AutoCAD Civil 3D General Discussion

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el_nath
Posts: 199
Registered: ‎04-14-2009
Message 11 of 15 (88 Views)

Re: Correcting Surfaces

11-06-2012 02:57 PM in reply to: caddirt

Try extracting the contours from the surface (SurfaceExtractObjects), modifying the resulting polylines, then add them to the surface definition as contours (AddSurfaceContours).

Nathan Selles-Alvarez, P.E.
Senior Civil Engineer
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autoMick
Posts: 225
Registered: ‎07-19-2012
Message 12 of 15 (75 Views)

Re: Correcting Surfaces

11-07-2012 01:03 AM in reply to: el_nath

Excellent suggestion - extract the contours, just delete the bits you aren't happy with, redraw these particular contours to add back into a surface.

 

A twist on this is that you can also add "contours" that aren't actually isolines, this works just like adding in breaklines, but if you are drawing 3D geometry that you want to create a surface out of, this can be handy.

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troma
Posts: 2,519
Registered: ‎05-21-2008
Message 13 of 15 (65 Views)

Re: Correcting Surfaces

11-07-2012 06:06 AM in reply to: el_nath

Extracting contours and adding them again will make your surface look good, but won't make it right.  Are you aiming for a pretty picture or a model you can use for earth volumes or other real uses?  I wouldn't go by this route if you want to use the surface as a model in a real sence.  It makes the surface 'right' at certain locations only, only where there happens to be a contour.

 

I suggest the feature line route.  When you add the feature line to the surface set the 'supplementing distance' to a low number (and 'weeding' to even lower) so that you get regular triangulation points along the feature line.  This will lead to a nice smooth surface that is also acurate.


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el_nath
Posts: 199
Registered: ‎04-14-2009
Message 14 of 15 (55 Views)

Re: Correcting Surfaces

11-07-2012 06:32 AM in reply to: troma

You are correct. Modifying contours will not make a correct surface, as all contours are derived data, and as Joe-Bouza pointed out, they are "lies". However, the original post asked about "correct contours", not a correct surface. At first, I wanted to respond as you did, but then I realized that there may be situations, where correct contours are the aim. As a design engineer, I rarely (never) use contours to build a surface. Maybe the end result is not a design surface, but to model an existing surface for watershed analysis, thus the desire for "correct contours". I can only guess.

Nathan Selles-Alvarez, P.E.
Senior Civil Engineer
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troma
Posts: 2,519
Registered: ‎05-21-2008
Message 15 of 15 (51 Views)

Re: Correcting Surfaces

11-07-2012 06:48 AM in reply to: el_nath

I'm not exactly sure of the intent either, but from reading the original post, I think the real problem is not enough data.  When you are in that situation, the triangles become too large and then the contours wander significantly in a zig-zag fashion.  The OP was addressing that by adding cogo points, I assume at interpollated elevations.  My solution is to draw a feature line between the elevations you have, and use the 'supplementing distance' option to let C3D do its own interpollation: more accurate and much quicker.  More TIN lines, smoother surface.  Still a guess really, but probably a better guess than otherwise.

 

I agree there are certain situations where the contours method might be better.  (Rarely).

 

Surely contours are not "lies" (deliberate attemps to deceive), but rather they are all just rough guesses and fudge.


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