Decision makers need quick responses to events that take place at a continually increasing rate and they should incorporate an enormous amount of knowledge such as data, choices and consequences. Also they must have fast access to consistent, high-quality knowledge to compete
[Kim 2005].

In our context, the DSS is used as an emergency management system, able to assist actors in urban disasters

![0_image_0.png](0_image_0.png)

mitigation and to prevent them about potential future critical consequences. The system includes a body of knowledge which describes some aspects of the decisionmaker's world and that comprises the ontology of the domain and past known scenarios. The kernel of the DSS is a multiagent system with three layers. Agents of each layer have their own way of behaving and communicating.

Representation layer : **This layer is composed by factual agents and has as essential aim to represent dynamically and in real time the information of the current situation. Each new entering information is dealt by a factual**
agent that intends to reflect a partial part of an observed situation. Agents interactions and more precisely, aggressions and mutual aids reinforce some agents and weaken some other.

Characterisation layer : **This layer has as aim to**
gather factual agents, emerged from the precedent layer, using clustering algorithms. We consider a cluster of agents, a group of which agents are close from dynamic and evolution manner point of view. The goal here, is to form dynamic structures, where each one is managed by a characterisation agent.

Prediction layer : **This layer is made up of prediction agents. Each one represents an observed scenario**
originally from the current situation. The task of the prediction agents is to compare their scenarios by past ones to provide a closed one of which result may be a potential consequence. This mechanism is based on the case base reasoning, the latter differs from a classic one by its ability to manage a dynamic and an incremental development.

## 3 Taxonomic Organisation Of The Studied Environment

Our perception of the environment focuses on two aspects: on the one hand, we observe the concrete objects of the world, the changes of their states and their interaction. On the other hand, we observe the events and the actions that may be created naturally or artificially. We have defined therefore, three categories of objects (Figure 2): Concrete object, Action object and Message object.

Concrete object : **Three types of concrete objects are**
distinguished. The first type is the Person object, which represents an actor of the environment. It is the only object that has the ability to act and to interact and of which behaviour and state evolution are usually predictable. The second type is the Passive object. Two subcategories are identified: immobile objects as buildings and roads networks, and mobile objects like the means of transport. The observation of these objects is the simplest one, because they do not have any behaviour. Their observation is reduced only to the description of their current state. The third type is the Mean object.

It is created at a given time and for a particular purpose. Its existence duration varies in time, according to the objective for which it is created. For example, a car is considered as a mean since it is driven by a driver, otherwise, it is considered as an immobile object.

Action object : **This type is divided into activities and**
phenomena objects. Both are created at a given time and are limited temporally without a priory knowledge of the bounds. Phenomena are unpredictable events that start at a given time. Their observation is the most complex because of their uncertainties and their rapid evolutions. Activities are the actions sequences performed by actors. Generally, they are ordered and emitted for a particular purpose.

Message : **Messages represent the interactions between**
persons and more precisely the information flows exchanged between the actors. The impact of a message is not easily measurable and it depends on its sender, its receiver and its performative. If the message is stored, its impact will be deferred. The observation of an object in the environment may concern a persistent, temporary or punctual state. A persistent state can become invalid following a rupture. For example, a building with ten floors is a persistent state as long as is not destroyed. A blocked road is also