What does the flood impact tool tell me?

For one district and an event window, it maps land that appeared flooded in Sentinel-1 radar images, gives the flooded area in square kilometres and estimates how many people live in that area, with a range. It also lists what the radar could not see.

Which dates should I choose?

Set the window to end on or a few days after the date you are interested in. Sentinel-1 passes over most places every 6 to 12 days, so the default window of 12 days usually contains at least one pass. The dry-season reference is chosen automatically from the driest month of a normal year, or you can choose a month yourself.

Does the people figure count those harmed or displaced?

No. It counts how many people live, according to the GHSL 2025 population grid, in the cells where water was seen. Some will have been unaffected and others outside the mapped area will have been affected. Treat it as an order of magnitude to guide where to look, not as a casualty or displacement figure.

What if my district is too large?

Large districts are mapped at a coarser cell size (20, 40, 80 or 160 metres) so that each answer stays within the server's limits. Districts more than about 277 km across are split into parts, and you choose one. For a whole large district or a series of dates, contact us about a community or NGO project.

Can I use the map in a situation report?

Yes, provided the "Generated at TechnicalGIS.com" line and the data credits are kept. Answers are kept for 90 days and can be shared by link. For operational emergency mapping, also consult the Copernicus Emergency Management Service and UNOSAT, which produce validated maps on request from authorised users.

Flood impact

Choose a country and a district, set the date the event window ends, and the service compares the latest Sentinel-1 radar images with a dry-season reference. It answers with the flooded area, the number of people living in it and a map. Radar sees through cloud, so it works during the rains.

1. Choose the place

Country

2. Choose the event

Window ends on Window length

The latest radar pass inside the window is compared with a dry-season reference.

Dry-season reference

Choose a country and a district first.

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Choose a country to see its districts. You can then pick a district by clicking it.

District boundaries from geoBoundaries, simplified for the web.

Looking for the other question? Try Season condition.

How the answer is worked out

Radar images. The Sentinel-1 passes in the event window are compared with the median of several passes from the dry-season reference month, using the same satellite track where possible.
Water. Calm open water reflects radar away from the satellite and appears dark. Land is marked as flooded where it is dark after the event and has darkened clearly since the reference. Isolated single cells are removed.
Usual water. Rivers, lakes and reservoirs that hold water in at least half of all years, from the JRC Global Surface Water record, are removed so that only new water is counted.
People. The GHSL 2025 population grid is summed over the flooded cells. Stricter and looser versions of the water rule give the range.

How reliable is this?

Radar flood maps made automatically are useful but imperfect, so no answer from this tool is rated better than use with care. The rating falls to low reliability when much of the district was not covered by the radar passes or when no suitable dry-season reference was found.

Radar does not see water under dense forest or tall crops, so flooding there is missed.
Flooding between buildings is usually missed; towns are under-represented.
Steep slopes, radar shadow and wind-roughened water can cause false or missed detections.
A pass shows the water at one moment. Floods that rose and fell between passes may not appear.
The results are indicative. They are not a substitute for field assessment or official damage and needs assessments.

Data sources and licences

Each source was checked on its provider's own page before use; last checked 29 October 2026.

Source	Licence	Credit
Sentinel-1 radiometrically terrain corrected backscatter	CC BY 4.0	Contains modified Copernicus Sentinel-1 data, processed to RTC by Catalyst and provided by Microsoft Planetary Computer under CC BY 4.0.
JRC Global Surface Water occurrence	Free of charge, without restriction of use (Copernicus programme)	Source: EC JRC/Google. Pekel, J.-F. et al. (2016), Nature 540, 418–422. https://doi.org/10.1038/nature20584.
GHSL GHS-POP R2023A population grid (2025)	CC BY 4.0	European Commission, Joint Research Centre: GHS-POP R2023A. Pesaresi, M. et al. (2024), Int. J. Digital Earth 17(1). https://doi.org/10.1080/17538947.2024.2390454. CC BY 4.0. Resampled and summed over the flooded area by TechnicalGIS.
geoBoundaries administrative boundaries	Per country (CC BY, ODbL or open government licence)	Runfola, D. et al. (2020) geoBoundaries: A global database of political administrative boundaries. PLoS ONE 15(4): e0231866.

Questions

What does the flood impact tool tell me?: For one district and an event window, it maps land that appeared flooded in Sentinel-1 radar images, gives the flooded area in square kilometres and estimates how many people live in that area, with a range. It also lists what the radar could not see.
Which dates should I choose?: Set the window to end on or a few days after the date you are interested in. Sentinel-1 passes over most places every 6 to 12 days, so the default window of 12 days usually contains at least one pass. The dry-season reference is chosen automatically from the driest month of a normal year, or you can choose a month yourself.
Does the people figure count those harmed or displaced?: No. It counts how many people live, according to the GHSL 2025 population grid, in the cells where water was seen. Some will have been unaffected and others outside the mapped area will have been affected. Treat it as an order of magnitude to guide where to look, not as a casualty or displacement figure.
What if my district is too large?: Large districts are mapped at a coarser cell size (20, 40, 80 or 160 metres) so that each answer stays within the server's limits. Districts more than about 277 km across are split into parts, and you choose one. For a whole large district or a series of dates, contact us about a community or NGO project.
Can I use the map in a situation report?: Yes, provided the "Generated at TechnicalGIS.com" line and the data credits are kept. Answers are kept for 90 days and can be shared by link. For operational emergency mapping, also consult the Copernicus Emergency Management Service and UNOSAT, which produce validated maps on request from authorised users.

Read how the community tools were built and tested, why we provide them, or contact us about a community or NGO project.