I guess by now everyone knows about the troublesome volcano under the Eyjafjallajökull glacier. And most have heard the speculation that Katla is the next volcano to go off. If that happens, things in Iceland will be a lot more dire than now, as Katla is a notorious troublemaker in the volcano world.
In the event of the eruption, a system like Ushahidi would help greatly in the evacuation and search and rescue efforts. I'm attempting to draw a road map of the eruption.
First there would be a massive flash flood. At the peak of an eruption in 1755 the flood discharge has been
estimated at 200,000–400,000 m³/s (7.1-14.1 million cu ft/sec); for comparison, the combined average
discharge of the Amazon, Mississippi, Nile, and Yangtze rivers is about 266,000 m³/s (9.4 million cu ft/sec).
The information needed here would be the location, direction and size of the flood; which roads and bridges have been destroyed or rendered impassable by the flood; which areas have been isolated and are now out of reach for terrestrial transport; which areas are the worst affected and in need of help; what are the closest safe spots for evacuation; what is the status of the rescue operations.
Volcanic ash would then start to envelop the region, depending on wind direction. This can prevent some or all aircraft from flying in the affected area and can cause illness or death in animals and humans. Useful information in this situation would be the wind direction; which areas are currently unaffected and safe to evacuate to; the location of first aid posts where people can get breathing masks; the quality and density of the ash fall; the location of the worst-affected areas; the location of the S&R teams; the status of the evacuation and rescue operations; the extent of the no-fly zone; the status of various airports and heliports in the area.