12d Model > AutoCAD > Inventor? And this is for a "civil" design. It begs the question...why aren't you using a program like Civil 3D in the first place? Civil design in Inventor just doesn't seem like a good fit to me.
Maybe this will help?
Re: Inventor 2013
"Autodesk® Inventor® 3D mechanical design software includes CAD productivity and design communication tools that can help you reduce errors, communicate more effectively, and deliver more innovative product designs faster."
Does this sound like a program geared to civil design?