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Registered: ‎10-14-2008
Message 1 of 23 (712 Views)
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River bottom surface from 3dpolyline sections?

712 Views, 22 Replies
09-04-2012 07:48 AM

Hello,

 

Can anyone tell is there an easy way to create a tin surface from 3d polylines that are generated by actual measurements of a river/ditch or imported from Hec-Ras (River Analysis -extension)?

 

The problem seems to be that the triangels should be long and narrow (like the ditch bottom in the example) instead of normal triangels the triangulation method normally generates. (The triangels in the example are generated by 3dwin -program).

 

When using Hec-Ras one way to go around this problem is to interpolate sections (XS:s) with very short distance. Hec-Ras does the interpolation very well, but I would like to have a better solution without tricks with other programs. Also in the long rivers the result is a very large file.

 

I'm using C3D 2012

 

Anyone?

 

 

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wfberry
Posts: 1,610
Registered: ‎09-05-2006
Message 2 of 23 (696 Views)

Re: River bottom surface from 3dpolyline sections?

09-04-2012 10:07 AM in reply to: Yami72

Not sure what you are looking for?  In Civil 3D making a tin from 3D polylines cannot be much easier.  All you are showing in your dwg is the TIN.  One can only assume is that all your triangle corners are related to a field collected point.  Perhaps you can explain more.

 

Bill

 

 

Contributor
Posts: 19
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Message 3 of 23 (667 Views)

Re: River bottom surface from 3dpolyline sections?

09-04-2012 11:23 PM in reply to: wfberry

Thank you wfberry,

 

Yes, maybe my explanation was a bit inaccurate. I made another example. I added two surfaces , #1 with 4 sections and #2 with 3 sections. In #1 there is not problems because sections are close each other so I can easily make a tin that represents actual river bottom quite well. This situation is found more or less only in daydreams...

 

In #2 there are 3 sections with longer distances and also curved river. Also section 2 has fewer points than sections 1 and 3. And also measured polylines are not straight in x,y plane. This situation is the every day situation.

 

In surface #2 you can see the problems. If you have different amount of points in polylines, triangulation "jumps" for example from river bottom to river bank. If you have not straight polylines, some triangels are formed only with one polyline itself. And the most diffucult situation is if you have a curved river without measured sections.

 

In Hec-Ras the section interpolation is pretty good. As you see from the attached picture, the river bottom is formed quite smoothly. Of course Hec-Ras has also limitations.

 

Now what I ask is that is there an easy way to create a tin surface from measured 3dpolylines in a way that the surface represents the actual bottom surface?

 

The ideal solution would be some kind of section interpolation along river centerline like in Hec-Ras (suggestion to the Autodesk).

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neilyj
Posts: 3,475
Registered: ‎08-01-2008
Message 4 of 23 (662 Views)

Re: River bottom surface from 3dpolyline sections?

09-05-2012 12:18 AM in reply to: Yami72

The method you are employing is fundamentally flawed when there is a large distance between sections or there is a curve in the stream. The triangular TIN is forming correctly according to the rules but it isn't necessarily what the river looks like.

 

If you have simple sections i.e. top of bank left, bottom of bank left, thalweg, bottom of bank right and top of bank right and these are the same along the river and closely spaced, then the TIN will form pretty much as expected.

 

If there are other features recorded in the sections then how does the TIN know how to form correctly? You will need to add breaklines (in plan) joining the relevant tops and bottoms of banks etc, swap triangle sides etc etc.

 

I can't see an automatic way of achieving this

neilyj
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Contributor
Posts: 19
Registered: ‎10-14-2008
Message 5 of 23 (658 Views)

Re: River bottom surface from 3dpolyline sections?

09-05-2012 01:10 AM in reply to: neilyj

Yes, the triangulation does as it is programmed to do. In this case the method is not good and something else is needed.

 

In case of long and diverse rivers adding breaklines etc. manual work is a hard work to do. I also tried the new Array-command. Using the Path -method it is quite easy to copy 3dpolyline sections along river centerline. But of course you need a centerline with correct xyz-coordinates. For simple sections and rivers this is quite ok.

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neilyj
Posts: 3,475
Registered: ‎08-01-2008
Message 6 of 23 (650 Views)

Re: River bottom surface from 3dpolyline sections?

