AutoCAD Tip: Create watertight polylines and regions with BOUNDARY (BO)

Turn enclosed areas into precise polylines or regions with the BOUNDARY command (BO). This is the fastest path to watertight shapes for hatching, takeoffs, and Boolean modeling.

When to use BOUNDARY:

• Generate clean perimeters from messy linework for quantification or hatching.
• Create regions for Boolean operations (UNION, SUBTRACT, INTERSECT).
• Extract closed polylines for CNC profiles, laser-cut paths, or export workflows.
• Replace manual tracing with automated, point-and-click enclosure detection.

Workflow (fast and reliable):

• Prep the geometry:
  • Remove duplicates with OVERKILL.
  • Close tiny gaps using PEDIT (Join) or FILLET with radius 0.
  • Work in the correct plane: set UCS then run PLAN to ensure a true top view.
• Limit the search: if the drawing is large, create a “Boundary Set” by choosing New and selecting only the relevant objects.
• Run BOUNDARY (BO):
  • Object Type: choose Polyline (editable, great for CNC/export) or Region (mass properties and Booleans).
  • Island Detection:
    • Normal: returns outer and inner loops (holes).
    • Outer: outermost boundary only.
    • Ignore: treats islands as solid—no holes.
  • Pick Points: click inside each enclosed area you want to convert.
• Place results on a dedicated layer (e.g., BOUNDARY) for control and plotting.
• Verify closure and data:
  • Polylines: check Area/Length in Properties or via AREA.
  • Regions: use MASSPROP for area, centroid, and moments.

Pro tips and troubleshooting:

• Speed: using a tight “Boundary Set: New” selection can reduce detection time from minutes to seconds in complex files.
• Noisy drawings: temporarily freeze heavy hatches/images/Xrefs to avoid slowdowns or ambiguous enclosures.
• Gaps you can’t see: run HATCH with a small gap tolerance, then use HATCHGENERATEBOUNDARY to extract a closed polyline when BOUNDARY struggles.
• Curves and arcs: prefer true arcs over many tiny segments; if needed, convert with PEDIT (Fit) for smoother CNC or plotting.
• 3D linework: project to the current plane first; boundary detection is 2D in the active UCS.
• Need regions from polylines later? Use REGION to convert closed polylines for Boolean operations.

Power workflows:

• Holes and voids: set Island Detection to Normal to capture both perimeters and openings in one pass, or create regions and use SUBTRACT for precise voids.
• Automated annotations: place areas on layers by type, then use FIELDS to report region areas in schedules or tags for drawings that auto-update.
• Batch picking: you can pick multiple interior points before closing the dialog to generate many boundaries at once.

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