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I am trying to merge two tiles of the worldclim data in geotiff format, which I want to subsequently work with in the raster package in R.

When I use gdal's gdal_merge.py script: gdal_merge.py -v -o tmean1_06_16.tif tmean1_06.tif tmean1_16.tif

I get a merged file, but it seems as if the oceans have all been set to 0. see pic enter image description here

So I thought that the no data value is not set correctly when merging. Therefore I looked up the no data value in one of the original tiles with gdalinfo.

It seems the no data value is set to NoData Value=-3.39999999999999996e+38

So i tried to use that value when merging: gdal_merge.py -v -n -3.39999999999999996e+38 -o tmean1_06_16.tif tmean1_06.tif tmean1_16.tif

But that results in a file that seems to either have lost all information or have a legend that is shifted so far, that every temperature value is set to the max. Everything is a uniform green now. enter image description here

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I think I may have figured it out.

If one resets the Nodata value in the original files to -9999 with: gdalwarp -dstnodata -9999 tmean1_16.tif tmean1_16_error999.tif

and then uses gdalmerge specifying both the input nodata value and the output nodatavalue: gdal_merge.py -v -n -9999 -a_nodata -9999 -o tmean1_06_16.tif tmean1_06_error999.tif tmean1_16_error999.tif

Then everything looks ok:

enter image description here

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