While I’m on a roll here, here’s another problem I encounter frequently and wonder what the best approach is.
That is where I have a triangular zone to fill. The smoothest think I have tried is to do a NetworkSrf and make it tangent to the edges. The down side to that is that I find networksrf output has a hard time joining and it makes a lot of control points.
Problem Triangle.3dm (70.0 KB)Tangency is not possible with the surface on the left side because the edges of the other two surfaces are not tangent to that surface. A basic requirement from geometry (not just Rhino) is if a surface is to be tangent to two other surfaces at a corner, the edges of the two other surfaces must be tangent with the other surface. Similar for curvature continuity.
hey Jim as David already said, your geometry is not hitting each other smoothly, you can use EdgeSrf and try to match some of it together to get some tangency, but it will still create unevenly bumpy geometry either way.
if there is no absolute necessity to stick to this geometry you have now, then maybe try to create smoother situations right from that start.
Thanks,
The problem I have is that there are a lot of these things in plans that come to triangular points. I have to deal with the cards I am dealt. I get this situation frequently.
i can understand that, but sometimes its good to understand what the curves are for, to change them to your needs. just reusing given geometry may not always bring you were you want to be. so if its not a structural security reason that those curves hit each other nasty as they are, then for the sake of a better living and a better optic just fit them together properly 
In the example shown, those are three curves taken off of plans. Maybe there is some way to get the same effect using different curves and slicing—
@bigjimslade The problems with your example file are not the edge curves themselves, it is the combination of two of the curves and two of the surfaces.
Attached are the same boundary curves with different surfaces. The triangular edge curves are tangent to the surfaces. Problem Triangle DC.3dm (251.9 KB)
I would just use sweep 1 with the simple option ticked. The real problem is your imput surfaces/curves. Always start with clean simple/smooth geometry. Garbage in garbage out.
I probably should have done a simpler example. The two vertical curves are simple. The reason the bottom one is complex is that it is a part of a larger curve where the shapes above are different from the shapes below.
If I do not have high precision these curves, they end up not matching (a nightmare section to build).
What seemed to create a smoother surface was to extend the vertical curves downward and then bend them back so they joined up.
While taking into consideration the points raised by others, assuming you have tangent geometry - take a look at this example using Rhino for windows: https://youtu.be/SkrrtM17_nE
It should give you enough information to fill in the gap on Rhino for Mac.
Andy
That worked pretty well when I adapted it.
THX.
