Desert areas

[planetmaker]

During normal play, the ground type cannot be modified.

Changing ground type can only be done in the scenario editor. For an existing map, rename the savegame file extension from .sav to .scn and then open it with OpenTTD's scenario editor. Save the file after you made the modifications and start playing the scenario you just saved (you can rename it to .sav, but that is not necessary)

I removed one of your postings as they were identical (probably posted accidentially due to connection problems)