I created this very small tool that can easily convert terragen terrains to format that sandbox can read (.raw and .pgm). It is very simple to use and it takes only one step to get terragen terrain file to sandbox.

You can also use it with 3dem program to easily convert almost any DEM file (digital elevation model) to format that sandbox reads. (See the small tutorial at the bottom.)

I included the sourcecode in package. You need .net framework 2.0 to run this program.

Fileformats that tool reads:
* Terragen terrain file (.ter)
* 16-bit grayscale raw images (.raw) (Not really needed I use it for previewing some raw files.)

Fileformats that tool writes:
* 16-bit portable graymaps (.pgm) (I recommend using this instead of raw format. Because sandbox might crash if you read wrongly crafted raw file.)
* 16-bit grayscale raw images (.raw) (If you want to edit your heightmap in image-editor e.g. Photoshop you have to use raw file.)

If you want to open your .raw heightmap in photoshop use the following raw settings:
* Channels Count: 1
* Depth: 16 Bits
* Byte Order: IBM PC
* Header Size: 0

Remember to use the same sandbox map size as your heightmap is. Sandbox notifys you if map sizes are unequal. You can see the heightmap size in tool window when converting .ter file.