I have downloaded SDTS dem files from the USGS. These extract from tar.gz files as ddf files. I have tried importing these files into Qgis using "add raster layer and choosing .ddf or .DDF from the file type list. But the program returns "Unsupported Data Source: C:\tmp\RockyHills_dem\1694744.DEM.SDTS\9948CEL0.DDF is not a supported raster data source".
Can anyone help?