Civil 3D surfaces are based Triangulated Irregular Networks (aka "TINs"). Since the surface consists only of triangles, any curves used to define it must be approximated by a series of triangles with one edge being a cord along the arc. The shorter the cord length, the closer the fit to the arc. When a curve is added to the surface definition (usually as breaklines) the software automatically applies the cords using user defineable settings at the time they are added. You can learn more about the settings by searching the help system on the topic "tesselation" in the context of adding brealines to a surface.
neilyj (No connection with Autodesk other than using the products in the real world)
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