 Hello, this is Hans van der Kwas, Senior Lecturer at IHE Delft Institute for Water Education. In this video I'm going to show you how to mosaic raster images that are in different UTM projections, different UTM zones. Often when we download global data it's in the geographic coordinate system, but sometimes the tiles are in UTM coordinates. In this example we have 12 images from the globlant 30 dataset, and we see in the layer properties that some are in zone 42, some in 43 and some in 44, and that poses a problem when we want to mosaic. The easiest way to mosaic large raster datasets is to build a virtual raster. So you go to raster, miscellaneous, build virtual raster, and there you select your input data. So here we select all, click OK, and always make sure that place each input file into a separate band is unchecked, and for the rest we can use the defaults, and in this case I don't save it to a disk but keep it as a temporary layer, and here we see the result and this doesn't look like the mosaic that we want because it should look exactly the same as the input. So let's remove this layer and do it in the correct way, and that is that we convert all these layers to the geographic coordinate system, EPSG4326, and that would be a lot of work if we would go clicking, so I made a little batch file here, and it says that for each file in and then all the TIFF files that it finds in the folder do, and then echo means print the name of the file, the TIFF part is the part of the file name before the dot, and then execute GDOLWARP which is the reprojection, T underscore SRS is the projection to which we want to reproject, which is EPSG4326, and I define a no data value that's needed to mask the filled up areas after reprojection, and I use the value 0, and then the input file is %f, that is the file, and then I save it to a folder reprojected, and I use %tildeNF for the part before the dot, and then I add underscore GCS.TIFF, and that forms then the output file name. So in order to execute this GDOL command, GDOLWARP, we need to run it in the OSGO4W shell that you can find with your QGIS installation. I go to the ZDRIVE where I stored it into the folder merge, and there I find all the TIFF files, and the world files, the world files are the TFW files, and I made a folder reprojected, and then I simply run the batch file, and go for a coffee because that will take a while, and here I have done it 10 times faster, but it's still faster than doing this for 12 layers manually in the QGIS interface, so these kind of scripts are always very useful. So let's see the result, go to the reprojected folder, and there I see my reprojected files. Let's open them in QGIS, I remove the original ones, not to get confused when I want to later mosaic it, select it, and there it is, and it looks good. So now we can build the virtual raster again, select all, okay, and then I can run it. You see it's pretty fast, this virtual layer, and there it is, and you can further process it from here as one layer.