• Environment Agency Flood Warning Areas — 3,426 MultiPolygon features
• National Grid Substations — 592 Polygon site footprints
• National Grid Overhead Lines — 763 LineString features
• National Grid Underground Cables — 3,996 LineString features

All map features, counts and risk classifications derive from these uploaded shapefiles. No synthetic assets, flood areas or risk results are generated.

The source shapefiles use British National Grid (EPSG:27700). They are reprojected once, during preprocessing, to WGS84 (EPSG:4326) for web mapping. Flood Warning Area geometry is simplified with a 15 m tolerance in projected metres — small enough that intersection results remain reliable, large enough for smooth rendering of 3,426 polygons.

• High — substation polygon footprint intersects a Flood Warning Area.
• Medium — overhead line or cable geometry intersects a Flood Warning Area.
• Low — no intersection, but within 250 m of a Flood Warning Area.
• Safe — no intersection and beyond 250 m.

Substation exposure is tested against the real polygon footprint, never the centroid; centroid markers exist for visualisation only. Classification uses geometric predicates and planar distance in metres — no scores, no models, no AI.

Shapefile parsing, reprojection, simplification and the intersection / proximity analysis run once in a preprocessing step (scripts/prepare-gis-data.ts) using an R-tree bounding-box index over the flood polygons. The application loads optimised GeoJSON and renders it with canvas layers, so the demo starts quickly and stays responsive.