[gdal-dev] GeoTiff 16bit to 8bit with GDAL 11.x

[Christine Waigl]

Hi all,

I have been using the GDAL command line tools to pre-process various kinds
of remote sensing imagery datasets in GeoTIFF format. This includes
resampling 16 bit images (eg. from Landsat 8) to 8-bit and to combine bands
into RBG composites.

In GDAL 11.4 (and previous 11.x that I tried), this doesn't work any more:

gdal_translate -ot Byte -scale [sceneID]_Bn.tif [sceneID]_8bit_Bn.tif

​I usually apply this to the already clipped RGB composites, but digging
deeper, even for single-band images what seems to be happening is that the
-scale option doesn't correctly calculates the source range:


   - If I use the command above, or with -scale_1, the whole output file
   has a value 0 for each pixel
   - If I use the "-b 1 -scale", the whole output file has a value 255 at
   each pixel
   - If I use "-scale src_min src_max" with a manually inserted src_min and
   src_max, I get a reasonable result (though not necessarily identical to the
   GDAL result with automatically calculated min and max.


What is driving me absolutely bonkers, though, is that *occasionally* the
command "-b 1 -scale" DOES produce a correct 8-bit file. It's not
reproducible, though: if I delete all 8bit files and re-run the script over
a whole set of 16 bit files, a whole different file may be resampled
correctly.

With GDAL 2.0.2, my old script (written when GDAL was at 1.8.x on my
machine) seems to be working at the moment, but my main system is currently
at 11.4, and I have a reason not to upgrade right away. Is there a way to
make it work with 11.4? I could run gdalinfo first and extract min and max
from the stats, and then feed this into gdal_translate, but I'd like to
avoid this path if possible. A test file (a file subsetted with "gdal_translate
-projwin... " from a whole Landsat 8 band) is here:
http://www2.gi.alaska.edu/~cwaigl/images/LC80660142015170LGN00_B5_clip.tif
(3.8 MB).

Thanks for your help,

Chris


-- 
Christine (Chris) Waigl - cwaigl at alaska.edu -  +1-907-474-5483 - Skype:
cwaigl_work
Geophysical Institute, UAF, 903 Koyukuk Drive, Fairbanks, AK 99775-7320, USA
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