I am attempting to mosaic 2 DEMs from northern Illinois, USA with QGIS. One DEM has a 4m resolution while the other has 2m. Both DEMs have been clipped and converted to .geotiff from larger .adf rasters. The resulting file size for both DEMs is between 300-400mb. I am using WGS 84 16n for the coordinate system.

I have tried the merge and align tools under raster options with no luck. The operation fails with a memory error or system timeout after about 15 minutes of running.

Is there a step I am missing in this workflow? Do the DEMs need to be resampled at the same resolution/scale? Do I need to manually geo-reference the rasters to align them?

Or is there a more appropriate tool/method within SAGA or GRASS?

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