Okay, I'll check that out first. What I did yesterday is point an orto cam down on an area just in front of the main cam, kill all lights and atmo, render to dsik after setting micro export. Turned out I still had the stones and wrong cam, so a piece of geo was a 'strip' towards horizon, with all polyspheres also present. I'll have a better go today.
@WAS , yes, without reflections a piece of ground renders pretty fast, but I was initially deterred by the long rendertimes due to (I think) low roughness (<0.2) reflective ground. And that is not necessary after all. But anyway interesting to see how exporting works, and whether rendering indeed speeds up over an obj.
Also interesting to be able to sculpt additional stuff into the geometry, which might be needed for some work.