Problem converting 3D polylines to 2D polylines

[lgallantHCE9F]

In trying to convert 3D polylines (DXF exported from Helioscope, which may or may not be relevant) to 2D polylines using the FLATTEN command, my straight lines are turning into crooked lines. Anyone have any suggestions on what might be going on here and how to retain straight, closed, 2D polylines?  It may be important to note that I've tried both removing and not removing hidden lines (though I'm not clear on what exactly that means) and have gotten the same result either way. Thanks so much!

[Kent1Cooper · Accepted solution]

Make sure you're in plan view first.  Since a 3D Polyline doesn't define a drawing plane or coordinate system, I think FLATTENing it may be affected by your direction of view.

If that doesn't solve it, try Pline3Dto2D.lsp, available >here<.  It has some benefits over FLATTEN -- read that post, and comment 6 at the Cadalyst CAD Tips link.

[lgallantHCE9F]

Thank you @Kent1Cooper, your .lsp worked! Much appreciated.