></p>
<p class="MsoNormal">Using external overviews (.tif.ovr) would add additional files to the directory, is more specific to GDAL.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">As I expect many users to be using GDAL to access these files, I would recommend that we use the external overviews. I don’t see the additional files as a burden.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">One other potential concern (and I’m hoping Frank can answer this) is the tile size of the overviews. GDAL appear to always use a tile size (BLOCKSIZE) of 128. It would be more optimum if the file size could be set to 512.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">_Peter<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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