I thought this would be extremely basic task but obviously it's not. In QGIS3 GDAL Merge (GUI), I am trying to merge two rasters which overlap and each of them has a different distribution of nodata values. I want the rasters to be mosaiced together such that nodata values are ignored if one the rasters does not have nodata value on that spot. In ArcGIS it is that simple, why not QGIS?
QGIS – How to Merge Overlapping Rasters Ignoring No Data Values
gdalmergenodataqgisraster
Related Solutions
I managed to find a workaround for this issue by converting the data format to Int16 from Float32. The minimum value is then -32768 and can be processed as a nodata value. The following command did the trick:
gdal_translate -ot Int16 -a_nodata -32768 input.tif output.tif
There's probably a better solution, but this solves my immediate problem at least.
Instead of using NODATA, in this case it is better to use a mask:
As the NODATA value is 0, I can easily create the mask with -co NBITS=1 as everything above a value of 1 will be cut to 1.
# Create the mask
gdal_translate -of GTiff -ot Byte -co NBITS=1 band1.jp2 mask.tif
# Create VRT
gdalbuildvrt -separate bands_merge.vrt band1.jp2 band2.jp2 band3.jp2 band4.jp2 mask.tif
# Create the tif
gdal_translate -of GTiff -ot Byte -b 1 -b 2 -b 3 -b 4 -mask 5 -scale 0 4095 0 255 bands_merge.vrt bands_merge.tif
# Remove the temporary files
rm bands_merge.vrt mask.tif
Unfortunately QGIS cannot handle GeoTiff mask files yet, but I need it for GeoServer.
Best Answer
Probably best performed using the command line. Yes there is the gdal_merge script, but an alternative approach is to use gdalbuildvrt with the component rasters and use gdaltranslate to create the final raster. See the answers to this question:
Merge thousands of rasters with gdal_merge.py
For my application, I had a lot of no-data values and did not have any problems. I had to use custom merge scripting though. So I created a python script which updated the VRT and applied the required Python functions.