Vectorizing building perimeters at variable terrain altitude

[bat21]

Hi Daniel and the CC Users!

I came across a task where I stuck. I need to vactorize building perimeters for several buildings of different shapes and heights localized in a terrain of variable altitude.
I tried sectioning as mentioned here http://www.danielgm.net/cc/forum/viewto ... 6f6#p12483. I tried also rescaling Z vector to zero "0" to have a flat pointcloud.

But then I get some results in one part of an area but not in the other, where the terrain is higher.
The most suitable would be the perimeter of a shape 70cm-1m above the ground level, as teh roofs use to go off the perimeter like 0.5m-1m.
I attache the suitable image to picture the situation.

Any of you guys have an idea how to get the polygons/polilines of building perimeters in this situation? I want it to be automated not manual as tehra are a lot of objects - too many for being resonable to manually vectorize them.

regards,
Marcin