So it is not possible using this method.

Any other ideas? 

Seems strange that there is no way to flatten part of a procedural terrain. 

Thanks, great idea!

However, I couldn't figure out how to make an object the center of the flat area, instead of the world origin. I tried editing the metanode and setting an external dependecy for the object, but it won't connect to anything.

Any ideas?

Ok, I figured it out eventually. I used a round black & white image as a texture map node in the function editor. I then used a subtract blending mode in a kombiner with the rest.
That gave me a round elevated terrain, retaining exactly the same terrain features I had before.

Finally I used clipping until the square floor got cut off completely. 

Thanks everyone for the inspiration!

 Thanks, I know about the clipping technique.

Unfortunately I don't think this can work with my terrain - a procedural one driven by terrain fractals.

You see, my goal is not a round mountain. I want a round platform to be the basis for large scale desert canyons.

I can't figure out what's wrong with my scene. Even if I delete all of my objects and leave only one simple terrain, or water, it still crashes when rendering to RPF/RLA.

Maybe there's a way to script a batch render that automatically reload Vue on crash and resume rendering?

OK , I upgraded to build 283512 and that didn't solve the problem.

However I tested rendering with another scene and it didn't crash, so I guess there's something in my scene that causes Vue to crash when rendering to RLA or RPF.

FuzzyVision- I tried all the techniques above but nothing helped..

IvanB - Thanks alot , that seems like a great way to get around this problem! I will try that later.

Leo.

In the meantime I think I figured out something. The smaller the populating objects are, the less they will invade the forbidden space. I think it has to do with how Vue treats their geometric bounding box. It seems that the ecosystem allows objects to show beyond their defined area, as long as part of them is still inside.

I can't think of a way to tell Vue that the whole object should stay inside the defined area.

Talley - the foreign object idea sounds like a smart workaround, I'll try that later.

Leo.