I believe that shouldn't be like that.
I've got a line set to exactly zero (not undefined) and the surface doesn't form. Only if I set the line a little bit different than zero.
This only applies to CAD Polylines at Elevation 0 - we know that Elevation 0 is a real place in at least Coastal Areas, but we also know that a vast amount of the CAD data we get from Engineers has 0 when they mean undefined or unknown. If you convert Polylines to linestrings before using them (which most of our workflows drive towards) then we assume that a Linestring at Elevation 0 is actually a real elevation 0 and we make the surface. We assume that a CAD Polyline at Elevation 0 9/10 times will be incorrect - and we had at one point a lot of people telling us that they did not think that polylines with Elev 0 should be used in a surface for this exact reason.
So the rule is CAD Lines and Polylines at Elevation 0 = assume they meant no elevation
Convert a 0 elevation Polyline to a Linestring - it will be given Elevation = ? (undefined)
Make a linestring Elevation 0 and it is real data and will be used in surface TINs.
Right or wrong - there are good reasons on both sides to argue this either way.
Change the elevation of 0 polylines to 0.001 and you are good to go
If AutoCAD had the concept of undefined Elevations rather than using -999 or 0 or some very large negative number to define no elevation the world would likely be a much better place and we could assume that 0 actually meant 0.
Thanks for the explanation Alan,
good to know though, since the help section "Understanding Lines, Linestrings, and Alignments" doesn't mention it.
Retrieving data ...