Convert 3D Polyline to 2D Polyline

There are various polyline types in AutoCAD®, the most common of which is usually the LWPolyline (lightweight polyline). It can be confusing because the LWPolyline and the old Polyline entity used way back in the early days of AutoCAD® are both shown as a “Polyline” in the Properties window. Chances are though, unless you’re working on some nasty generated drawing, you’re probably using LWPolylines.

The third polyline type is the 3D polyline, which as the name implies is a 3D version. Both the LWPolyline and the Polyline only permit the creation of geometry on a flat plane (UCS), but the 3D polyline allows points anywhere in 3D space.

Converting between the various types therefore has obvious difficulty, because going from 3D to 2D means you’re going to have to remove some of the 3D information from the polyline, and AutoCAD® could interpret how to do this in many ways. Therefore historically there have not been native commands to convert between the polyline types.

The FLATTEN command is an option for making the polyline flat (i.e., visibly the same as before, but drawn as a 2D line on whatever UCS you’re working on). I have had troubles with FLATTEN in the past however, because I think it does some odd stuff sometimes with merging lines and approximating… so I tend to avoid it where possible.

In most cases if I wanted to convert 3D to 2D, I’d do this:

  1. Explode the 3D Polyline ( ! )
  2. Select all of the Line entities we just created
  3. Go to the properties window
  4. Set the Z elevation of the start and end points of all lines to 0 (or whatever elevation you want)
  5. Use PEDIT to join all the lines together again

And then you’re left with a nice 2D Polyline again.

If at this point you then want to convert back to a 3D Polyline again, do the following:

  1. Gather up some raw materials
  2. Use them to build a time machine
  3. Go back in time to just before you did all of the above, and tell yourself to CREATE A COPY OF THE 3D VERSION FIRST!
  4. Return to the present, and use the line that now magically exists in the drawing

Alternatively you can avoid the inconvenience of mastering the laws of the space-time continuum by planning what you’re doing before you make any irreversible changes…


That’s all for today – if you haven’t already subscribed to my blog, join the other 340 who have! Holy-moly that’s a lot of people listening to my nonesense… !


Oops, I deleted it again

We’ve all deleted stuff we didn’t want to, and you probably got it back by mashing the good old CTRL+Z key combo until you’ve got back the objects you deleted. This however undoes anything you did after deleting these objects. So for example if you deleted something and then went off and did some other work, to get back the stuff you deleted you’d probably end up undoing all that work.


Whilst “oops” is an apt comment, I’m actually referring to the OOPS command. This will reinstate the objects removed by the last ERASE command, and is really handy in the right situation.

This one isn’t just for mistakes either. There have been many times that I’ve deliberately used OOPS – for example, if I had certain objects on top of each other I could select the top objects, erase them, move the objects underneath somewhere else, and then invoke OOPS to retrieve the deleted objects.

I hope knowing this command helps you as it has helped me!


P.S. Oops, I’m accidentally going to slip in a reminder to subscribe below… 😉

AutoCAD® 2013 Free Download

As with last year, Autodesk have kindly allowed us to download their latest version of AutoCAD® for free. Visit the download link at the end of this post to get your free copy of AutoCAD® 2013!

But as expected, it’s only free for a limited time. 30 days is your quota, but that’s certainly enough time to evaluate the new features that it contains.

Be careful to save your drawings using a version of AutoCAD® that you own, or you could end up in the unfortunate predicament I was in last year where I’d saved my work using an AutoCAD® trial, which of course expired leaving me with drawings I could no longer edit! Luckily I was due an upgrade anyway… but that’s a sure way to lose some work (or at least time spent recovering it!).

Click here to download AutoCAD® 2013


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