Fill Enclosed Area — JOSM plugin

Adds a map mode (toolbar toggle button, "Fill Enclosed Area") that turns a click into a new closed way, but only when the click point sits inside a region that is genuinely enclosed by existing, connected way geometry.

What it does and does not do

• ✅ Only fills a region if the surrounding OSM geometry forms a closed boundary made of already-shared nodes.
• ✅ No gap bridging — if two ways don't actually share a node, the boundary is not considered closed there.
• ✅ No snapping or guessing — coordinates are never invented, moved, or approximated.
• ✅ No freehand approximation — the new way is built from the exact existing node sequence of the enclosing boundary.
• ✅ If the clicked point is not inside a closed boundary, nothing happens and you get a short notification.
• ✅ Reuses existing nodes/ways — the new way references the same Node objects already in your dataset; if an identical closed way already exists, that way is selected instead of creating a duplicate.
• ✅ The result is a completely normal Way, added via JOSM's command stack (AddCommand), so it's fully undoable with Ctrl+Z like any other edit.

How it works

1. Every non-deleted way in the current edit layer is decomposed into a line string using each node's real projected (EastNorth) coordinates.
2. All line strings are handed to a JTS Polygonizer, which computes the faces of the planar arrangement formed by that geometry. A face only closes where segments share an exact endpoint coordinate — i.e. an actual shared OSM node. Ways that merely run close together, cross without a shared node, or leave a gap are never joined by this step.
3. Among the resulting faces, the smallest one that contains the clicked point is chosen (the innermost enclosing region).
4. Every coordinate of that face's boundary ring is mapped back to the original OSM node it came from. If any point can't be matched to a real node (which shouldn't happen, since the polygonizer never invents coordinates), the plugin aborts and creates nothing rather than guess.
5. A new Way is built from that exact node sequence and added to the dataset via AddCommand.

Building (Ant)

Requires a JDK and Apache Ant — that's it, no build-tool version matrix to fight.

1. Get the two dependency jars in place — see lib/README.txt for exactly where to get each one:

   • your local JOSM install's own jar (e.g. josm-tested.jar)
   • jts-core-1.19.0.jar (used for the enclosed-face detection)

2. Optionally copy build.properties.sample to build.properties and adjust the paths/Java version for your machine (not required if you just drop both jars into lib/ under the default names).

3. Build:

   ant dist

   The plugin jar is written to dist/FillEnclosedArea.jar.

4. Optionally, build straight into your local JOSM plugins folder:

   ant install

   (adjust josm.plugins.dir in build.properties first if you're not on the Linux default path).

Other useful targets: ant clean (wipe build output), ant compile (compile only, useful for a quick syntax check).

Note on the bundled JTS dependency: since JOSM plugins are loaded as a single self-contained jar (no external classpath resolution at runtime), the build merges JTS's classes into the plugin jar itself rather than shading/relocating them (Gradle's shadowJar did the relocation before; plain Ant doesn't have an equivalent built in without pulling in an extra tool like jarjar). In the unlikely case another installed plugin also bundles a different JTS version, you could hit a class conflict — let me know if you want the jarjar relocation step added.

Installing

1. Copy the built jar into your JOSM plugins directory:
   • Linux: ~/.local/share/JOSM/plugins/
   • Windows: %APPDATA%\JOSM\plugins\
   • macOS: ~/Library/JOSM/plugins/
2. Restart JOSM, or enable it via Edit → Preferences → Plugins.

Usage

1. Click the "Fill Enclosed Area" toggle button in the map-mode toolbar (left side of the JOSM window).
2. Click anywhere inside a region bounded by connected way geometry.
3. If the region is genuinely closed, a new closed way is created and selected. If not, you'll get a short "not inside a closed boundary" notification and nothing is changed.

Known limitations

• The whole edit layer's ways are scanned on every click; for very large loaded areas this may take a moment.
• If the enclosing face contains holes (e.g. an island fully inside the boundary, itself formed by connected geometry), only the outer ring is turned into a way — inner rings are not currently assembled into a multipolygon relation. This could be added as a follow-up if needed.
• Ways with missing/incomplete node coordinates are skipped, as they can't contribute exact geometry.