Government guide
Mapping a flood in six hours
When the Benue and Niger rise, cloud covers most of the country. Here is how radar imagery gets emergency teams a ward level flood map before the end of the working day.
Why ordinary images fail in a flood
Floods in Nigeria arrive in the rainy season, under thick cloud. Optical satellites, the ones that take photo like images, cannot see through cloud. On a typical September day less than a third of the Benue valley is cloud free.
Radar satellites send their own signal and measure what bounces back. Water is smooth and reflects the signal away, so it shows up dark. Cloud and night make no difference.
The six hour workflow
Hour 0: the radar pass. Our partner constellation passes over Nigeria several times a day. Hour 1: the data comes down through Ilorin. Hour 2: we compare it with a dry season image of the same place.
Hour 4: the model marks new water, and an analyst checks the edges against roads and settlements. Hour 6: the map goes to the emergency team as a phone friendly web map and a PDF by ward.
Radar sees water through cloud and at night. That is why it works in September.
What emergency teams do with it
Teams use the maps to decide where to send boats, which roads are cut and which camps need supplies first. Road status is the most used layer: drivers check it before leaving the depot.
After the water goes down, the same maps help count affected buildings and farmland for relief and compensation.
Getting ready before the rains
The fastest flood maps come from teams that prepared: boundaries agreed, contacts named and a dry season baseline captured in March or April. We set this up free for state emergency agencies.
Figures in this guide are example figures for a COVORN demo site.