Contours from draped surface

I have been trying to get contour lines from a site model. The starting point was a mesh which was then converted to nurbs surfaces and finally, I used drape to create a clean single surface topography.

Not matter what I try, I cannot get contour lines from it.

I have used contour tool but it creates only a few contours, not at every metre like I need. It is inconsistent in that it creates some but by no means all.

I have tried to do it manually by creating a plan and intersecting it with the surface but some contours are impossible to create (“intersect failed”).

I had a concern that the surface was too big so I cut it into 8 smaller bits and still it does not work properly.

I have tried using boolean tools instead to create a new edge that I could then use dupedge to extract the curve. That didn’t work.

I have tried split.

I have tried trim.

I have tried on both Mac and Windows versions of Rhino (latest releases).

If anyone has any tips it would be most appreciated. Attached is the draped surface which I have then trimmed around the perimeter to deal with the way the draped surface tends to ‘fall off’ at the edges (please note I have tried all of the above on a non-trimmed surface).

topo_for rhino forums.3dm (4.1 MB)

Unfortunately this is a known bug with the Contour command - actually anything that intersects a plane surface with a NURBS surface that has a lot of control points. It can work OK, but often it misses an odd contour here and there, or sometimes even whole groups of them. And sometimes it even works better after saving and closing the file and then re-opening it. I don’t know of any real workaround - except to use Contour on a mesh and not a surface.

Unfortunately contouring a mesh results in polylines, if you want smooth curves you then have to rebuild them.

In playing with your model, I also see that the result depends on the starting height. If you start at 0, the result is bad. If I just click anywhere, I get a lot more, but unfortunately, the contour lines are then not at even meter heights.

If I mesh your model, and contour the mesh, the result is practically instantaneous and good. That’s why I pretty much always start with mesh data.


topo_for rhino forums-msh.3dm (16.9 MB)

1 Like

Thanks, that’s very helpful. I have tried rebuilding the surface which seems to help too. In the future I will try meshing first and then rebuild the lines as curves. I think if rebuilding the surface works (as it seems to here), that is preferable to having to rebuild all of the lines!

Do you happen to know if the bug is being dealt with at all by McNeel?

I have reported this a long time ago - but it doesn’t appear to be in the bug tracker and it doesn’t seem to have been improved (yet) for the next version. I will create an item in the bug tracker to make sure this is on the radar.