[gdal-dev] FW: gdal_merge.py

[Ian Reese]

HI Gdal,

Just wanted to follow this up.

Cheers,

Ian

From: Ian Reese
Sent: Monday, 12 March 2018 6:55 p.m.
To: gdal-dev at lists.osgeo.org
Subject: gdal_merge.py

Hi Gdal,

I am having some issues with gdal_merge.py.

When I merge two tifs together, the original nodata value is converted to 0, no matter what that original no data.  Here is my test case:

Data:

https://data.linz.govt.nz/x/9iitN4 DEM_BA32_3501_2013

https://data.linz.govt.nz/x/WLurgo DEM_BA31_3550_2013

Running gdalinfo on both files reveals a 'nodata' value of -9999.  When I view the data as individual files in QGIS, the 'nodata'  regions are confirmed as 'nodata' or '-9999' and are rendered transparent.

Next I run gdal_merge.py:

gdal_merge.py -o test_merge.tif *.tif

I also try:

gdal_merge.py -n -9999 -a_nodata -9999 -o test_merge_wnodata.tif *.tif

No matter which variation of merge I run, those regions that once contained 'nodata' or '-9999' are converted to '0' and the nodata value is not retained after the merge.  When I load the test_merge_wnodata.tif into QGIS, those regions once containing 'nodata', are now filled with 0 and rendered as such.

Maybe this is a bug or am I using gdal_merge.py improperly.

Cheers,

Ian


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