[gdal-dev] Downsample with averaging?

[Jay Jennings]

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 Jennings
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