How AutoCAD and Civil 3D exchange data

Civil 3D is built on the AutoCAD engine and shares DWG, so AutoCAD 2D linework and base drawings open and reference directly in Civil 3D with no conversion. The return leg is narrower than the shared format suggests: opened in plain AutoCAD, Civil 3D's AECC objects appear as non-editable proxy objects with the civil commands disabled, so moving an editable drawing back means running ExportToAutoCAD (or Export Civil 3D Drawing), which writes a new DWG with every civil object exploded to AutoCAD primitives — the geometry survives, the design intelligence does not. Autodesk documents LandXML as the alternative when the design objects themselves have to travel.

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