HelloI need to create a 3D Model of a tunnel in AutoCad.  I think that the  tunnel should be made of 1 or more surfaces, and that a 3D polyline should be the center line of the tunnel.  The 3D pline should be composed of lines and arcs, with the arcs tangential to the lines, like the center line of a road.  I also need to determine the shortest distance between any x y z point inside the tunnel model, and the nearest wall (surface) of the model.I have experimented with SWEEP using a straight 3D pline and a semi-circle.  The first problem is that snap to nearest does not work as expected on the swept surface, it only snaps to certain lines on the surface.  The second problem is that I don't know if it is possible to create a 3D polyline as described, like a road centerline.Thank youJohn

Why 3Dpolylines?  What exactly are you constructing?  Why surfaces and not solids?

HelloThanks for writing back.  I need a 3D model of a tunnel in national grid coordinates.  I need to model surfaces because I want to connect a laptop to a surveying instrument so that when I measure a point inside the tunnel while it is being constructed, I can compare the measured point to the design surface.   In order to see if the tunnel has reached design location at any measured point.  The model does not have to be based on a 3D polyline, but tunnel and road design is generally based on such a line.  I am beginning to think that it may be necessary to use AutoCad Civil 3D.  I would prefer to use AutoCad, if it is possible.Thank youJohn    

I think you would be better off using Civil 3D.