October is still the rainy season in Thailand, and this month many people will open a map that shows water. A map that shows where water is and an announcement that tells you what to do are different things, and the difference matters most on the day the rain is heaviest.
Water on the ground is not always a flood
From above, a freshly planted rice paddy and a flooded field look almost the same. Farmers let water into their paddies on purpose every season. A satellite sees the water; it cannot see whether anyone wanted it there.
So a layer that colours surface water measures water, not harm. Reading it as a flood map means deciding, pixel by pixel, something the sensor never measured.
A forecast is not a warning
Global flood forecasting systems such as GloFAS, run by Europe's Copernicus Emergency Management Service, estimate how much water rivers may carry over the coming days. Its own terms of use say the data "does not constitute in any way an early warning", which only national or regional institutions are authorised to issue.
That is not modesty. A warning is a decision: someone with authority weighs a forecast against local knowledge, such as dams, embankments and what happened last time, and then tells people what to do. A model output carries none of that.
Who issues warnings in Thailand
Disaster warnings in Thailand come from the Department of Disaster Prevention and Mitigation (DDPM). The Thai Meteorological Department issues weather warnings, and provincial and local authorities direct evacuations. The DDPM hotline is 1784; for medical emergencies, call 1669.
If any map, ours included, disagrees with an official announcement, follow the announcement.
What data is good for
Data is good at the questions before and after an emergency: which districts flood in most years, how this season compares with earlier ones, where water stayed longest. Those answers help people plan drainage, harvest dates and insurance.
On the day itself, a data company has only a short thing worth saying: this is information, not an instruction, and here is where the instruction comes from.