Extruding the bottom face on different polysurfaces at varied lengths

Grasshopper
1

Hello all. I’m very new to grasshopper. I’m attempting to use grasshopper to extrude the bottom face of a series of different polysurfaces (which represent buildings) downwards into a terrain/topography. The faces are to be extruded at a length that directly corresponds to the height of each individual building. This is so that I can then boolean difference the extrusion and the topography and obtain recesses into the topography that correspond to the building heights.

So far, I’ve used deconstruct brep so I can extract the face I would like to extrude and I’ve also extracted the two vertices at the top and bottom of each polysurface that correspond to the height but I’m now unsure of how to use this extracted information to perform the extrusion. I’ve attached a sample of the topography along with the gh file…any help would be greatly appreciated!

topography test
topography test3537×3665 1020 KB

topography test.gh (477.1 KB)
topography test.3dm (7.5 MB)

2

topography test_2025Feb12a
topography test_2025Feb12a1503×382 129 KB

topography test_2025Feb12a.gh (480.5 KB)

I see topography in the Rhino file (with some divots/holes) but not in the GH file?

3

By the way, the first error I saw in your GH file was this:

missing
missing387×203 12 KB

A bounding box has only six sides, not ten :exclamation:
It worked only because List Item ‘W’ (Wrap) defaults to True, so i=10 gives you the 4th face.

The second error is that you got the bottom of the bounding box, not the building. :roll_eyes: