Creating surface boundaries from multiple objects.

[Demesne]

Hi

 

How can I create a boundary of multiple 3d faces automatically?

 

I have a drawing representing a topographical surface which is built up from almost 30,000 3D faces. Usually I would turn osnaps off and draw around the group of 3D faces to create my own boundary which runs close to but doesn't overlap the faces. This dosen't produce a very accurate boundary but does mean that when I create my own TIN of the surface (for cut & fill calculations) I'm creating a surface that almost covers the same area.

 

Attached is an extract of a DWG which was received from a 3rd party. The green 3D faces are what we are given and the red 2D poly is what I have drawn to show what I'm trying to achieve. I have tried to produce this using the BOUNDARY command, but it doesn't want to work.

 

Does anyone have a LISP that would find this boundary or a way to create it with AutoCAD commands?

 

Thanks for your help.

 

TIN existing.dwg

Edited December 6, 2010 by Demesne

[Mark_CV3D]

I created a tin by selecting the 3dfaces and then trimmed the boundary so it got the same boundary as the outer one of the 3dfaces, and then I extracted Border. The resulting file is to big to attach here.. maybe pm?

[Demesne]

Thanks for your help Mark but I don't have Civil 3D. I use 2008 (vanilla) and Quicksurf to produce TINs and perform volume calcs. This is a frequent problem that I encounter when creating my own models from 3rd party data.

 

I've even tried flattening the 3d faces, converting to polylines and using the boundary command. For some reason, the polylines created (several crossing boundaries are created) contains errors where triangles are missed and a true boundary isn't acheived. This incorrect boundary also takes time to compute!

 

It seems like it should be an easy task but whatever I try never produces perfect results. I'm wondering if one of the LISP gurus would have the answer?

[scj]

Try to find the F2S.lsp (by Bill Gilliss). It will produce the "shadow 3DSOLID" under your cloud of 3Dfaces.

Explode this resulting 3DSOLID and you will get the boundary (as a REGION).

In case of problems - please continue asking.

Regards from good ©old Germany

Jochen

[ReMark]

F2S.lsp available here.

 

http://www.accustudio.com/index.php?option=com_content&view=article&id=48&Itemid=58

[Demesne]

Thanks Jochen!

I've tried your suggestion on the drawing extract I posted earlier and it works fine. I'll try it on a surface of 30,000 3D faces later on and see how long it takes.

 

Thanks for the link ReMark.

 

PS. The North of England is rather cold too. -12°C here the other day. Brrrr! Slippers!