QGIS: Merging Rasters (Digital Surface Models)

Question

I got several Raster Layers as Digital Surface Models (DSM) in QGIS. I now want to merge them by using Raster -> Miscellaneous -> Merge, but if I do, the resulting merged raster layer is just blank space, as seen in the picture below. Does anyone know what's the problem here? Changing the data type doesn't d anything, I used float32. I also tried merging only two, also didn't work...

UPDATE I got it working by using Raster -> Miscellaneous -> Build Virtual Raster but I'd still like to know what's the problem here. Does the virtual raster have any disadvantages vs the merged one?

Answer 1

A virtual raster is basically just a text file containing the filepaths to the original raster(tiles). But you can export your .VRT to any format (GeoTIFF, xyz..)!

Answer 2

Since you got the virtual raster to work, you should now be able to Export it (right click the layer in the layer tree) as a single combined raster file.

As I look at your merged output, I see faint whitish ghosts of your input, not quite a blank area as you describe it in text.

That might be that you are displaying your blank merged raster with a bit of transparency and so the underlying input is washing through. Or it may mean that somehow the merge algorithm is interpreting 16 (or at least some) of your rasters as white and averaging them with the meaningful 17th raster in a region. That's why you see nearly white.

If that hunch is correct, the issue is probably non-recognized NODATA pixels in the input rasters. Identify what data value they have (for instance by using the Identify Features tool - i icon in the toolbar) and then specify that in the Merge tool's Advanced parameters \ Input pixel value to treat as nodata and see if that fixes the issue. Not sure why Merge would fail while VRT would work in this way, though.