How to convert 3d polyline to polyline in AutoCAD?

Converting a 3D polyline to a 2D polyline in AutoCAD removes elevation (the Z-axis) so the geometry lies on a single plane. This guide gives clear, beginner-friendly steps for several methods, explains common errors and fixes, and offers practical tips to ensure a clean 2D result.

Why convert a 3D polyline to 2D?

• Some workflows, prints or 2D drafting tasks require geometry with no elevation data.
• A 2D polyline is simpler to edit with standard 2D tools and produces predictable results when plotting.
• Removing the Z-axis avoids issues with layering, dimensioning, and data exchange to 2D-focused software.

Quick checklist before you start

• Work on a copy of your drawing or the selected objects to preserve the original.
• Check which AutoCAD version you’re using — some command names and tool placements vary slightly.
• Turn on Object Snaps (OSNAP) and set an appropriate UCS if you plan to redraw manually.

Method 1 — Use the “Convert 3D to 2D Polylines” tool

1. Open the Modify tab and find the Design panel (or search the command by name).
2. Choose “Convert 3D To 2D Polylines”.
3. Select the 3D polyline(s) you want to convert and press Enter.
4. The tool creates a new 2D polyline that adopts the elevation of the polyline’s start point (usually Z = 0 if the start point is at elevation 0).

Pros:

• Fast and preserves shape including arcs/polylines.
• Keeps segment types where possible.

Cons:

• If start-point elevation ≠ 0, result may not be at Z = 0 (check elevation after conversion).

Method 2 — Use the FLATTEN command

1. Select the 3D polyline(s).
2. Type FLATTEN in the command line and press Enter.
3. Confirm selection(s) when prompted and press Enter again.
4. FLATTEN projects the geometry to the current XY plane and removes Z values, producing a 2D polyline.

Notes:

• FLATTEN is quick and ideal for multiple objects.
• If FLATTEN is not available in your AutoCAD flavor, use the EXPLODE + PEDIT method below.

Method 3 — EXPLODE + PEDIT (when FLATTEN is unavailable or to control segments)

1. Type EXPLODE and press Enter. Select the 3D polyline and press Enter. The polyline becomes individual 3D line/arc segments.
2. If segments still have varying Z elevations, run FLATTEN on them or manually set their Z to 0 (see Errors/Fixes).
3. Type PEDIT and press Enter. Select one of the line segments.
4. When prompted to convert it to a polyline, choose Yes.
5. Use the Join option and select the other line segments to combine them into a single 2D polyline.
6. Press Enter to finish.

Pros:

• Gives control over each segment; useful if you must clean or reorganize geometry before joining.

Cons:

• More steps; arcs and splines may require additional handling.

Method 4 — Manual redraw with PLINE

1. Type PLINE and press Enter.
2. Snap to the existing vertices of the 3D polyline using Object Snaps (OSNAP) to trace the shape on the XY plane.
3. Finish the polyline (Close or Enter) to create a 2D polyline.

When to use:

• If the 3D polyline is complex and automatic conversion misbehaves, or if you want to simplify geometry while redrawing.

Cons:

• Time-consuming for complex shapes.

How to verify the result (check for remaining Z values)

• Select the converted entity and open the Properties palette. Check the Elevation (should be 0 for 2D).
• Use the LIST command: select the object and inspect coordinate values — Z should be 0.
• Zoom in and verify visually that the polyline lies flat on the XY plane.

Common errors and fixes

• Problem: FLATTEN command not found.

  • Fix: Use EXPLODE + PEDIT workflow, or check if FLATTEN is part of an Express Tool or requires enabling in your AutoCAD profile.

• Problem: Converted polyline still has nonzero Z values or vertices with different elevations.

  • Fix: Select object → open Properties → set Elevation to 0. If vertices keep Z values, use FLATTEN on the segments or rebuild with PLINE.

• Problem: PEDIT Join fails because segments are not coplanar or endpoints don’t match.

  • Fix: Ensure endpoints are snapped together; use ENDPOINT OSNAP to match vertices, or use JOIN only after flattening segments.

• Problem: Loss of arc geometry or segmentation after explode/join.

  • Fix: Use the “Convert 3D To 2D Polylines” first (it preserves arc segments better). If arcs are lost, re-create arcs manually with ARC or use SPLINE/PEDIT fit options.

Practical tips and best practices

• Always duplicate objects before modifying them.
• Work with an appropriate UCS (usually World UCS) so flattening projects to the desired plane.
• If converting many objects, select them in groups to avoid processing delays.
• If you need a plot-ready 2D drawing, use FLATTEN then PURGE and OVERKILL to clean duplicate geometry.
• Use OSNAP and GRID to improve accuracy when redrawing with PLINE.
• Save often and consider using version control or incremental saves when performing large conversions.

FAQ

Can I convert multiple 3D polylines to 2D at once?

Yes — most methods (FLATTEN, “Convert 3D To 2D Polylines”, EXPLODE + PEDIT) accept multiple selections. Select all desired polylines before running the command.

Will converting to 2D remove arc segments or change the shape?

It can, depending on the method. The dedicated Convert 3D To 2D Polylines and FLATTEN typically preserve arcs. The EXPLODE + PEDIT route may convert arcs to approximated segments—check the result and, if needed, recreate arcs.

Does AutoCAD LT support these commands?

FLATTEN and PLINE are available in AutoCAD LT. Some specialized tools may differ; if a command is missing, use EXPLODE + PEDIT or manual redraw.

How do I check if any vertex still has a Z value?

Select the polyline and open the Properties palette to view Elevation, or use the LIST command to view vertex coordinates (look for nonzero Z values).

Will converting to 2D lose 3D information permanently?

Yes — converting removes elevation data from the new 2D polyline. Always keep a copy of the original 3D polyline if you might need the 3D information later.

My converted polyline is at a nonzero elevation. How do I move it to Z = 0?

Select the polyline, open Properties, set Elevation to 0, or use MOVE with an appropriate Z offset (e.g., move by Z = -currentElevation).

After flattening, I see duplicate overlapping lines. How can I clean them?

Use the OVERKILL command to remove duplicate/overlapping segments, then PURGE to clean unused items.