[gdal-dev] problems with gdal_merge.py

Volker Wichmann Volker.Wichmann at uibk.ac.at
Wed Apr 23 14:59:11 EDT 2008

Frank Warmerdam wrote:
> Volker,
> You will need to file a ticket on this, and provide as much supporting
> information (ie. .HDR files) as you can.
> I'm pretty sure we have already gone through several iterations on this
> so there is clearly something that isn't being properly understood.  I'm
> not going to take your fix on faith, I need to know I'm actually fixing
> things, not just stirring the pot again.

I will do so, and started to build up a test case. But first of all I 
may need some advice, I'm a little surprised by my first tests.
I build a simple 4 by 4 grid, geotiff format. Running gdalinfo on the 
file I get:

gdalinfo 4by4.tif
Driver: GTiff/GeoTIFF
Files: 4by4.tif
Size is 4, 4
Coordinate System is `'
Origin = (10.500000000000000,13.500000000000000)
Pixel Size = (1.000000000000000,-1.000000000000000)
Image Structure Metadata:
Corner Coordinates:
Upper Left  (  10.5000000,  13.5000000)
Lower Left  (  10.5000000,   9.5000000)
Upper Right (  14.5000000,  13.5000000)
Lower Right (  14.5000000,   9.5000000)
Center      (  12.5000000,  11.5000000)
Band 1 Block=4x4 Type=Byte, ColorInterp=Gray

The Origin / Upper Left matches with what I expected (the grid BOUNDARY 
is: N 14  S 10  W 10  E 14). I'm not sure to understand the values 
reported for the Lower Right - isn't this off by 1 cell in x and -1 cell 
in y??
I don't think it makes sense to go on before I get this ...
>> Other issues I'd like to see mentioned on the gdal_merge.py page 
>> (http://www.gdal.org/gdal_merge.html):
>> - if the output file is already existing, the behaviour of the module 
>> is indetermined as it tries to append to the file (?)
> I don't know why you say this. If the output already exists, then the
> script attempts to mosaic into it.
ok, I see it's a feature. But I think it should be mentioned in the docs
>> - the -ul_lr arguments reference the cell CENTERS
> The -ul_lr arguments are (to the best of my knowledge) outer pixel edges,
> not cell centers.
not for me, but as written above: first of all I need to get that right 
to build a proper test case.

your help is really appreciated,

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