[gdal-dev] problems with gdal_merge.py

[Volker Wichmann]

Hi,

I'm checking gdal_merge.py for generating subsets and mosaics from one 
or more input rasters. I found the following problem using esri ascii 
grids as input:

gdal_merge.py -o clip.tif -ul_lr 672898.75 5125166.25 674323.75 
5123911.25 -v dtm_utm01111.ascii

Filename: dtm_utm01111.ascii
File Size: 1281x1121x1
Pixel Size: 2.500000 x -2.500000
UL:(672198.750000,5125601.250000)   LR:(675401.250000,5122798.750000)
Copy 280,174,571,503 to 0,0,570,502.


using the same file in geotiff format I get:

Filename: utm01111.tif
File Size: 1281x1121x1
Pixel Size: 2.500000 x -2.500000
UL:(672200.000000,5125600.000000)   LR:(675402.500000,5122797.500000)
Copy 280,174,570,502 to 0,0,570,502.


In case of the ascii, the file is resampled (and thus the values in the 
output subset differ from the original). This doesn't happen with a 
geotiff as input (see the "Copy" lines above).
It's also strange, that in both cases the reported UL and LR don't match 
the input coordinates and the extent of the output files (the latter 
match the coordinates passed with -ul_lr).

I'm still using GDAL 1.4.2.0 but haven't found any major changes in trac 
regarding gdal_merge.py. It would be nice if somebody could try to 
reproduce this with GDAL 1.5 to see if any of the underlying functions 
have changed.

Thanks,
Volker