Mesh or Polysurface to Nurb, making a solid mass from a topography surface

Any idea how to change the mosh or polysurface to a nurb?
Secondly, I need to make solid mass from this topo for 3D printing it. do you have any better idea?
Thanks for your help
Help- (1000.2 KB)

I’m seeing “Invalid Brep” in your model?

The method used in the previous post won’t handle the holes in your mesh. Patch doesn’t make a perfect seam to close them (the blue areas). (1012.3 KB)

thanks for you reply,
The holes are not important at the moment since I have the profile edges of the wholes. I just need the topography to make a solid mass. Do you have any idea?
If I would be able to make a clean topography, I can use solid difference to make a mass with a cube.

Yes I do. Did you look at the thread/model I referred to above? Do you have questions about it?

I remember doing something very similar to what you are asking on the old forum but can’t find it at the moment. I found these two but they’re not the one I had in mind:

If you have the terrain mesh without the holes, that would be the place to start.

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I was looking at it, yes that’s it, thank you so much for your help

Sorry for bothering you.
I have another issue, I need to make a trench (excavating the topography) on the perimeter of the area instead of making a wall.
I tried to make an extrusion in Rhino and use Boolean2Objects, but there was an error.
can you suggest another solution please?
I attached grasshoper file with the brep and perimeter curve.
Help- (1.3 MB)

I used the Clipper offset plugin to get offset curves projected on an XY plane at the lowest point of the trench curve, extruded those vertically and used them as cutters with Split Brep (twice, one after another) to cut and extract the surface of the trench. Then extruded that down to make a “Closed Brep” of the trench and used SDiff to subtract it from your brep topagraphy. A little surprised that it worked, it might be better to raise the trench “solid” slightly above the surface to make sure SDiff isn’t confused by the co-planar surfaces.

The bottom of the trench is exactly as jagged as the surface. Making it flat somehow is another step that I didn’t take (sorry). It’s complicated… (1.3 MB)

By the way, when you created your brep topagraphy, you didn’t account for the fact that the terrain’s lowest point is below the World XY plane so part of it protrudes from the bottom.

that’s it, thank you so much, I tried sth similar but no result.
your knowledge of grasshopper is amazing man.
thanks for your help.