My youngest daughter, Evie, turned 3 last week and to celebrate I can talk about 3D things.
How do you create a 3D Line in AutoCAD using the ArcInfo EXPORT TO CAD geoprocessing tool?
In Microstation the default linear element that gets created by the EXPORT TO CAD tool is a complex line string, which for 3D Microstation files is already a 3D element. In AutoCAD there are different types of complex lines; the Polyline, the Lightweight Polyline and the 3D Polyline. The EXPORT to CAD tool by default creates lightweight polylines.
Therefore if you have 3D linear features in ArcGIS and you’d like them to be 3D linear features in AutoCAD you have to override the default behavior of the EXPORT TO CAD tool by including a field in the exported feature classes’ attribute table called [CADType] and then populate that field with the string ‘3D POLYLINE’ for all the linear feature that you want to be created as 3D Polylines in AutoCAD.
For more information on creating CAD data in ArcGIS
you can view several linked articles found in the ESRI Knowledge Base.