Generating a closed terrace topography from a mesh

Hi,
I’m trying to create a closed brep in the shape of a terraced topography from a mesh surface so I can 3D print it. Like this, Terraceterrain|582x500

My method:

In Grasshopper I find the edges of the mesh surface (using Mesh NakedEdge) , project them on a plane underneath the mesh and make sidewalls between the mesh edge and the projected curve - the expectation being that I will get closed curves or atleast curves I can use after I Contour, with Boundary Surface to make all the contours into surfaces that I then Extrude down the same distance as my contour distance. Then Boolean Union the resulting breps.

I’ve been using two different approaches in making the mesh surface:

  1. Convert a DEM file to Heightmap with QGIS and using the heightmap with the Heightfield command

  2. Convert a DEM file to ESRI grid (ascii) with QGIS and using dtm ascii to mesh component (using all ascii points as resolution)

When I reference the mesh from the first approach it works, when I use the second approach the Boundary Surface component only makes one surface. Using the second approach I also tried joining what contour curves I was able to and running Boundary Surface on just those, but once again it only creates one surface (input is flattened).

The questions I’m stuck with are:

A. I would expect contouring a surface with sidewalls would leave me with closed curves, or atleast curves that I could join into closed curves, or which would work with Boundary Surface - is there a better way to do this?

B. Why won’t the Boundary Surface component make more than one surface even when I have culled the input and I’m only sending it planar curves?

I’m new to grasshopper and rhino so I’m assuming I probably missed some of the nuance in how these components work “under the surface”, so any help on that is also much appreciated!

Screenshot of .gh file: https://i.imgur.com/e7307gT.png

Attachments: (The different surfaces I’ve used are in the .3dm file)
3d topograhpy.3dm (1.4 MB)
Terrace_topography.gh (116.7 KB)
asciitest.zip (19.2 KB)

Not sure why yours doesn’t work, anyway using solely mesh works well in this case…


Terrace_topography_re.gh (385.0 KB)