GAPFLAG-bgol (Flag Open Polygons)

Command Type: AutoLISP routine (works in AutoCAD, BricsCAD, ZWCAD and most AutoLISP-compatible CAD platforms)
Developed by: BGol Community, bgol.in

Inspired by the general concept of open-polygon detection utilities used in CAD/survey drafting. This is an original, independently written routine, new file name, new command name, new source code, created so the CAD community has a free, editable, open-source alternative.

Overview

GPF scans a selection of polylines and detects which ones are open (start point does not coincide with end point). Each open polygon's exact gap is flagged with a connecting line plus a circle marker, making it fast to spot and fix boundaries before running area, hatch, or boundary operations that require closed geometry.

How to Use

1. Load GAPFLAG-bgol.lsp into your CAD session (drag-and-drop the file onto the drawing window, or use the APPLOAD command).
2. Type GPF at the command line and press Enter.
3. Select the polygons (closed or potentially-open polylines) to check.
4. Every open polygon found has its gap marked with a straight line between the two endpoints plus a circle centered on the gap, on a dedicated layer, BGOL-GPF (colour red). Properly closed polygons are left completely unmarked.
5. The command line reports how many polylines were checked and how many open gaps were flagged.

Why Use It

• Improved workflow, quickly identifies open polygons before running further operations, saving time versus manually zooming into each shape.
• Enhanced accuracy, avoids errors caused by treating an open polyline as if it were closed (e.g., inaccurate AREA or BOUNDARY results).
• Streamlined operations, ensures a batch of polygons is properly closed and ready for tasks that require closed geometry.

Where to Use It

• Drawing cleanup: sweep through a batch of polygons and immediately spot which ones need their gaps closed.
• Quality assurance checks before running area calculations, hatching, or boundary-dependent operations that require fully closed polygons.
• Preparing survey or site boundary drawings for further analysis, ensuring no unintended gaps remain in the polygon set.

Notes

• A polygon is flagged only if its DXF Closed flag is not set and its actual start/end points don't coincide within a small tolerance, this avoids false positives on entities that are closed but whose flag happens to read oddly.
• The marker circle's radius scales with the gap size so tiny gaps are still visible and large gaps aren't overwhelmed by an oversized circle.
• Source code is fully open, feel free to study, modify, and redistribute it under the same open, free-for-all spirit.

Credits

Developed and shared for the CAD community by BGol Community. Website: https://bgol.in/ Free and open-source, use it, modify it, share it, for personal or commercial work alike.