you'll need a geodetic frame of reference for the coordinate system definition. look up "Low Distortion Coordinate Systems." Richard Sincovic (AKA Sinc) has put stuff up in the Civil3D forum & taught a class on this at Autodesk University in 2010. If you're a subscription customer, you have access to past AU documents through the Autodesk University website.
![Autodesk Design & Make Report](https://forums.autodesk.com/t5/image/serverpage/image-id/1350519iCB55845EBFC43016/image-size/large?v=v2&px=999" "Autodesk Design & Make Report")
