Count exposed properties inside a hazard zone
Exposure work starts with two questions: how many locations sit inside a hazard footprint, and where they cluster. This example counts Christchurch's rated properties inside the council's 200-year flood extent and maps them per H3 cell.
Christchurch properties inside the 200-year flood extent
Data: Christchurch City Council's rating units, its 200-year flood extent, tsunami inundation and liquefaction management areas, all public council layers (CC BY 4.0)
Method: keep the rating units inside the flood extent (simplified to 5 m), then count them per H3 cell at resolution 8
Receipt on "Flood extent, 200-year"
1. simplify 5 m
Receipt on "Rating units"
2. intersect with the result of 1
Receipt on an H3 resolution-8 grid over Christchurch
3. count_points of the result of 2, per cell → point_count
4. filter point_count > 0
The public map shows counts per cell, never a single property. The grid covers the city, so units on Banks Peninsula are counted inside the extent but not in a cell. Tsunami and liquefaction areas are drawn for context; the count is for the flood extent only.
Run it on your data
Upload your locations as a CSV or spreadsheet with coordinates, and run the same intersect and H3 count against the hazard layer you use. Counting per cell keeps individual locations off a shared map. See spreadsheets.
Frequently asked questions
Are these real properties?
Yes, the council's public rating units. This map shows no address or value for any of them.
What about locations on a zone boundary?
ST_Intersects counts a point on the boundary as inside the zone.