That depends on what you mean by objects [considering also the possibility of things other than Lines] being stretched on the Z axis.  If, when you select a Line, the Properties box shows a "Delta Z" value of zero, but it's "tall" into the Z direction when viewed from a direction other than plan view, then it must have non-zero Thickness [not to be confused with Lineweight, or with Width in Polylines -- Thickness is "up off the page" in the Z direction].  Other entity types show it differently than the Delta Z value for Lines, but like Lines, they will all have a non-zero value in the Thickness slot in the Properties box.  FLATTEN is not needed to fix that [though it will] -- just change their Thickness to 0, either in the Properties box or with the CHPROP command, to flatten that kind of extension in the Z direction.  And unlike Flatten, this approach will leave things that should remain at different elevations [such as contour lines] where they are, instead of taking them all to the 0 level in the Z direction.

Edited by Kent Cooper - 09.Sep.2014 at 16:14

philippe JOSEPH wrote: .... move all the objects from point 0,0 to 0,0,1E99 then move all the objects from 0,0,1E99 to 0,0,0. ....

Edited by Kent Cooper - 10.Sep.2014 at 15:17