I have NOT figured out how to mark custom map tiles as terraformable of any level. Depending on the world art used despite being a class 20 planet only has 15 tiles available, and none can be terraformed at any level.
I assume it has to be a custom map, and there is a line out there which can mark how many level 1/2/3 tiles it has. I can't seem to find anything that seems right in any of the other 'planet' files.
As far as I can tell from looking at and modding the game files, there is no such mark or designation for maps that tells the game what "level" of terraforming is allowed for each planet. Except for the major race homeworlds, all the planets are (somewhat) randomly generated by combining bits and pieces from pregenerated planet surface art. What all of the various terraforming projects do is look at the percentage of land vs. water to decide if that particular tile may be terraformed.
For example, Soil Enhancement has this line:
<LandPercentageMin>0.3</LandPercentageMin>
For the most part, as you develop additional terraform techs, that LandPercentageMin number decreases; Terraforming Plant is 0.25, Habitat Improvement is 0.2, Resequencing Station is 0.15, etc. The best techs (Biospheres and Universal Soil Adapter) have 0.0 listed, meaning they can terraform ANY single tile.
This may also explain why your custom class-20 planet has only 15 useable tiles - the Land percentage is just not high enough beyond those 15 tiles.
