Modeling from CAD Linework
The terrain surface is a single continuous form. Your design carves it up into discrete, purposeful areas.
Why this matters
Site design in plan is a set of boundaries and areas. In 3D, those boundaries become cutting lines that divide the terrain into independently controllable surfaces — paving, lawn, planting, water. The SPLIT workflow is how you turn your 2D CAD plan into a 3D model that actually represents the spatial conditions you designed. Do this step by step, layer by layer, and the model will be accurate. Rush it, and the model will fight you every time you try to assign materials or add grade changes.
Pre-split setup
Before splitting, verify that your CAD linework is positioned correctly relative to the terrain surface. In the Front or Right viewport, the linework should sit above the terrain surface — not on it, not below it. If curves are embedded in or below the surface, SPLIT will fail. Use MOVE to lift the linework layer slightly in Z (a small value like 0.1 units) if needed. This is a modeling convention, not a dimensional inaccuracy — the surfaces will be positioned correctly after splitting.
Create new layers to hold your split 3D surfaces. Use the same layer naming conventions as your CAD file to avoid confusion.
Splitting the terrain
| Command | What to do | Notes |
|---|---|---|
| Turn off layers | Turn off all non-hardscape layers (vegetation, furniture, annotations) | SPLIT will attempt to use every visible curve as a cutting object. Visible curves on non-hardscape layers will cut unintended shapes into the terrain. Work only with the layers relevant to the current split operation. |
| SPLIT | Select terrain surface → Enter → SELCRV → Enter | SELCRV selects all visible curves as cutting objects. If the command proceeds to the command line — your linework was clean enough to split the surface. |
| If SPLIT fails with SELCRV | Try SPLIT → SELCLOSEDCRV → Enter | SELCLOSEDCRV uses only closed curves. If this works, you have open curves in your visible layers that need to be closed before the full SPLIT will work. |
| If SELCLOSEDCRV succeeds | SPLIT remaining open curves separately | Select the new subsurfaces individually and SPLIT with the remaining open curve geometry. |
| If SPLIT still fails | Bad linework | Curves that produce no split result have gaps, self-intersections, or are not projecting correctly onto the surface. Redraw them in the Top view with Osnap End active. |
After splitting — surface organization
After a successful split, the terrain becomes many sub-surfaces. Move each to the appropriate layer immediately — don't continue splitting without organizing. A disorganized collection of unnamed surfaces is impossible to work with efficiently.
3D curbs and vertical elements
| Command | Settings/Notes | Result |
|---|---|---|
| DUPBORDER | Select a split surface (curb, step, raised bed) | Creates a closed curve at the perimeter of the surface. This curve becomes the basis for a vertical element. |
| PATCH | U and V both set to 100 | Creates a smooth NURBS surface from a boundary curve. Used for surfaces that need to be capped, covered, or given a top face. |
| EXTRUDESRF | Select surface → type height (negative Z = down) | Extrudes a surface to create a solid face. A curb surface extruded downward creates the vertical face of the curb. |
| MOVE | Type Z offset value | Positions the new extruded element vertically relative to adjacent surfaces. Use the Front viewport and snap to align tops and bottoms. |
| FILLETEDGE | Set radius before selecting edges | Rounds the top edge of a curb, retaining wall, or step. Keep radii small and consistent — 0.05 to 0.1 for curbs. |
| IMPORT | File → Import → select .3dm, .obj, .dwg | Imports external model content (provided building, downloaded site elements). Always check scale after import with DI. Scale to correct dimensions if needed. |
Try this
After splitting the terrain, select each sub-surface individually and check its area using the AREA command. Then go back to your CAD plan and estimate the area of the same element using LIST on the hatch or boundary polyline. The numbers won't match exactly due to grade, but they should be in the same order of magnitude. A major discrepancy means the terrain or the linework has an error worth finding before you continue.
What breaks
Linework below the terrain surface — curves that intersect the terrain from below produce unpredictable split results. Visible symptoms: the SPLIT command produces far more or far fewer surfaces than expected, or produces surfaces that extend far beyond the cut boundary. Always verify linework is above the surface before splitting.
Not organizing surfaces immediately after splitting — Rhino creates all split results in the active layer. If you continue splitting without moving sub-surfaces to their correct layers, you end up with an undifferentiated pile of geometry that cannot be assigned materials or selectively hidden. Organize as you go.