Rhino Topography Model from Contour Lines

Quick reference

• Software: Rhino 8 (Rhino 7 works; Grasshopper ships with both).
• Input: contour lines, ideally 3D polylines at their true elevation, from the QGIS export (DXF, or the DWG prepared from it).
• Two routes: native Rhino (Patch, Loft, MeshPatch) or Grasshopper (Delaunay Mesh).
• Rule of thumb: use a mesh for rendering and GIS work; use a NURBS surface only when you need smooth, sectionable geometry.
• Units: the Venice lagoon is modeled in meters (EPSG:3003). Set Rhino's model units to meters before importing.

1. Check your contours first

The most common terrain failure is contours sitting at the wrong elevation. Before you build anything, confirm whether your lines already have a Z height.

1. Import the DXF or DWG: File → Import, choose the `.dxf` or `.dwg`.
2. Select a contour and check Properties (or run the What command). Read the point coordinates.

• If the Z values are real elevations, your lines are already 3D and you can skip to section 2.
• If every Z is 0, the lines are flat and you must assign elevations first (below).

1. If contours arrived as many short segments, select all curves (SelCrv) and run Join so each contour is a single closed polyline. Closed polylines build far more cleanly.

Assigning elevation to flat contours

• Move by a known amount: select the contour and run Move, then type `0,0,5.5` to raise it to 5.5 m. Or drag the blue gumball arrow and type the value.
• Set to an absolute height: run SetPt, select the contour, choose the Set Z option, and type the elevation. SetPt moves every point to that one height while leaving X and Y alone.
• Many contours at once: read the elevation from a layer name or label. Doing this by hand is slow, which is exactly when the Grasshopper route in section 4 pays off.

2. Build the surface in Rhino

Three commands turn contours into a surface. Each surface type has different characteristics.

Patch

The most common starting point. Produces a smooth NURBS surface.

1. Run Patch.
2. Select all contours and press Enter.
3. Set the options:

• Sample point spacing: start around one twentieth of the site width. Smaller catches more detail.
• Surface U/V spans: more spans means a smoother but heavier surface.
• Stiffness: raise it to keep the surface close to the contours and stop it ballooning between them.

1. Press Enter and inspect the result against the contours in a side viewport.

Loft

Builds a NURBS surface that passes exactly through the lines.

1. Run Loft.
2. Select the contours in elevation order, bottom to top. Do not skip around.
3. Accept the default Normal style and confirm.
4. If the surface twists, simplify the contours with Rebuild first, or switch to Patch or MeshPatch.

MeshPatch

Creates a triangulated mesh. This is the best choice for rendering, GIS, and most landscape work.

1. Run MeshPatch.
2. Select all contours and press Enter.
3. Raise the density if close contours need more resolution. The result is already a mesh.

3. Check the surface

Before moving on, make sure the model is honest.

1. Switch to Shaded mode and orbit. Look for spikes, holes, and flat spots between contours.
2. Cut a section with Intersect or a clipping plane and compare it to the contours.
3. If a Patch surface bulges between contours, raise its stiffness or rebuild with MeshPatch.
4. Repair small holes or spikes by patching that region, or by trimming.

4. Build the surface in Grasshopper

Grasshopper is a good option when you have many contours, when elevations live in layer names, or when you want a clean triangulated mesh with no hand tuning.

1. Type Grasshopper in Rhino to open it.
2. Add a Curve parameter (Params → Geometry → Curve). Right-click it, choose Set Multiple Curves, and select the contours in Rhino.
3. Extract points from the contours. Two options:

• Discontinuity (Curve → Analysis): returns the corner vertices of each polyline. Fast and exactly on the data.
• Divide Curve (Curve → Division): samples points along each line when you need more resolution than the vertices give.

1. Triangulate with Delaunay Mesh (Mesh → Triangulation). Connect the points.
2. Optionally clean the result with Weld (Mesh → Util) at a small angle, and check the normals with Rhino's Flip or UnifyMeshNormals.
3. Bake the final mesh (right-click the component, choose Bake) to bring it into Rhino as a real object.

Reading elevation from layer names

This does not apply to the current base dxf or dwg but may be useful for other files. If each contour's elevation is stored in its layer name, a layer-name component (from the Elefront or Human plugin) lets you read it, parse the number, and move each contour to that Z in a single definition. One setup replaces hours of manual SetPt.

5. Mesh or NURBS?

• Mesh is the landscape default: it renders fast, exports cleanly to GIS or other software and render engines, and does not distort between contours.
• NURBS is for smooth, sectionable, machine-able geometry and clean drawings.
• Convert between them with MeshToNURB (mesh to polysurface) or Mesh (NURBS surface to mesh at any resolution).

6. Common problems and fixes

• Contours at Z = 0: assign elevations first, or the surface comes out flat.
• Patch balloons between contours: raise stiffness, or use MeshPatch.
• Loft twists: select contours in order, or simplify them with Rebuild.
• Holes or spikes: patch the region, or trim.
• Mesh normals inside-out: Flip or UnifyMeshNormals.
• Model too heavy: rebuild with fewer spans, or reduce the mesh with ReduceMesh.