Thanks Michael, that makes perfect sense. Pure laziness on my part for not scaling those values back. I'll fix that and try projecting again.
The end goal of this particular exercise is not the terrain itself. I'm modelling an airfield layout with MOI and need the terrain to project the 2D model onto so that the model follows the terrain when I place it on the landscape in the Unreal 4 engine. I'd much prefer to work with MOI but if I can't get it to handle these massive chunks of data, can you suggest an affordable polygon modeler that might handle it please?

I'll continue to use MOI for everything else as it's absolutely superb. You must have put a lot of time and thought into the interface as it's amazingly intuitive. The projected snap points are so useful and all the constructs/transforms are child's play compared to the overly complicated features in Solidworks. Top marks!

Hi Sutts, well for a polygon modeler you might check out Silo or Blender.

If you do a web search for "blender terrain", you can find some stuff:

https://artisticrender.com/how-to-use-blenders-free-terrain-generator-ant-landscape-add-on/
https://www.blendermarket.com/products/true-terrain-
https://www.youtube.com/watch?v=Tf9qUs0rqik
https://www.youtube.com/watch?v=jyvbRJJTzY4

- Michael

Or this free one who has an UI Moi's like! ;)

And direct MOi <-->Rocket 3F objects export / import to the screen in the 2 ways with just a press key on an icon !
https://rocket3f.com

Thanks for the pointers, will check those out.

Here the same cube drilled above on a volumical spiral with the Flow Function!
As you see in less 30 seconds absolutly no problem & difficulties! :)