Why Can't I Ever Trim or Split Any Curved Surfaces?

Hello. I’m new to Rhino but have researched high & low & don’t understand why this keeps happening. I don’t have any trouble splitting or trimming surfaces that are planar but the moment a surface is curved, I can’t trim or split it. It’s making me crazy.

In this file I’m trying to use the “streets” layer to split the “topography” layer.

Please see file.
McCambridge_Context.3dm (6.6 MB)


You’re topography is a mesh, which isn’t ideal for split or trimming operations like this. A Nurbs surface would be much easier to do this with. There’s a number of ways this can be achieved. In Rhino 7, you could for instance convert from mesh to SubD to surface.

Furthermore, if your topography surface is non-planar, but your street defining curves are, then it’s probably best to extrude the curves first so that the extrusions fully intersect the topography geometry. Then you can use them as splitters or cutters.


Thank you so much! I will try this right now. This was really driving me up the wall! I really appreciate your help.

See if MeshTrim command solves your problem.

That’s using hammer to drive in a screw! It’d probably work but the result wouldn’t be ideal.

Well it provides a solution to the problem the OP was having, trying to trim a mesh using the normal trim command. This won’t work on a mesh, as you pointed out. Meshtrim solves that problem, but whether it creates a result that is not ideal is another matter, and depends on the desired outcome.