Explore · Stage 2 · Real data
What actually happened?
Cyclone Ditwah flooded the lower Kelani in late 2025. These are published flood layers, not predictions. There is no animation, because nothing was mapped between these dates and this project won't invent it.
The mapped flood
Shown: Copernicus Emergency Management Service (© 2025 European Union), [EMSR851] Kelani Ganga River (AOI02): Delineation Map.
When each layer was mapped
| Copernicus image | Observed (ha) | Flood trace (ha) | Extrapolated (ha) | Total (ha) | Not analysed (ha) |
|---|---|---|---|---|---|
| 30 Nov 2025, 05:39 UTC, PlanetScope | 3,377.0 | None | 3,382.3 | 6,759.3 | 636.2 |
| 3 Dec 2025, 07:06 UTC, Legion | 688.3 | 122.9 | 2,108.1 | 2,796.4 | 7,122.3 |
The UNESCO layer is one merged extent from several sensors. Its file name gives 26 November to 2 December 2025, and its catalogue entry gives 28 November to 5 December. It has no date for each area, so it shows where water was mapped during the window, not when.
The fair comparison
Far more ground was not analysed on 3 December, so the headline fall overstates the change. Counting only the 16,071.0 ha analysed on both dates, observed flood went from 1,969.6 ha to 685.4 ha. The source doesn't say why areas were not analysed, and the sensors differ, so some of this change may still not be real.
How far to trust these layers
- UNESCO: its publisher describes it as a preliminary analysis that has not been validated in the field.
- Copernicus observed: extracted from satellite images, excluding permanent water. Stated position error is 6.25 m and 4 m.
- Copernicus extrapolated: observed flood extended with a terrain model into areas images can't see well, such as towns and forest.
- Depth: every depth polygon is marked as modelling. It is not a measurement.
- Alignment: the best match between UNESCO and Copernicus needs a shift of 0 pixels, at about 11 m per pixel. That shows no systematic offset, not that the layers agree: they cover different times.
Sources
- Cyclone Ditwah and Floods in Sri Lanka (November 2025), UNESCO IHP-WINS, doi:10.63253/hvu5r3bf. Licensed CC BY-SA. The derived map layers here are shared under the same licence.
- Copernicus Emergency Management Service (© 2025 European Union), [EMSR851] Kelani Ganga River (AOI02): Delineation Map, and Monitoring 1.
- Basemap: OpenFreeMap, with data from OpenStreetMap contributors.