Flow along surface problem

I’m fairly new to rhino so I may be making an obvious mistake here.

I am attempting to flow an extruded road system along a surface that has been shaped to the contours of a site I am working on using the FlowAlongSrf command. Each time i try this the result is severely warped. I have attached images showing the result. Any help would be greatly appreciated. I’ve attached the Rhino file at the bottom as well.

Screen Shot 2020-05-25 at 1.27.30 am|690x384
Screen Shot 2020-05-25 at 1.29.56 am|690x343

Rhino issue.3dm (5.3 MB)

Hi there,

Do you need the objects to be ‘projected’ downwards? As if you were dropping them from the flat plane and landing on the patch surface?


Yeah that’s exactly what I’m trying to do. Would you be able to show me how you did it?

Sure. Here is how I would do it. It might be considered a cheat actually.

  1. In blue, make a copy of your patch surface topograpy. Run UntrimAll. This is going to be the new Target Surface at the end.
  2. Select the untrimmed surafce. Run SetPt. Check only the Z value. Copy=Yes in the command line. Select your flat plane for moving it up to
  3. Now run FlowAlongSrf, Using the flat red surface as Base Surface, and blue as the Target Surface

Below is why you may, I’m guessing, want ConstrainNormal=Yes in step 3. Basically keeps everything ‘pointing’ vertically. Maybe it’s not so important.

Let me know how you get on - FlowAlongSrf took FOREVER for me (30 minutes or so) but then my laptop is nearing the end of its life.

This is also a good video for more about the intricacies of FlowAlongSrf. Once you get it, it makes a lot of sense really.

Can Extract the bottom surfaces
then duplicate the edges
create a copy of the Patch and Untrim
use the project command
and part that surface with the projected curves then extrude
Rhino issue.3dm (10.4 MB)

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Hi John,

Thanks so much for the help. It worked perfectly! The FlowAlongSrf did take a while but got there in the end. I’ll check out the attached video as well so I can improve my skills.