Just created a new terrain myterr and followed the folder structure getting my custom arrangement of 508 and 1016 tiles.
ah3terr
-------myterr
stdshape
terrset00
spdtree
swa
texsrc
texsrc
I opened myterr to the default Eu summer terrset00, then closed it. Then I created the folder structure and copied into terrset00 only the following:
atlas0.txt
atlas1.txt
catlas0.txt
catlas1.txt
objdet.txt
terrdet.txt
Then I opened atlas0 and atlas1 and copied into terrset00\texsrc all of the bmp files related to the listings in these two text files.
ATLAS0
atlas,
512,ImageSize
4,ChannelCnt
4,rows
4,cols
texsrc\sand.bmp
texsrc\dunes.bmp
texsrc\dryscrub.bmp
texsrc\drygrass.bmp
texsrc\erosiongrass.bmp
texsrc\grass1.bmp
texsrc\decidforest.bmp
texsrc\grassfeild0.bmp
texsrc\grassfeilds.bmp
texsrc\grass1.bmp
texsrc\roughgrass.bmp
texsrc\grass1.bmp
texsrc\rockgrass.bmp
texsrc\grass1.bmp
texsrc\pineforest.bmp
texsrc\pacrock1.bmp
ATLAS1
atlas,
1024,ImageSize
4,ChannelCnt
2,rows
2,cols
texsrc\farm1.bmp
texsrc\village.bmp
texsrc\mountain.bmp
texsrc\cliff.bmp
The next time I opened myterr, I had my custom palette And the terrain editor generated all the additional files and folders needed.