09-05-2012 02:03 AM in reply to: Yami72

Yami72 wrote:

Yes, the triangulation does as it is programmed to do. In this case the method is not good and something else is needed.



...short of employing mind reading techniques how is the software supposed to know how the river joins up??

 


Yami72 wrote:

Yes, the triangulation does as it is programmed to do. In this case the method is not good and something else is needed.

 

In case of long and diverse rivers adding breaklines etc. manual work is a hard work to do. I also tried the new Array-command. Using the Path -method it is quite easy to copy 3dpolyline sections along river centerline. But of course you need a centerline with correct xyz-coordinates. For simple sections and rivers this is quite ok.


This technique seems to be susceptible to "bad modelling" in that the cross section is assumed to be the same over long distances i.e. in an analogy to corridor earthworks volumes using very widely spaced cross sections in possibly varying ground topography this will derive totally misleading volumes.

 

As mentioned before, I'm not sure there is an easy way to solve this....:smileyfrustrated:

neilyj
(No connection with Autodesk other than using the products in the real world)


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Mentor
Posts: 187
Registered: ‎07-19-2012
Message 7 of 23 (645 Views)

Re: River bottom surface from 3dpolyline sections?

09-05-2012 03:39 AM in reply to: Yami72

As I understand it, HECRAS can utilise both the river path in plan view and the cross-sections to give you interpolated cross-sections that follow the river path.I don't have a solution unfortunately, however I have a few ideas that may lead in the right direction. 

 

1. "Loft" in the 3D Modelling surface tools works very nicely to interpolate (straight line path) between cross-sections. So if you have enough cross-sections along your river meanderings, then works nicely.

 

2. There are extensions for other software (e.g. sketchup) that permits "loft by rails". Once again you do it in sections, but what you do is to define the "rails" which are the left and right banks for the river and then loft the 1st cross-section to merge into the 2nd cross-section. This then gives you an accurate path of the river in plan view, and interpolates between the two cross-sections. Is there a similar way to do this in C3D?

 

3. Can't you export the hecras interpolated (as well as measured) cross-sections? If you can get these then the rest is straightforward (although only as accurate as the original data).

 

Cheers

 

- Mick

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AllenJessup
Posts: 5,599
Registered: ‎05-21-2003
Message 8 of 23 (637 Views)

Re: River bottom surface from 3dpolyline sections?

09-05-2012 04:57 AM in reply to: Yami72

Maybe coming from a Surveying background I look at this a little differently. To me the answer to a problem where you don't have enough field data to model a Surface correctly is - You don't have enough field data. So in order to get a good model, you need more field data.

 

I understand field time is expensive and for a long river in may be impractical to get tight sections for the whole length. But if there are areas that can't be modeled with sections that are far apart, those areas should have sections more often.

 

If I couldn't get any more field data. I'd try using a curved Featureline for the centerline, use a stepped offset to create bottom and top of bank in the areas you don't have data. This doesn't make the model any more correct. It just makes it look better.

 

Allen

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neilyj
Posts: 3,475
Registered: ‎08-01-2008
Message 9 of 23 (631 Views)

Re: River bottom surface from 3dpolyline sections?

09-05-2012 05:25 AM in reply to: AllenJessup

Totally agree

neilyj
(No connection with Autodesk other than using the products in the real world)


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Contributor
Posts: 19
Registered: ‎10-14-2008
Message 10 of 23 (610 Views)

Re: River bottom surface from 3dpolyline sections?

09-05-2012 07:19 AM in reply to: neilyj

Thanks for the comments.

 

I also totally agree that a land survey is the key to the good modeling. Being on the field is quite familiar to me. But you always have to balance between costs and accuracy.

 

Thanks Mick! The loft tools seems to be a pretty handy tool for this. At least in smaller models after a short test. I read from another forum that in Inventor there is also "loft by rails" -tool. Allen's stepped offset works well in modificated channels.

 

After these comments it seems that in longer rivers the Hec-Ras interpolation is the best solution at the moment. And thanks River Analysis -extension (beta) it is easy to read all sections from the original Hec-Ras project files to 3dpolylines. Hec has its limitations and this "channel interpolation by rails" could be a good thing to develope also in C3D.

 

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