Reasoning About Threats: From Observables to Situation Assessment

We propose a mechanism to assess threats that are based on observables. Observables are properties of persons, i.e., their behavior and interaction with other persons and objects. We consider observables that can be extracted from sensor signals and intelligence. In this paper, we discuss situation assessment that is based on observables for threat assessment. In the experiments, the assessment is evaluated for scenarios that are relevant to antiterrorism and crowd control. The experiments are performed within an evaluation framework, where the setup is such that conclusions can be drawn concerning: 1) the accuracy and robustness of an architecture to assess situations with respect to threats; and 2) the architecture's dependence of the underlying observables in terms of their false positive and negative rates. One of the interesting conclusions is that discriminative assessment of threatening situations can be achieved by combining generic observables. Situations can be assessed with a precision of 90% at a false positive and negative rate of 15% using only eight learning examples. In a real-world experiment at a large train station, we have classified various types of crowd dynamics. Using simple video features of shape and motion, we have proposed a scheme to translate such features into observables that can be classified by a conditional random field (CRF). The implemented CRF shows to classify successfully the crowd dynamics up to 80% accuracy.