I started in Terragen around 2002AD, and the Terragen scenario was flat, and Terragen had files *.tgw (worlds) and *.ter (terrains), and I got used to that fornat.
About December 2014 on a later computer I got back into Terragen with a new downloaded free version, and the Terragen scenario was very different, set on a spherical planet, and it had no *.tgw or *.ter files, but instead a Terragen world was saved on a *.tgd file, which could be used to call up the job again (like *.pz3 for a Poser job). I got used to that format, and I made some images with it.
Recently my computer needed repair, and its main disk was reformatted and re-installed. Fortunately I had saved my files on a terabyte offload disk pack. I looked in Wikipedia for information about Terragen, and downloaded the free version of Terragen 3, and its scenario is flat, and it again has files *.tgw (worlds) and *.ter (terrains), as in 2002, and it does not seem to recognize *.tgd files.
How can I make any use of my *.tgd files on Terragen 3? What has happened?