Hi All
I finally got my head around QGIS a bit more and have made a couple of short videos of how I work with DEMs in GlobalMapper and how you can run a similar workflow in QGIS (which is free).
GlobalMapper to TG: https://youtu.be/kHqa9CE7Wyw (https://youtu.be/kHqa9CE7Wyw)
QGIS to merged DEMs: https://youtu.be/pSyS2y7tvRA (https://youtu.be/pSyS2y7tvRA)
One thing to remember is that QGIS is a GUI tool that uses GDAL as a backend. The actual processing is performed via a GDAL script generated by QGIS. As such, some functions do not actually require you to load any data at all.
WARNING It's very easy to create very large files very quickly. Manage your disk space when working with QGIS.
Merging DEMs is quite easy.
Raster > Miscellaneous > Merge
Select files (or a folder of files) to merge, specify an output file and hit OK.
By default it loads the merged result
Multiple level of detail is a 2 step process:
1: Select areas to resample
Load a single DEM (e.g. your merged DEM)
Raster > Extraction > Clipper
Specify an output file
Specify an area to export (enter coordinates or draw a rectangle)
Hit OK
2: Resample each DEM
Raster > Conversion > Translate
Select a layer or file to resample
Specify an output file
Tick "Outsize" and enter a percentage to resample the file
Hit OK
Repeat for each file.
Then in TG load the DEMs from lowest to highest resolution.
To use the same methodology as my GlobalMapper workflow requires some manual calculation of coordinates if you want to maximise the reduction in file size. As a rough guide you can use 0.0001° as the size of a pixel in the USGS 10m NED.