KEEPIN-bgol (Erase Entities Outside a Polyline Boundary)

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 boundary-based selective erase utilities used in CAD 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

KIN ("Keep Inside") is the inverse of a "clear inside a boundary" tool: it erases every entity located *outside* a selected closed polyline, keeping only what falls within it. Useful for isolating a specific area of interest from a larger, densely-populated drawing.

How to Use

1. Load KEEPIN-bgol.lsp into your CAD session (drag-and-drop the file onto the drawing window, or use the APPLOAD command).
2. Type KIN at the command line and press Enter.
3. At the Select boundary polygon: prompt, click the closed polyline that defines the area you want to keep.
4. The routine tests every other entity in the drawing against that boundary and erases anything whose reference point falls outside it. The boundary polyline itself is left untouched.

Why Use It

• Efficient Isolation, quickly extracts/isolates a specific area of interest from a larger drawing without manually deleting everything outside it one entity at a time.
• Accuracy, uses the exact defined polygon boundary (even if irregular) to precisely determine which entities survive, avoiding accidental removal of entities meant to be kept.
• Streamlined Workflow, simplifies preparing a focused sub-drawing or extract from a larger, cluttered source drawing.

Where to Use It

• Isolating a specific parcel, grid section, or area of interest from a larger survey drawing by keeping only content within a chosen boundary.
• Preparing a focused extract or "crop" of a drawing for a specific deliverable, without manually deleting everything outside the area of interest.
• Any drawing cleanup task requiring content outside a (possibly irregular) boundary to be removed while preserving everything inside it.

Notes

• Entity "location" is approximated using its insertion point (for TEXT/INSERT/CIRCLE/POINT) or start/first vertex (for LINE and polylines). Very large or oddly-shaped entities are tested by that single reference point, not their full extents.
• Uses a standard ray-casting point-in-polygon algorithm against the boundary's vertex list, so it correctly handles irregular (non- rectangular) boundaries.
• See also the companion tool CLRIN-bgol (CIN) which performs the opposite operation, erasing entities *inside* a boundary instead.
• 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.