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I have two raster DEMs in same resolution I want to bring together. I was using the merge-function in SAGA, GRASS and QGIS but I always get the following result:

enter image description here

Where could the fault be?

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    Are the DEMs still in their original CRS? Looks like a reprojection gap. – AndreJ Jan 24 '13 at 6:19
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    Or possibly a tiling artifact in the original data. I've seen this sort of thing many times with data from Ordnance Survey and the error is in how OS tiled the data and not in the merging process. It could be a similar thing here. – MappaGnosis Jan 24 '13 at 8:23
  • It might be worth making a virtual raster (*.vrt) from the two DEMs to see if you still get the same problem with this (Raster -> Miscellaneous -> Build Virtual Raster). N. – nhopton Jan 24 '13 at 10:35
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    Same with vitual raster...one of the DEMs are reprojected.... – MAP Jan 24 '13 at 12:33
  • Can you test if this is the same problem? osgeo-org.1560.n6.nabble.com/… – nickves Jan 25 '13 at 9:56
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You should first mosaik the map tiles, then perform the reprojection. This will avoid the gaps unless they already exist in the original data.

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Not enough rep to comment but I believe markusN is refering to running:

gdalbuildvrt OUTPUT.img.vrt DEM_1 DEM_2 DEM_N

FWIW, this is what I do in my workflow to avoid visible seams at tile boundaries.

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