I just installed ImagJ to look at the height.raw heightmap file from my bowlma project. It can import and export in 8 and 16 bit raw format. I had to set the import to Little-endian byte order so white was the 28,000ft elevations and funny, black is showing negative depths under the water. Now I see if I can create a 16bit grayscale png file and export it to 16bit raw for the terrain editor. I suspect the TE will see "black" as sea level, I hope.
I started reading other game's terrain creation forums, and creating a grayscale heightmap file, then converting to raw is pretty standard. Some only use a png format grayscale heightmap. But then, their land area is much smaller than the AH MA. It's getting interesting digging into this, and hopefully the 16bit raw file export from ImagJ will be compatible with the TE when I generate my test file. I can always do the export from Krita as an .r16 and change the extension as Greebo described.