Build from Existing Contour Polylines

[eckdahlrd]

I have never really had good luck building a dtm surface from existing contours I receive as polylines. I know it's not the most accurate way to build an existing surface, but when that's all you have...I'm sure you know what I mean.

I always seem to get a lot of triangulation that cuts contours off, and the contours of my new surface are only about half way close. For the work I'm doing, it's probably close enough, but I would like to improve it.

It takes forever to swap edges and modify the triangulation to make the contours more exact, is there a quicker and easier way to get a better result from contour lines?

Again, I know this is not the best way to build a surface (too bad the drawing didn't even come with the tin triangulation!), but I have to work with what I have.

Thanks for any advice in advance.

[dgordon.68443]

Have you tried adding the contours as break-lines instead of adding them as contours.

I find that at least the triangles don't cross any of my contour lines when I do it this way.

you still have the problem with flattening deep bends. One suggestion would be to use add line instead of swap edge in order to fix those areas a bit faster.

[sinc]

What version of the software are you using?

I think they made improvements to this in 2007. At least, in both C3D 2007 and 2008 I've been able to generate surfaces that have almost exactly the same contours as the ones I start with. Sometimes it takes playing with the parameters a bit, but the results are typically quite good. Far better than the results we used to get in Land Desktop.

Can you post a drawing where you can't get a "good" surface built from contours?

[MHultgren]

No matter how close you get using contours for the base of your surface, you still end up with an extrapolation of an extrapolation. It all depends on how accurate you need your surface to be.
A case in point...

In Phoenix, I have run street grades as flat as .3% that is .003 ft\ft. If I even thought about using contours as a base for my design on a 100 acre parcel, and then had the surveyors go out and stake it, I would normally end up with double to triple the amount of earthwork that the design estimated. Developers don't like it when that happens :roll:

[elebron]

Adding feature lines as breaklines on ditches and ridges helps reduce the interpolation, and the resulting surface should follow the original polylines very closely.
good luck!
el