I started to work with CryEngine Sandbox eight years ago. (damn, already 11 years?) I also work with Game Maker.
Finally managed to read Zs from the terrain. Originally I wanted to read it directly from colors of a heightmap but it appeared to not be possible. After consulting with Game Maker forum member kburkhart84, I've managed to reach it using different way. There's still a lot of work around terrain to be done but this 3 weeks long fight is finally over.
BTW, that building is only in 20% of its real size. Just telling you for an idea how miniature this testing terrain is.