When thousands of people gather to celebrate or to mourn, authorities have to be aware of the risks connected with such crowds.

In the German city of Duisburg during a recent music and dance event, a crowd of several hundred thousand spectators ran out of control. Thousands tried to enter a festival area while at the same time thousands were attempting to leave the area through a narrow underpass. The outcome was twenty-one dead, many people trampled underfoot, and some five hundred hospitalised.

These crowd-related accidents happen all over the world. For example, the annual Hajj, when two million pilgrims visit Mecca in Saudi Arabia, is also notorious for such tragic incidents.

Crowd-control techniques, which have become critical to the organisation of mass events, are partly based on estimations and assumptions. How many people may be expected, and of what kind? How will they react to events? What kind of weather is forecast? And in the case of a football match or other game, which side is likely to win?

In contrast, some other aspects are fixed, and that is where geomatics comes in: maps are needed of the location and surrounding area. The exact location of obstacles and other objects which could influence crowd behaviour. Dimensions of roads, tunnels, bridges, stairs, and the like are invaluable, and aerial and satellite photographs could give additional information like, for example, Google street-view pictures.

However, to be of any use this information has to be up to date, really up to date. Directing a crowd into a passageway through stands recently narrowed but not yet shown on maps is, literally, a recipe for catastrophe. Crowd control then rapidly morphs into disaster management. Constant updating of information should be a first priority for organisers.

Communication is also very important. Next to internal connections between police and security officers at the site, organisers should communicate with the crowd by means of displays, sms and other mobile methods. Many people in a crowd will be using their mobile phones in the case of a threatening situation, either to make a call, to send a text or to tweet. As more and more mobile phones are equipped with GNSS receivers, this information is location-based and could thus be used to gain better insight into the actual situation.

The photograph shows a crowd being directed into a narrow tunnel during the Love Parade in Duisburg (Germany).