Is it possible to overwrite the raster generated using gdal_merge.py? When I ran the command:
% gdal_merge.py -init 255 -o out.tif in1.tif in2.tif
it works fine, but if I ran the same command and out.tif was already previously generated, it overwrites out.tif to a uniform raster (i.e., all values equal to 255).
