Making a Flat Surface

[tgreeneWA89M]

I'm working on an excavation project that involves creating a large depression with a flat bottom. I'm using polylines to create the surfaces I need. I draw 2D polylines in the shapes that I want and then program their elevation like topo lines, so in top view it is in 2D, but if I rotate it appears 3 dimensional. When I create a surface using these polylines, AutoCAD draws the surface as having a lot of elevation change in the areas I want to be flat. I only want my polylines to be contours, but it seems like AutoCAD is interpolating all across the drawing. Is there a way to minimize the amount that the program interpolates elevation changes? I've tried to turn off the "minimize flat areas" options when creating the contours, but nothing changes.

[Anonymous · Accepted solution]

have you tried using feature lines to create the grading you wish to attain?

[jeff_rivers]

Seconding what JohnGordon said.  Use Featurelines.  They might first appear more complex to work with than 2D polylines, but you will get the surface you want without any weirdness.  

[Joe-Bouza]

I’ll third @tgreeneWA89M & @jeff_rivers plus set a tiny depression or rise elevation in the center

[neilyj666]

And fourthing for @Joe-Bouza's suggestion...😉

[tcorey]

Can you post the drawing?

[samir_rezk]

Hi @tgreeneWA89M 

using Polylines as contour lines should be  treated as the last case resort for surface modeling. It is probably the most inaccurate way you can choose to work. You as the designer want to define the parameters and constraints of the design then let Civil 3D generate the contour lines for you. Civil 3D is a design program that’s what it does best; plain vanilla AutoCAD is a drafting program you can draw polylines to resemble your design, but it is not a design object, just a representation of your thoughts! Do not treat Civil 3D like AutoCAD - they are vastly different.

To define the perimeters and constraints of the design you may want to investigate using feature lines like has been mentioned in the thread several times, grading by projection using the grading tools as well as corridor modeling for linear design might be options for you as well.

hope this sheds light about the intended workflows of working with Civil 3D.

[neilyj666]

A polyline at a constant level defining, for example, a pond base will cause issues when added to a surface and subsequently displaying contours. This is most obvious for a pond base as it is generally closed and thus civil 3d gets "confused" trying to determine the contour lines. Adding 0.001m to the polyline elevation will get around the problem without affecting volumes to any significant degree.