[Gdal-dev] gdal_merge.py

[Xiaodong Zhang]

Hi,

I'm using gdal_merge.py to merge 3 gtiff images (MODIS) with exactly the same
spatial extents but each only cover different parts of the area. Each input
image (byte) has areas of no data of values 0.

gdal_merge.py -o out.tif -pct -v -co compress=packbits input1.tif input2.tif
input3.tif

I had expected the final image should be the "merge" of the three, taking data
from three images. But instead, the final image has the same data cover as the
last input image, input3.tif. 

I also tried -n nodata_value (as below), but it gave me an error message,
ImportError: No module named Numeric

gdal_merge.py -o out.tif -pct -v -n 0 -co compress=packbits input1.tif
input2.tif input3.tif

By the way, what is the algorithm used by gdal_merge.py? Is it possible to
specify an algorithm, say, take the maximum.

Many thanks
Xiaodong