The GPS trajectories of mobile phone users can be collected by various software packages installed on mobile phones. However, these GPS data often do not record transportation modes (e.g., walking, riding a bus, or driving a car). We present a collaborative isolation forest (Co-IF) model to identify the transportation modes of mobile phone GPS trajectories. Unlike previous models that identify multiple transportation modes simultaneously, the proposed Co-IF model builds a single-class classifier for each transportation mode and then combines their results. Compared to existing models, the Co-IF model offers competitive performance and shows improved reliability with noisy trajectories. Keywords: Transportation Modes, GPS Trajectory, Isolation Forest, Fréchet Distance

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