idk

pipeline/transform/postcode_boundaries/README.md

@@ -77,9 +77,9 @@ The output of `process_oa` is `list[(postcode, polygon)]` — the per-OA fragmen
 
 ### Phase 4: Merging and writing
 
-**Fragment merging** (`output.py:merge_fragments`): Groups all fragments by postcode, unions them. If the result is a MultiPolygon (meaning the postcode has disconnected pieces — either from spanning OAs with a gap, or algorithm artifacts), applies a 5m buffer-then-unbuffer to close tiny gaps from floating-point mismatches at OA boundary edges. If still a MultiPolygon after that, keeps only the largest polygon — postcodes are contiguous delivery routes, so detached fragments are artifacts.
+**Fragment merging** (`output.py:merge_fragments`): Groups all fragments by postcode, unions them. If the result is a MultiPolygon (meaning the postcode has disconnected pieces — either from spanning OAs with a gap, or algorithm artifacts), applies a 5m buffer-then-unbuffer to close tiny gaps from floating-point mismatches at OA boundary edges. If still a MultiPolygon after that, keeps the largest part **plus any other part ≥ `_MIN_DETACHED_PART_AREA` (100 m²)** (`_keep_polygon_parts`); only sub-100 m² noise slivers are dropped. Keeping substantial detached parts matters because a postcode genuinely split across an OA seam (by a railway, river, or main road wider than the 5m buffer) would otherwise lose a chunk — measured at ~1.8% of merged area left as uncovered gaps (often 3000–5000 m² building blocks) before this change.
 
-**GeoJSON output** (`output.py:write_district_geojson`): Groups postcodes by district (the outward code, e.g. `SW1A` from `SW1A 1AA`). For each district, converts every postcode polygon from BNG to WGS84 using pyproj, simplifies with 1m tolerance (Douglas-Peucker), rounds coordinates to 6 decimal places (~0.1m precision), and writes a single `{district}.geojson` FeatureCollection. Each Feature has `postcodes` (formatted like `"SW1A 1AA"`) and `mapit_code` (no space: `"SW1A1AA"`) in its properties.
+**GeoJSON output** (`output.py:write_district_geojson`): Two passes. Pass 1 converts every postcode from BNG to WGS84 (pyproj), simplifies with 1m tolerance (Douglas-Peucker), and snaps to 6 decimal places (~0.1m precision); multi-part postcodes become `MultiPolygon` (`to_wgs84_geojson_multi`, each part handled independently), single-part stay `Polygon`. The whole set is then made a **partition** (`_resolve_overlaps`): each postcode is trimmed by the union of its higher-priority overlapping neighbours, where **priority = ascending area** (smaller postcodes win contested ground). That single rule handles both seam overlap *and* containment — an enclosed postcode is always smaller than its container, so it keeps its area while the container gets a hole (the query uses both the `overlaps` and `contains` predicates, since `overlaps` alone excludes containment). This runs last, so nothing re-introduces overlap; a postcode that would be emptied keeps its original geometry, so no active postcode is dropped. Pass 2 groups postcodes by district (the outward code, e.g. `SW1A` from `SW1A 1AA`), rounds coordinates to 6dp, and writes a `{district}.geojson` FeatureCollection. Each Feature has `postcodes` (formatted like `"SW1A 1AA"`) and `mapit_code` (no space: `"SW1A1AA"`) in its properties.
 
 ## Memory architecture
 
@@ -103,10 +103,10 @@ Key design choices:
 
 ## Key invariants
 
-1. **Every square meter of every OA is assigned to exactly one postcode** — the combination of INSPIRE claiming + Voronoi fills the entire OA, and overlap resolution ensures no double-counting
+1. **No two postcodes cover the same ground in the output** — within an OA the INSPIRE claiming + Voronoi tile it with no overlap, and a final `_resolve_overlaps` partition pass removes the thin overlap strips that the merge buffer + per-postcode simplification introduce across OA seams (measured residual overlap ~0.01% of area)
 2. **Every postcode that exists in the UPRN data gets a polygon** — unless all its UPRNs share coordinates with another postcode's UPRNs (handled by jitter) or it has zero UPRNs
 3. **Postcode polygons never extend outside their OA(s)** — all geometry is clipped to OA boundaries
-4. **Output is always single Polygon, never MultiPolygon** — the largest-polygon extraction in both `merge_fragments` and `to_wgs84_geojson` ensures this
+4. **A postcode split across an OA seam keeps all its substantial parts** — `merge_fragments` keeps every part ≥ 100 m² and the output is emitted as a `MultiPolygon` (the Rust server `postcodes.rs` and `loader.py` both parse MultiPolygon); only sub-100 m² noise slivers are dropped
 
 ## Module structure